mmmwm ► PRACTICAL WIRELESS February, 1955 ICROMITE' DRY ELECTROLYTIC CONDENSERS These small but high quality electrolytics have proved so popular that the range has been greatly extended. The use of high-gain etched foil electrodes keeps size and weight down, making the condensers suitable for suspension wiring. Conservatively rated; long shelf life ensured; green plastic insulating sleeving prevents short-circuits. Capacity in rv.F. Peak Wkg. Volts Surge Volts Dimns. Length in Ins. Diam. Type No. List Price Each 50 12 15 W 4! CE87B 2/9 25 50 60 it CE88DE 3/- 1 350 400 U HI 3 CE86L 2/6 8 350 400 il CE99LE 3/3 16 350 400 2,1 B CE9ILE 41- 32 350 400 2* !* CE93LE 6/- 4 450 550 Hi Hi CE99PE 3/3 8 450 550 CE90PE 3/6 16 450 550 I \i CE92PE 5/- 32 450 550 2\l 1 ,v CE94PE 7/6 THE TELEGRAPH CONDENSER CO. LTD RADIO DIVISION: NORTH ACTON • LONDON • W.3 • Telephone : ACOrn 0061 THE WORLD-FAMOUS CAMBRIC CONE ^enforian HIGH FIDELITY UNITS • A most notable advance in loudspeaker design acclaimed by experts (including F. J. Camm, John Gilbert, P. Wilson, H. J. Barton-Chappie and L. Ormond Sparkes). It provides instant matching of the speech coil impedance at 3 ohms, 7.5 ohms and 1 5 ohms, and is available on the models marked with an asterisk. This development has added immeasurably to the great popu- larity already achieved among Hi-Fi enthusiasts. Your dealer can show you these new units, or they may be heard at our London Office, 109 Kingsway, any Saturday between 9 a.m. and noon. BASS REFLEX CONSOLE CABINET Easily assembled polished walnut veneer cabinet, designed AT REMARKABLY LOW to give maximum reproduction from either the 10" or 12" Cambric Cone unit. Packed flat, with screws, ready for assembly. Size 32" x 22" x 16 . Price £ I 0 - I 0 - 0 Corner Console Cabinet (8 unit) £5.10.0 WHITELEY ELECTRICAL RADIO CO. LTD MANSFIELD • NOTTS Model HF5I0 . .. 5" Steel Unit „ HF5I0 . .. 5" Die cast Unit „ HF6I0 . .. 6" Steel Unit „ HF6I0 . .. 6" Die cast Unit „ HF8I0 . .. 8" Steel Unit „ HF8I2' . .. 8" Die cast Unit „ HF9I2* . .. 9" Die cast Unit „ HFI0I2*. ..10" Die cast Unit* „ HFI2I4 . ..12" Die cast Unit COST ...£1.17.6 ...£1.19.6 ...£2.10.6 ...£2.12.6 ... £3. 0.6 ... £3. 5.6 ... £3. 9.6 ... £3.17.6 ...£9.15.6 (Tax paid) February, 1955 PRACTICAL WIRELESS 65 OSMOR radio products ltd. (Dept. P.56) 418 BRIGHTON ROAD, SOUTH CROYDON, SURREY. Telephone: Croydon 5148/9 These really powerful units in compact form give "quality and performance right out of proportion to their midget size and modest cost. Osmor " Q " Coilpacks have everything that only the highest degree of technical skill can 'ensure— extra selectivity, super sensitivity, adaptability. Size only 1? x 3 J x 21 with (variable iron-dust cores and Polystyrene formers. Built-in trimmers. Tropicalised. Prealigned Receiver-tested and guaranteed. Only 5 connections to make. All types for Mains and Battery Superhets and T.R.F. receivers. Ideal for the reliable construction of new sets, also for conversion of the 21 Receiver, TR1196, Type 18, Wartime Utility and others. Send to-day for particulars ! SEPARATE COILS 4/- A full range is available for all popular wavebands and purposes. Fully descrip- tive leaflets and connection data available. (Optional) new simple fixing 2d. extra. Just note these "5 Star" Features. ♦ Oftly liiu high. • Packed in damp-proof conjiujiewt, * Variable iron- dust cores. ♦ Fitbefljpgs for easy connec- tion. * Low loss Uroly- styrene formers. L. or M.W. T.R.F. REACTION COIL TYPE QFl 11-12 4/9. A range of coils for F.M. Receivers shortly avail- able. A special design of coils now available for reflex circuits. OSMOR STATION SEPARATOR The Separator may easily be tuned to eliminate any one station within the ranges stated and fitting takes only a few seconds. Sharp tuning is effected by adjusting the brass screw provided. NO. METRES 1— 141-250 2— 218-283 3— 267-341 4— 319-405 5— 395-492 6— 455-567 7— 1450-1550 8— 410-550 k'c. COMPLETE CHASSIS CUTTER Type Hole Sizes Prices lin. xllin. fin. xliin. Jin. xllin. Uin. x2in. 1916 18/9 22/8 27/3 Illust. list on request. I I.F.s. 465 k/c. Permeability-tuned with I flying leads. Standard size Uin. x liiin. x 3-iin. For use with OSMOR coilpacks I and others. 14/6 pair. Midget I.F.s 465 * k/c. fin. x 3 in. x 23in.. 21/- pair. PRE- | ALIGNED, 1/6 extra, both types. — I ^.^^ i Send 5d. (stamps) for fully descriptive r|JJL # literature including " The really efficient ^Tm a w 5-valve Superhet Circuit and practical ~ W H m» Drawings." 6-valve ditto, 3-valve (plus W # rectifier) T.R.F. circuit. Battery portable Superhet circuit, Coil and Coilpack leaflets, Chassis Cutter leaflet, and full radio and component lists, and interesting miniature circuits, etc. DIALS— VARIOUS DIALS CALIBRATED TO COILS Metal dials, overall size 52 in. square. Cream background. 3- colour Type MI, L.M.S. waves, M2, L. & M. waves, M3, M. and 2 S. waves. Price 3/6 each. Pointer 1/6 : Drum, Drive, Spring and Cord, 3/2. Type A glass dial assembly, measuring 7in. x 7in. (91 x 91 overall). Mounts in any position. Choice of two 3-colour scales. 24/6. P. & P. 1/6. OUR TECHNICAL DEPT. WILL BE PLEASED TO ANSWER (BY LETTER ONLY) ANY ENQUIRY RELATING TO CIRCUITS WHICH OSMOR COILS OR COIL PACKS ARE USED OR ARE INTENDED TO BE USED- 1 WE ENDEAVOUR TO KEEP ABREAST " WIRELESS WORLD," " PRACTICAL " PRACTICAL WIRELESS " Coronet Four : Beginners' Superhet ; . Modern High Power Amplifier 2 ; Attache Case Portable ; R1155 Converter ; - A.C. Band-Pass 3 ; Modern 1-Valver ; 3-speed Autogram, modern reflex, etc. OF THE TIMES BY BUILDING THE VARIOUS CIRCUITS PUBLISHED L\ WIRELESS," " RADIO CONSTRUCTOR," ETC. WE KEEP STOCKS OF THE COMPONENTS SPECIFIED M WIRELESS WORLD *' " No Compromise " TRF Tuner. " Midget Mains Receiver." Sensitive 2- valve Receiver. Television Converter (special coils in cans available), Midget sensitive T.R.F., etc. "RADIO CONSTRUCTOR" Converting the TR1193 receiver to a general purpose s'het receiver simple crystal diode set. Radio feeder units. Economy 8 W.P.P. Amplifier. Circuit and details available for adding push-pull to the 5/6 valve Osmor superhet. A LIST OF FIXED CAPACITIES AS REQUIRED FOR SWITCH TUNING AVAILABLE ON APPLICATION "Q" COIL UNITS MAKE EASY SWITCHING Wavebands may [be added or changed in a few minutes. "Switching arrangements can be increased as required. Multi-waveband Coilpacks may be easily made up. The Coil Unit consists of Aerial and Oscillator Coils ajid. Trimmers wired and ready to connect to switch. . THE SIMPLEST AND MOST CONVENIENT METHOD OF SET BUILDING SO FAR DEVISED FOR THE AMATEUR. 15'- PER including 4 UNIT foolproof drawings DESIGNERS ARE ASSURED OF FULL CO-OPERATION FLEASE LET US KNOW YOUR REQUIREMENTS NEWCOMERS TO RADIO. WE HAVE A NEW DEPARTMENT READY AND WILLING TO HELP SEND US YOUR PROBLEMS 66 PRACTICAL WIRELESS February, 1955 IMPROVED TWO WAVEBAND T.R.F. COILS Ti h ^ Lon W d Med A u J?\, waveband T.R.F. Coils are an improvement on our REGD. fip^iXiH^if SS 0 ^ 11 ^ i ?°r! l %-* If y £ u wjsn to lm Prove the performance of 1ftcm th « Midget Three Valve A.C. Mains Receiver'\by S. W. Amos (Wireless World February, 1950), we strongly recommend the use of these coils. C*T^ ' The following points are worth noting : 1. Wound on Polystyrene Colour Coded Formers— C2 R.F. Aerial Transformer Colour Coded Blue— C3 Detector with Reaction, Colour Coded Green ' ^°iour ^oaea 2. Litz windings on Long waveband. 3. Single 4 B.A. hole fixing. 4. Coverage : Long wave 800/2.000 metres. Medium wave 190/550 metres 5. Windings are terminated to tinned copper spills which ensure ease of soldering „ , . , . • „ Retail Price— 9/- per pair. bend 1/- in stamps for General Catalogue. Obtainable from all reputable stockists or in case of difficulty direct from works. 500 pF. TWIN GANGED ",7C \ RC U I T A PPU C AT I O N DENCO (CLACTON) LTD., 357/9 Old Road, Clacton-on-Sea, Essex STOP PRESS : Osram " " 912 " high quality amplifier chassis, 14/6. Front panel finished in bronze complete with control markings, 6/6. " Mullard " " Five-Ten " amplifier chassis, 14/6. Front Panel, 6/6. lilllllllllllllllllllllllilllllllllllllllllllllllllllilllllllil SETS OF VALVES Ten EF50 (Ex-Brand New Units), 5/- each 45/- Set 6K8G, 6K7G, 6Q7G, 5Z4G.6V6G ...37/6 „ 1R5, 1S5, 1T4, 1S4, or (3S4 or 3V4) ...26/6 ., TP25, HL23/DD. VP23. PEN25 (or QP25) 25/- „ 6K8G. 6K7G, 6Q7G. 25A6G, 25Z5 or 25Z6G 37/6 ,. 12K8GT, 12K7GT, 12Q7GT, 35Z4GT, 35L6GT or 50L6GT37/6 ,. 12SA7GT, 12SK7GT, 12SQ7GT, 35Z4GT, 36L6GT or 50L6GT37/6 ,. CRYSTAL MICROPHONE INSERTS POST FREE POST FREE Ideal for tape recording and amplifiers. No matching trans- former required. Brand New R.F. CNITS RF24 20-30 mc/s 15/- post free RF25 40-50 mc/s 10/6 post free RF26 50-65 mc/s 35/- post free RF27 60-80 mc/s 35/- post free HENRY'S (RADIO LTD.) We have over 20,000 American and B.V.A. valves in stock. ALL VALVES NEW AND GUARANTEED. 1D8GT 10- 1A7GT 12/6 1H5GT 10/- 1N5GT 10 - 1C5GT 10- 1Q5GT 10/- 1A5GT 10- IS5 7/6 IS4 7/6 7/6 IT4 IR5 7/6 3S4 7/6 7/6 3V4 1LN5 8/- 1LD5 8/- 5U4G 8/6 5Z4G 8/6 5Z3G 8/6 MU14 8/6 7/6 6B8 6K8G 9/- 6K7G 6/6 6J7G 6/6 6N7GT 7/6 6L7 7/6 6AM6 42 43 75 . 80 6L6G 6Q7GT 6SQ7GT 6SG7GT 6SJ7GT 6SK7GT 6SN7GT 6SL7GT 6SC7 6V6G 6V6GT 6F6G 6AC7 6AG7 6C5GT 6J5GT 12A6 12K7GT 12K8GT 12Q7GT EY51 EF41 -EL4A EZ40 EM34 35W4 12/-IUL41 11/- UY4.1 11/- UF41 10/- DK40 10/- 50B5 8/-|50C5 9/- 12SA7GT 8/6 8/6 12SQ7GT 8/6 8/6 8/6 12SJ7GT 8/6 12SK7GT 8/6 8/6 12SR7 7/6 10- 6D6 6/6 8/6 6C6 6/6 8/6 6A7G 8/6 6/6 6A8G 8/6 8/6 TZ40 37/6 8/6 0Z4 7/- 9/- 25Z5G 8/6 9/- 25Z6G 8/6 10/- 35Z4GT 8/6 8/6 7/6 35Z5GT 7/6 35L6GT 86 7/6 50L6GT 8/6 6/6 25A6G 8/6 10- 12/6 KT33C 5/- KT66 12/6 5/- EBC33 8/6 7/6 EF54 6/- 8/6 EB34 3/6 8/6 EA50 21- 8/6 Dl 21- 11/- EF80 10/6 11/- EABC8010/- 11/- ECC85 10/- 10/- EZ80 9/- 10/- EM80 10/- 10/- ECL80 12/6 6/6 6/6 EF36 EF39 EK32 EL32 7/6 EF50 (Red Syl.) 10/ HL2 LP2 KT2 VP2 SP2 TDD2A VP2B 215SG 866A 354V 4D1 9D2 8D2 PEN46 AC6PEN 6/6 VP41 7/6 TH233 10/- 41MP 7/6 SP61 4/- 3/6 4/- 5/- 8/6 8/6 8/6 8/6 4/- 15/- 5/- 4/- 4/- 4/- 7/6 SP41 D41 VP23 4/- 5/- 6/6 8/- 6/6 6/6 8/- 8/6 4/- HL23DD 6/6 TP25 PEN25 QP25 QP21 TP22 ATP4 MS/PENB 7/6 MS/PEN 7/6 VP4 (5 or 7) AC/PEN 10/- FC13C 10/- FC13s/c 10/- 42SPT 6/- PENDD4020 12/6 VT501 7/6 U19 10/- ML4 6 - PL81 PL82 PY81 PY82 11/-I12AT6 10/- 12AT7 10/- 12AU6 10/- 12BA6 PCC84 12/6 12BE6 PCF82 12/6|6X4 8/- 9/- 9/- 9/- 10/- 8/6 MORSE PRACTICE BUZZER Complete with tapper and 4 volt ' buzzer on baseboard, 6/-, brand new. Post paid. 931A PHOTO-CEEES as described in V P/T " Nov. issue. 50/- each. Data supplied. Equivalent to Mazda 27M/2. INDICATOR UNIT TYPE 182A This unit contains VCR517 Cathode Ray 6in. tube, complete with Mu-metal screen, 3 EF50, 4 SP61 and 1 5U4G.valves, W/W volume controls, resistors and condensers. Suitable either for basis of T/V or Oscillo- scope. " Radio Construc- tor " 'Scope constructional circuit included. 67/6 (plus 7/6 carr.). CRYSTALS 200 kc/s. 2-pin (U.S.A.) ... 10/- 465 kc/s. 2-pin (U.S.A.) ... 10/- 500 kc/s. 2-pin (British) ...15/- T.C.C. .1 5/7,000 v. wkg. Type CP58QO. Bakelite Case. 7/6 each. B.I. .1 2,500 v. wkg. Bakelite Cased, 4/-. CATHODE RAY TUBES (Brand New) VCR97 (slight cut-off ) 15/- VCR97, guaranteed full T/V Picture ... 40/- VCR517C, guaranteed full T/V Picture... 35/- VCR139A, guaranteed full T/V Picture ... 35/- 3BP1, guaranteed full T/V Picture ... 30/- Carr. & ' packing on all tubes, 2/-. 28-page Catalogue, 3d. Open Mon.-Sat. 9—6. Thurs. 1 p.m 5, HARROW ROAD, PADDINGTON, LONDON, W.2 TEL. : PADDINGTON NEON INDICATOR STRIKING 80v. 2 6 1008/9, 0401. February, 1955 PRACTICAL WIRELESS 67 iiiu.n Youn own r radio: We can supply all the parts (includ- ing- valves, 5in. moving- coil speaker, > cabinet, chassis, and everything [_ down to the last nut and bolt) to enable YOU to build a professional- looking radio. The chassis is punched and drilled ready to mount the components. There is a choice of any of three attractive cabinets 12in. long, Sin. wide by 6in. high, as follows : either ivory or brown bakelite, or wooden, finished in walnut, and easy-to-follow point-to-point and circuit wiring diagrams supplied. MODEL T.R.F. RECEIVER Complete rains supplied. MODEL 2. SUPERHET RECEIVER This is a powerful midget 4 valve plus metal rectifier Superhet Receiver with a valve line-up as follows : 6K8, 6K7, 6Q7, 6V6. The dial is illuminated and coverage is for the Short Wave bands between 16-50 metres, the Medium Wave bands between 190-540 metres, and the Long Wave bands between 1,000-2,000 metres. Operates on 200/250 volts A.C. mains. Plus 2/6 Packing, i>7 Carriage, Insur. *f ■ ■ ■ ■ W T.R.F. RECEIVER. We can supply this Receiver ready built at £6 15. 6. plus 3/6 p.c. ALL COMPONENTS SUPPLIED ARE GUARANTEED FOR ONE YEAR Instruction Booklet and priced Parts List for either of the above available separately at 1/-. This money will be refunded If circuit diagram is returned as NEW within 7 days. When ordering please staW Model No. This is a 3 valve plus metal rectifier T.R.F. receiver with a valve line-up as follows : 6K7 (IIF). 67.1 (Det) and 6V6 (Output). The dial is illuminated and when assem- bled the receiver presents a very attrac- tive appearance. Coverage is for the Medium and Long- Wave bands. Operates on 200/250 volts A.C. Mains. Plus 2/6 Packing. Carriage, Insur. £5.10.0 MAINS NOISE SUPPRESSOR KIT Consisting of 2 specially designed chokes and 3 condensers. Extremely effective, cuts out all mains noise. Can be assembled in existing receiver or separately as desired. Complete jy j-y -| plus 1/- with circuit diagram BATTERY CHARGER KIT Incorporates metal rectifier. Trans- former is suitable for A.C. mains 200/250 volts. Charges either 12, 6 or 2 volt accumulator at 1 amp. Complete with circuital a- 1Q/-I-I plus 1/6 post 1 ^/ 1 1 and pack ing. g ram . Price TERMS OF BUSINESS : Cash with order (or C.O.D. Post items only) ; all orders for small items totalling over £2 post free unless otherwise stated. Money back Guarantee on all goods. This is a 3 valve 3 stage Amplifier for use with Gramophone, Microphone or Radio. Valve line-up is as follows : 6SL7. 6V6. 5Z4. Negative feed-back. Tone Control. Voltage adjustment panel incorporated. 4 watts output. For operation on A.C. Mains 200/250 volts. The complete Kit includes every item down to the last nut and bolt, drilled and punched chassis, and comprehensive point-to-point wiring circuit diagram. Chassis dimens. : 8in. x 6in. x 2lin. ALL COMPONENTS SUPPLIED ARE GUARANTEED FOR ONE YEAR PRICE £4.5.0 The Output Transformer supplied is for use with a loudspeaker of 3 ohms impedance and we would suggest that the output of the completed amplifier justifies the use of one of the latest W.B. H.F. Speakers which can be supplied as follows: 8in., 60/6 ; 9in., 67/- ; 10in., 73/6. All plus 2/6 pkg., carr., ins. Circuit Diagram only, available separately at 1/-. To those who require this Amplifier ready-built we can supply it at £5.1.0 plus 3/6 pkg., carr., ins. Office : (Dept. P.W.), 34, Montpelier Rise, London, N.W.ll. Telephone : Meadway 1736. Works and Despatch Dept.: 44, Tottenham Street, London, YV.l. Tel: Langham 1151. Open 9 to 6 p.m. Daily , Saturdays 1 pm. PERSONAL CALLERS WELCOMED NEW LEARN THE PRACTICAL WAY. | With many of our courses we | supply actual equipment. Courses | include: Radio, Television,! Electronics, Draughtsmanship, i Carpentry, Photography, and J Commercial Art, etc. POST THE COUPON TODAY FOR OUR BROCHURE ON THE LATEST METHODS OF HOME TRAINING FOR OVER 150 CAREERS & HOBBIES PRIVATE AND INDIVIDUAL TUITION IN YOUR OWN HOME City and Guilds Grouped Certificates in Telecommunications : A.M. Brit. I.R.E. Examination, Radio Amateur's Licence, Radio and Television Servicing Certificates, General Radio and Television Courses, Radar, Sound Recording, etc. Also Courses in all other branches of' Engineering and Commerce. The advantages of E.M.I, training. ■^The teaching methods are planned to meet modern industrial requirements. *We offer training in all subjects which provide lucrative jobs or interesting hobbies. *A tutor is personally allotted by name to ensure private and individual tuition. "jlrFree advice covering all aspects of training is given to students before and after enrolling with us. Courses from 15/- per month The only Po si world-wid tal College which is part of e Industrial Organisation. POST THIS COUPON TODAY Send without obligation your FREE book. E.M.I. INSTITUTES, Dept. 32K 43 Grove Park Road, London, W.4. Phone: Chiswick 44I7/8. NAME ADDRESS.. FEB. SUBJECT(S) OF INTEREST . I ICI6 68 PRACTICAL WIRELESS February, 1955 or SUPPLY UNIT RECTIFIER FOR NO. 43 TRANSMITTER Ex Canadian Army, in original wood case. Input 110 v. A.C. 50/60 c/s. 1.7 kVA. Output (HT1), 2,100 v. 375 mA. (HT2), 500 v. 400 mA., plus H.T. lines 450 v.. 265 v., also 383 v. regulated and neg. bias. 250 v., 150 v., 80 v. Making three complete power supplies all fed via double choke condenser, input circuits. Valves are 4/866A/866, 5Z3. 6SJ7, 2/6A3, VR150/30 (stab.) and IV (time delay). The complete unit mounted in metal case with lid shock mounted. Dim. : 2ft. 6in. x 1ft. 6in. x lit. Finish olive drab. Wgt. 4201b. Ask for £15 Carriage P/H26. 15/- extra. BC-456 SPEECH MODULATOR UNITS Part of SCR-274-N. " Command Equipment" (U.S.A. made.) Complete with valves 1625, 1215 and VR150/30 transformers, relays, etc.. less dynamotor. Overall Dim.: lOiin. x 7]in. x 4Jin. Loose stored, etc. Ask for 17/C Carriage P/E42. 11/0 each. Paid. Also available, BC-456, as above in original carton. Ask for 07 /C Carriage P/E42A. L 1/0 each. Paid. Circuit, 1/3. ELECTRONIC IGNITION TESTER Type V.E.D. Patt. 563562, by English Electric in original wood case. A Cathode Ray tester for checking ignition of internal combustion engine while engine is operating, will oper- ate from 6, 12 or 24 volts D.C. or 230 v. A.C. Built into black crackle case with hinged front and carrying handle. Dim. : 15 \ in. x 8 Jin. x ll]in. No leads or instruction book available. £"| 5 Carriage P/H535. each 10/- extra. RECEIVER UNIT TYPE 25 Ref. : 10P/1L. Part of TR1196, range 4.3-6.7 mc/s, with valves, 2/VR53 (EF39), 2/VR56 (EF36), VR55 (EBC33), VR57 (EK32), 2/1 F.T. 460 kc/s, etc., in m£tal case, 8Jin. x 6?in. x 61in. Ask for OC I Post & Packing P/H299 L*r each. 3/6. EX U.S.N. TEST OSCILLATORS, TS-24/ARR2 Low/High frequency, battery powered for TBX alignment, H.F. signal 245 mc/s. I. F. signal tunable 540 to 830 kc/s with valves, 2/955 acorn triodes and clockwork time switch with calibrated dial 0/30 minutes. Unit Dim.: 9 Jin. x7iin. x7in. Finish black. Ask for 97 /C Carriage P/H364. 41/0 each 3/- extra. T1154B TRANSMITTER UNIT Medium/High powered for C.W.-M.C.W. R/T 3 ranges. 10-5.5 mc/s, 5.5-3 mc/s, 500-200 kc/s. Complete with 4 valves,' etc., in metal case, 14in. x 16 Jin. x 8 Jin. External Power Supply required. Ask for QQAfi Carriage P/E5A. aa/U each. 7/6 extra. SUPPRESSOR UNIT 5C/870 *> Contains 4 H.F. chokes and 4 tubular con- densers 0.1 mfd. 250 v. D.C, carrying 5 amps. (2 sets on each lead), each choke and con- denser separately screened in compartments of aluminium alloy box, 4£in. x 4in. x 2in. 4-hole fixing. Ask for 0 /£ Post P/H907. */0 each 1/- extra. VISUAL INDICATOR TYPE 1 Ret : 100/2. / Dual reading left/right D.F. meter for R1155, 2lin. Scale overall Dim. : 3 J in. x 2 Jin. In used condition. Ask for 1 0 /IS Post P/H862A. l*/0 each. Paid. JEFFERSON TRAVIS UF-2 TRANS- CEIVER CHASSIS (U.S.A. made). Less valves and partly stripped by the M.O.S. Ask for 1 7 /C Carriage P/H518. I '/0 each Paid. Circuit at 2/3. 'indicator unit TYPE 6 With VCR-97 tube and valves, 4/VR91 (EF50), VR54 (EB34), 3/VR92 (EA50), VR78 (Dl), etc. Dim. : 18in. x 8£in. x7iin. Wgt. 211b. In or iginal wood case. Ask for D/H524. INDICATOR UNIT TYPE 6H With VCR-97 Tube and valves 4/VR91 (EF50), 3/VR54 (EB34). Dim. : 18in. x 8J.in. x 7Jin. Wgt. 221b. In original wood case. Ask for CQlfi Carriage P/E777. *»y/° each. 5/- extra. R.F. UNIT TYPE 24 In original carton With valves 3/VR65 (SP61). etc. Range 20-30 mc/s switched tuning. Dim. : 9 1 in. x 7iin. x 4|in. Wgt. 71b. Ask for 111/ Post P/H850. IW /" each. 1/6 extra. R.F. UNIT TYPE 25 In original carton Range 40-50 mc/s, otherwise as R.F. 24. Ask for 14/C Post P/H847. , MCI Carriage •*«>/ " each. 5/- extra. 1 each 1/6 extra. Please note Carriage and Postal Charges refer to the U.K. only. Overseas Freight etc., extra. Order direct from : — CLYDESDALE 2, BRIDGE STREET, GLASGOW, C.5. SUPPLY CO. LTD. Phone: South 4451/7 ALPHA FOR VALVES GUARANTEED NEW AND BOXED. AC/P 6/9 H30 5 7.9 PEN 220 A UBC41 11 VU111 3 6 3Q5 AC/ PI ii- HG3 4' UF41 11 VU120A 3/- 3D6 ACfi/l EN HL2 5 PL81 14 6 VR53 6 6 VV77 8/6 3S4 sie HL2.if>D7/6 PL82 11 6 VR54 2 W81 10/ 3V4 ATP4 6/6 HP2I0 6 9 PL83 13/ VR55 7 6 X65 10/- 4D1 CV173 10/ HR2I0 6 9 PM2B 5 VR5<> 6 X66 11/6 42 CV280 7/6 K !'.( 182 8 6 PM221) 5 VR57 8 X79 11/- 5U4 DH73M 10- KF35 8 6 PM12M 10 VR65 3 9 Y63 9/- 5Y30T KB4 1 11/ KL35 8 6 PP225 5/ VR65A 3 6 Z21 10/- 5Z3 BBP60 11/6 KLL32 8 6 PY80 11 VR66 3 9 Z22 6/9 5Z4G BOW' 9/- KT2 ft PY81 11 6 VR91 6 1A3 9/- 6A7 BC4 '.'(I 9/- KT33C 11 6 PY82 10 6 VR91 8 1A5GT 6/6 6A8G BCH42 10/6 KT0L 13 PX25 15 7 6 VR92 2/ IA7 11/6 6AC7 K«!L80 14/6 KTflfi 13 QP21 VR105/30 1C5GT 8/- fiAG3 imvAi 11/- KTW61 7 9 SI 30 8 6 5 6 1M 7/6 6AJ5 KP4 1 10/ RTW63 7/ - SP22 8/- VR116 4 1LD5 6/9 6AK5 KP80 13/6 KT74 87 HP220 6 9 VR119 4 1N5 lQ Ja 7/6 6AL5 EL2 126 KTZ4L 6 9 SH2I0 5/- VR123 6 6 1R5 6AM5 BL33 13/- LD2M 6 9 TP22 6 9 VRI36 11- 1S4 7/6 6AM6 KM I 11/6 8/- LP220 6 9 TP26 9/ VRI37 6 3 IBS • 7/6 6AQ5 KML31 MH4 5 6 U10 B/- VC150/30 1T4 7/6 6AT6 UY5L 14'6 MHM 6 9 U22 8/ ■ 9 1U5 8/- 6B4 EZ40 10/ MS/PEN5/- U25 14:6 VP23 8/ 220 VSG 6/9 6B8G KZ4L 11/ P41 9 U281 10 6 VT52 8 2A3 6/9 6BAH Ell 48 21 P215 5 8/ U403 10 VT75 7 6 2X2 5/- GBE6 FW 4/500 PEN 25 UAF42 12 VT501 6 3A4 8/- 6BR7 10/- PEN 46 8 6 PB41 9/ VU39 8 6 3Q4 9/- 6BW6 10/- 5/- 8 6 8/- 3/ 8/- 8 6 8 6 8,6 8/6 10/6 10/- 6/6 7 6 •/- 9 »/- 7 6 76 8 6 8; 6 TERMS : Cash with order or C.O.D. Postage to be added to orders as follows : 9d. up to I0j- ; //- up to 20/- ; 1/6 up to 4©/- ; 21- up to IS. MAIL ORDER ONLY : Send 6d. in stamps for illus. catalogue. WHEN ORDERING PLEASE QUOTE "DEPT. P.W. 6C4 6C5GT «C6 6C9 6D3 6D6 6F6G 6F«M 6F8G 6F13 6F15 6G6G 6H6 6J5G 6J5GT 6J5M 6J6 6J7G 6K6GT 6K7G 6K7GT T.K7M 6K8G 6K8GT 6L6G 6L7M 6L18 6N7 6Q7G 6Z4 6Q7GT t3R7G 6SA7GT 8/ 6SG7 7/6 6SH' (5SJ7GT 8/ 6SK7 6/3 6SL7 8/- 6SN7GT 9/- 6SQ7 91- 6SS7 8/- 6ST7 : 7/6 6U4GT 15/- 6U5G 8 6 6U7G 91- 6V6G 7/6 6V6GT 7/6 6X4 8/- 6X5GT 7/9 B6 D8 7H7 7Q7 80 807 8D2 9001 9002 900:: 9004 9006 8/6 8/6 8/6 9/- 9 8 8/- 8/6 8/6 8 6 10/ 8/- 8/6 7/6 5 6 5 6 5/6 5/6 954 21- 4/9 956 3/6 10F1 10/- 10LD11 11/- 10P13 11/6 10P14 11/6 12A6 6/9 12AH8 11/6 12AT7 9/- 12AU7 9/- J2AX7 10/- P2C8 8/- P2H6 5h 12J5 91- V2K7 91- 12K8GT 91- 12Q7GT 91- 12SC7 7/6 12BH7 " 12SJ7 12BK7 12SL7 12BQ7 12SR7 20DI 20F2 20PI 210VPT 5/ 2201 PT 5/ 25A6G 9/ 25L6GT 8/6 25U4GT12/- 25 Y5 91- 25Y5G 9/- 25Z4G 9/- 25Z5 9/- 25Z6GT 8/6 35L6GT 8/9 35 W 4 10/- 35Z4GT 8/6 50L6GT 8/6 5/6 8/6 6/6 91- 8/6 7/6 10/6 12/6 15/- HEADPHONES Type CLR. Low resistance 120 (l , 7/6 pair. Type CHR. High resistance 4,000$), 11/6 pair. Type DHR, a super job, 13/9 pair. Headbands, wide type, 1/9 eah earpiece, light headband complete with lead and Igranic jack plug, 13/6 pair. WIRE WOUND RESISTORS Available in the following values : — 25 £2, 50 f2, 100 fl, LSe-Qi 200 f), 250 Q, 350 O, 500 fi, 1,000(2, 1.500Q, 2,000 0. 2,5000, 3,500 ft, 5,00012, 6,80012, 1 0,000 Q. Toler- ance plus or minus 10 per cent. Ratings 5 watt, 1/- ea. ; 10 watt, 1/3 ea. ; 15 watt, 1/9 ea. MAINS TRANSFORMERS 3-way Mounting Type MTl. Primary 0-210-220-250 v. Secondary 250-0-250 v. 80 mA., 6.3 v. 4 amps., 5 v. 2 amps., with taps at 4 v. on filament Minding. Price 17/6 each. MT2. Primary 0-210-230-250 v. Secondary 350-0-350 v. 80 mA. 6.3 v. 4 amps. 5 v. 2 amps. Both filament windings tapped 4 v. Price 17/6 each. MT3. 30 volt 2 amp. tappings as follows : 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24 v. 17/6 each. - f( | ))AtPHJi RADIO SUPPLY CO. \^SLy ild Winces/ chambers,! victoria square, leeds 1. February, 1955 PRACTICAL WIRELESS 69 PREMIER RADIO COMPANY OPEN TILL P.M. SATURDAYS B. H. MORRIS & CO., (RADIO) LTD. Telephone : (Dept.P.W.) 207, EDGWARE ROAD, LONDON, W.2 r J ^£gffilP&& BUILD THESE NEW PREMIER DESIGNS 3-BAND SUPERHET RECEIVER MAYBE JP7 iQ C Plus 2/6 Pic. BUILT FOR *i ■ I^.O & Carr Latest type Superhet Circuit using 4 valves and metal rectifiers for operation on 200/250 volts A.C. mains. Waveband coverage — short 16-50 metres, medium 180-550 metres, and long 900-2,000 metres. Valve line-up 6KB freq. changer. 6K7 IF, 6Q7 Detector AVC and first AF. 6V6 output. The attractive cabinet to house the Receiver size 12in. long, 6Jin. high. 5Jin. deep can be supplied in either WALNUT or IVORY BAKELITE or WOOD. INSTRUCTION HOOK 1- (P<»sl free) TRF RECEIVER MAY BE O C i C A Plus 2/6 Pk. BUILT FOR o&Ob IObU & Carr. The circuit is the latest type TRF using 3 valves and Metal Rectifiers for operation on 200/250 A.C. mains. Waveband coverage is 180/550 metres on medium wave and 800/2,000 metres on long wave. The dial is illuminated and the Valve line-up is 6K7 H.F. Pentode 6J7 Detector and 6V6— Output. Which includes Assembly and wiring diagrams, also a detailed Stock List of priced components. CABINETS— PORTABLE Model PC/1 Brown Rexine covered, 22/6. Overall dimensions 151n. x 13,1 in. x 5in. Clearance under lid when closed ?Hn. Model PC/2 Grey Lizard Rexine covered, 45/-. Overall dimensions 15in. x 13in. x 6in. Clearance under lid when closed 2i in. Model PC 73 Rexine type covering in various colours, 69/8. Overall dimensions 162 in. x 14 Jin. x 10Un. Clearance under lid when closed 6{in. All the above Cabinets are supplied with Panel, Carrying Handle and Clips. Packing and Postage 2/6. Send for details of the Premier Wide angle Televisor design which may be built for £30. DECCA MODEL 37A DUAL SPEED RECORD PLAYER Includes turnover crystal pick-up with sapphire stylus and a light-weight plastic spring-balanced arm. Heavy gauge pressed steel case with brown enamel finish in good quality for operation on A.C. mains 200/250 v. 50 c.p.s. Supplied complete, £6.19.6. Plus pkg. and carr. 5/-. GARRARD S5 GARRARD Rim Drive 78 r.p.m., com- plete with magnetic pick-up and turn- table, £4.19.6, plus 2/6 pkg. and carr. TERMS OF BUSINESS : CASH W ITH ORDER OR C.O.D. OVER £1. 4-WATT AMPLIFIER MAYBE £M -g A A Plus 2/6 Pk. BUILT FOR 36*Ib IU.U & Carr. Valve line-up 6SL7, 6V6 and 6X5, FOR A.C. MAINS 200/250 VOLTS. The output Valve is of the beam type and feeds 4 watts into a specially de- signed output Transformer which is suitable for' either 3-ohm or 15-ohm Speakers. Negative feedback is applied from the secondary of the out- put Transformer over the whole Ampli- fier to the input stage giving an excellent frequency response. Due to the high gain and wide range tone controls any type of pick-up may be used. Overall size 9 x 7 x 5in. Price of Amplifier com- plete, tested and ready for use, £5/5/-, plus 3/6 pkg. and carr. INSTRUCTION BOOK 1/- (post free) which includes Assembly and wiring diagrams, also a detailed Stock List of priced components. ^X^TTub£s~ VCR 517C 6Jin. picture. This tube is a replacement for the VCR97 and VCR517. Guaranteed full size picture. PRICE, 35/-. Plus 2/6 pkg., carr. ins. VCR 516 9in. Blue picture. Heater Volts 4, Anode 4 Kv.. in Manufacturer's original Carton. Plus 5'- pkg., carr., ins. PRICE, £1.19.6. COMPONENTS AVAILABLE FOR THE MULLARD AND OSRAM AMPLIFIER DESIGN RADIO SUPPLY CO. (L L E T E D DS) 32, THE CALLS, LEEDS, 2 Terms C.W.O. or C.O.D. No C.O.D. under £1. Postage 1/- extra under 10/-, 1/6 extra " 2/6 extra under £3. Open 9 to 5.30, Sats. until 1 p.m. ms ( under £1, 21- extra under £2. 2 6 extra Catalogue 6d. Trade List 5d. Open i S.A.E. please with enquiries. R.S.C. 4 - 5 WAIT HIGH GAIN QUALITY AMPLIFIER A5— For Mains input 200-250 v..50cc/s. Kit includes 4 valves, punched chassis crackle finished, point to point wiring diagrams and instructions, and every item required. Only 50 m.v. input required for full output, making it suitable for use with any type of pick-up. H.T. and L.T. supply available for Radio Feeder or Tape Deck Preamp Output for 2-3 ohm speaker. Separate Bass and Treble Controls. Suitable for Standard or Long-Playing Records in the home or small club. etc. Only £4/15/-. carr. 3/6, or ready for use, 25/- extra. chassis (Unpunehed) 18s.w.g. aluminium amplifier (4-sided). 12"x9"x2r 6/11 14". 10" -3" 7/11 14"x9"x2i" 6/11 16"xl0"x3" 8/3 18 s.w.g. aluminium receiver type. 6"x3r*ir 1/11 10"'5|"x2" 3/3 7rx4f"x2" 2/9 ll">6"x2r 3/11 16 s.w.g. aluminium, receiver type. 12"x8',x2.V" 5/3 20"x8"x2i" 8/11 16"x8"x2J" 7/6 16 s.w.g. aluminium, amplifier type, 4- sided. I 12"x8"x2r 7/11 20"x8'x2i." 13'6 16" , x8"x2i" 10/11 14*xl0"x3" 136 EX-GOVT. TRANSFS. 230 V. 50 C/CS. 0-11-22 v. 15 a., 35/9 ; 0-13-18-20 v. 35 a., 79/6 ; 7.7 v. 7 a. C.T. 4 times. 259. 48 v. 1 a., 9/9 I 460 v. 200 mA. 6.3 v. 5 a., 27/9 : 400 v. C.T. 150 mA, 4 v. 6a., 6.3 v. 6 a., 6.3 v. 0.6 a., 4 v. 6 a., 4 v. 3 a.. 4 v. 3a., 4 v. 3 a.. 5 v. 2 a., 229 : 1.220 v. 350 mA. 610-0-610 v. 150 mA. 300-0-300 v. 150 mA. 29/9 ; 865-775-690-0-690-775-865 v. 500 mA. 29 6 I 4 v. 2.5 a., 4/9 ; 4 v. 6 a.. High Ins., 7/9: 250-0-250 v. 200 mA. 6.3 v. 8 a.. 5 v. 3 a., 22'9; I 300-0-300 v. 150 mA. 6.3 v. 4 a., 5 v. 3 a. , 22/9- AMMETERS Moving t oil G.E.C. 0-5 amps, 21 n. scale. 11/9. EX-GOVT. AUTO TRANSFORMERS Double Wound, 50 e/cs. 10-0-200-220-240 v. to 10-0-275-295-315 v. 1,000 watts, 69/6 ; 0-230 v. to 0-230 v. tapped every 11 volts ; from 57.5 v. 5,000 watts (21 amps.). £6/15/-. 0-110-240 v. to 0-130-140-150-160-170 v. 1,500 watts, 55/-. Single winding, 15-10-5-0-195-215-235 v. 500 w., 27/9- EX-GOVT. SMOOTHING CHOKES 250 mA, 10 H 50 ohms 14/9 250 mA, 10 H 100 ohms 14/9 150 mA. 10 H 100 ohms 11/9 100 mA, 10 H 150 ohms Trop 6/9 50 mA, 5-10 H 200 ohms 2/9 L.T. type 1 amp. 2/9 EX-GOVT. METAL BLOCK (PAPER) CONDENSERS : 2 mfd. 1 3,000, v.. 2/9 ; 4 mfd. 500 v., 2/9 ; 4 mfd. 750 v., 3/9 ; 4 mfd. 1,500 v., 4/9 ; 4 mfd. 2,000 v., 7/9 ; 6-6 mfd. 400 v., 5/11 ; 8-8 mfd. 500 v., 6 11 ; 4 mfd. 400 v., plus 2 mfd. 250 v., 1/11 ; 8 mfd. 500 v., 5/9 : 15 mfd. 500 v.. 6/9. EX-GOVT. E.ILT. SMOOTHERS .02 mfd. 8,000 v. cans, 1/11 ; .25 mfd. 4,000 v. Blocks, 4/9 ; .5 mfd. 2,500 v. Blocks. 3/9 ; .5 mfd. 3,500 v. cans, 3/3 ; 1.5 mfd. 4,000 v. Blocks, 5/9 ; .1 mfd.. plus .1 mfd. large Blocks 8,000 v., 9/6- EX-GOVT. L.T. SMOOTHING CON- DENSERS.— 2,000 mfd. 12 v. Small Blocks, 1/11. RF26 UNITS, BRAND NEW, CARTONED. Only 39/6. Carr. 2'6. EX-GOVT. T R A N S M I T T E R - R E- CEIVERS. Type TR9D, complete with all valves, 47/6, carr. 5/-. E X-G O VT. ACC UMUL ATORS With Non-Spill Vents. Unused and guaranteed 2 v. 16 A.H., 5/9 each or 3 in wood carrying case 9x7x5ins., 14/9, plus 5/- carr. MIDGET MAINS TRANSFORMERS. Manufacturer's Surplus. Primary 220/ 240 v. Sees. 275-0-275 v. mA. 6.3 v. 2 a. Only 11/9. TV. PREAMPLIFIER (Plessey) For Fringe Areas. Brand New. Complete with 6F13 valve. Only 22/6. R.S.C. MASTER INTERCOMM. UNIT, with provision for up to 4 " Listen — Talk Back" units individually switched. A high gain amplifier allows speech emanating from the rooms containing remote control units to be heard at the master control. The unit is in kit form and point-to-point wiring diagrams are supplied. A walnut veneered wood or brown bakelite cabinet is included. Mains input is 200-250 v. 50 c/cs. H.T. line 300 v. CHASSIS IS NOT V ALIVE." Ideal also for use as " Baby Alarm." Sound amplification 4 watts Price only £5/19/6. " Listen— Talk Back units " in walnut veneer cabinets can be supplied at 35/- each. Full descriptive leaflet lOd. The Master Unit can be supplied assembled and tested for 30/- extra. EX-GOVT. VALVES (NEW) Each Each Each IT4 7/9 6Q7G 9/11 AC5PenDD IR5 7/9 6SN7GT 9/9 9/9 1S5 7/9 6V6G 8/9 EBC33 7/9 3V4 7/9 6V6GT 7/9 EF36 5/9 3S4 9/9 6X5GT 8/9 EB91 8/9 5U4G 10/6 8D2 2/11 EF91 7/9 5Z4G 9/6 807 7/11 EL33 9/6 6F6G 7/9 9D2 2/11 7/9 MU14 8/9 6J5G 5/9 12A6 MS/Pen 5/9 6J7G 6/6 12K7GT 10/6 SP4 5/9 6K7G 5/11 15D2 4/9 SP41 1/11 .6K8G 8/11 25Z4G 9/6 SP61 2/9 6SK7 7/9 35Z4GT 10/6 VU120 2/11 H.T. ELIMINATOR AND TRICKLE CHARGER KIT. Input 200-250 v. A.C. Output 120 v. 40 mA. fully smoothed and rectified supply to charge 2 v. acc. Price with steel case and circuit. 29/6. Or ready for use. 8/9 extra, SILVER MICA CONDENSERS. 5, 10, 15, 20, 25, 30, 35, 40, 50, 100. 120, 150. 200. 230, 300, 400, 500, 1,000 (.001 mfd), 2.000 pfd. (.002 mfd.), 6d. each ; 3/9 doz. One type. 70 PRACTICAL WIRELESS February, 1955 for LONG PLAYING RECORDS Recording techniques have made such strides over the last two years that if you are to get the really superb reproduction made possible by the latest L.P. records you need one of the Acos " Hi-g " Pick-ups. These have been specially designed to meet the very exacting demands of the new records with their shallow groove and microscopic sound " track This special design is necessary on two scores — firstly to do justice to the brilliant recording, and secondly to ensure the longest possible life from L.P. records. Such records are expensive ; even a single playing with an unsuitable pick-up can cause irreparable harm. If you want the best reproduction and value horn your radiogram or record player and L.P. records ask your dealer (or post the coupon below) for details of the " plug-in" ACOS " Hi-g " Pick-ups — specially designed to replace existing pick-ups on most famous makes of record playing equipment. The cost? 32/6CI. (plus io/5d. P.T.) . . * always well ahead ACOS devices are protected by patents, patent applications and registered designs in Great Britain and abroad. — " *~~ — ~~ " l To: COSMOCORD LIMITED, ENFIELD, MIDDLESEX Please send me details of ACOS "Hi-g" replacement pick-up heads NUMBER PLEASE! The new restricted form of London Telephone , Directory no longer includes subscribers, like Cosmocord, in "fringe" areas. Please note therefore that our number is ENFIELD 4022. ADDRESS PW February, 1955 PRACTICAL WIRELESS 71 EVERY MONTH VOL. XXXI, No. 580, FEBRUARY, 1955 COMMENTS OF THE MONTH EDITOR • F.J. CAMM al 'Radio Fees 23rd YEAR OF ISSUE BY THE EDITOR ACCORDING to the chairman of the Mobile Radio Users' Association, the Postmaster- General has illegally collected about £400 million in radio and television licences since 1904. He was referring to a recent judgment against the Postmaster-General for £143 6s. 8d. which had been paid in licence fees by a Colchester firm of engineers, who claimed repayment of these fees charged for mobile transmitters and receivers on their vehicles, on the grounds that they were illegal. It was a test case and the Association was responsible for the case being brought to court. Although the case was concerned with licences for mobile radio users, it applied also to domestic licences, because they are only collectable by the Postmaster-General if the regulation empowering him to do so were laid down. In fact, no regu- lation has been laid down under the relevant Act of 1904. A spokesman for the G.P.O. commenting on the case, in which the P.M.G. submitted to judgment by consent, said that nothing other than land mobile radio licences was in question. The question of the fees which have been extracted for domestic licences was not before the court, and it seems a great pity, as this was a test case, that this matter was not raised also. It is too late now, for the Govern- ment proposes by the usual trick of retrospective legislation to legalise what has been illegally done. If an ordinary citizen does something legal to-day, it can be made illegal by retro- spective legislation, and the late Cripps was one who made use of this politically dishonest method in the case of Sir Leonard Lord and Sir John Black. When the Government itself does something illegal it makes it legal by the same method. Thousands of people have been fined and their convictions recorded for the non-payment of wireless licence fees which, as it now turns out, were illegally demanded. The proposed retrospective legislation should contain a clause ordering these convictions to be expunged from the records and the fines refunded. Of course, current wireless licences of all kinds are operating under the Wireless Telegraphy Act, 1949, and no question arises as to the charges under that Act. The retrospective legislation proposed will validate licence fees collected between 1904 and 1949, and presumably, therefore, will also validate the convictions. The case in question was based on the Bill of Rights under which no British citizen can be charged a fee by a Government department without the authority of Parliament. Under the 1904 Act licence fees for mobile radios could be charged only if regulations were made by the Post Office and the consent of the Treasury obtained. This has never be£n done. The same omissions apply to domestic wireless licences, and it is clear that there has been no power since 1904 to make a charge for a wireless licence. BONA-FIDE RETAILERS •yHE annual report of the Radio and Tele- vision Retailers' Association makes it clear that they intend to compile and publish a list of firms who are bona-fide retailers, complying with the definition of a retailer already agreed between R.T.T.A. and other sections of the industry. The other sections, however, are not willing to co-operate in the compilation of this very necessary and much overdue list. We have many times drawn attention to incompetent radio dealers, but unfortunately the publication of such a list, in view of the present state of the law, cannot protect the public against the charletans, since it is a white list and not a black list, and the omission of a dealer's name from it cannot be taken as indicating that he is not a bona-fide dealer. However, such a list will go a long way towards removing some anomalies in radio trading, especially if wholesalers refuse to supply those not on the list. INDEXES FOR VOLUME 30 HpHE December issue concluded Volume 30 and the index for it will shortly be available price Is. Id., post paid. Arrangements have been made for Messrs. Hazell, Watson and Viney, of 52, Long Acre, London, W.C.2, to undertake the binding of your parts in complete volumes with the appropriate index. If you are desirous of having your parts bound, Messrs. Hazell, Watson and Viney will be pleased to let you have a quotation for the work on receipt of an inquiry. Those readers who prefer to have their copies bound by a local bookbinder can obtain not only the above-mentioned index but also an index for any previous volume from us direct. The order should be addressed to the Publishing Department, George Newnes, Tower House, Southampton Street, Strand, London, W.C.2. — F. J. C. 72 V.H.F. Station ■yHE opening date of the V.H.F. 7" station to serve East Anglia is not yet known, but it will have a temporary mast at first until a permanent one is erected. Printed Circuits Agreement TT is announced that Pye Limited A have acquired an interest in Technograph (Printed Circuits), Ltd., who hold patents for printed circuits all over the world. Both companies intend to work closely together in furthering the development and application of printed circuits, not only in Britain but throughout the world. September Sales Down T> ADTO sales figures for " September were slightly down on the previous month. 12.1 sets per shop were sold in September, compared with 12.3 in August. Radio from Rowridge HTHE new Rowridge television A station on the Isle of Wight is not intended to be used exclusively for TV. It has been officially announced that the BBC plan to transmit three V.H.F. sound pro- By "QUESTOR" grammes from the station when the 500ft. mast replaces the temporary aerial in about a year's time. Broadcast Receiving Licences '"THE following statement shows the approximate number of broadcast receiving licences issued during the year ended October, 1954. The grand total of sound and television licences was 13,701,205. Region Number London Postal ... 1,524,625 Home Counties ... 1,443,030 Midland . 1,207,949 North Eastern 1,587,978 Northwestern ... 1,224,725 South Western ... 992,626 Wales and Border Counties 618,900 Total England Wales Scotland Northern Ireland Grand Total and 8,599,833 1,040,028 219,676 9,859,537 Scriptwriters Alan Simpson {left) and Ray Gallon (right) show a disgusted Tony Hancock their offering for the latest edition of " Hancock's Half-Hour" heard weekly in the BBC Light Programme. Overseas Appointments \JR. HUGH CARLETON iVA GREENE, O.B.E., has been appointed Controller, Overseas Services and Mr. Oliver J. Whitley, Assistant Controller, Overseas Services. v Commercial Radio Possibility TT has been reported that should the new commercial television service to be introduced this year prove successful, a proposal for commercial radio may be put forward. The chief advantage of such a service would be the abolishment of licences. Radio Telescope PROF. V. A. AMBART- A SUMYAN, a Russian astro- physicist, visited Jodrell Bank experimental station recently as the guest of Prof. A. C. B. Lovell, professor of radio astronomy at Manchester University, who is head of the Jodrell Bank telescope scheme. He was shown the new £500,000 radio telescope near Chelford which is being constructed to be the largest in the world. Decline in Canada Th 1GURES issued by the Canadian A Radio Television Manu- facturers' Association reveal that sales of radio receivers in October last totalled 41,411, a decline of 14,031 on the total for October, 1953. Firm's Communication Station "TARSLAG, LTD., a Teeside A building and contracting com- pany, have been negotiating with the Northallerton Water Board for the lease of a site on one of the highest points of the Osmotherley Moors, near Northallerton. The firm intend building a radio station to keep in constant touch with their employees engaged on jobs in North Yorkshire and South Durham. # Radio Show Dates 'T'HE National Radio Show this L year will be held at Earls Court, London, from August 24th to September 3rd, with a preview February, 1955 PRACTICAL WIRELESS 73 for overseas and special visitors on August 23rd. Meanwhile plans are well in hand for a smaller exhibition, the Northern Radio Show, to be held at the City Hall, Manchester, from May 4th to 14th. Mobile Radio Used A/TOBILE radio was used by the Sunderland Transport Department to control bus opera- tions when the Queen and the Duke of Edinburgh visited Wearside. Two vans and a walkie- talkie set were used. The transport committee is now to consider using mobile radio permanently in view of its outstanding success on this occasion. Civil Defence Exercise NOTTINGHAM, Derby, ^ Nottinghamshire and Derby- shire civil defence forces made extensive use of mobile radio for their largest exercise since the war. It was assumed that all telephones were out of action and all communication had to be by radio or field telephone. Obituary TT is with deep regret that we re- cord the death of Mr. "Jimmy" James, production manager of Whiteley Electrical Radio Co., Ltd., who collapsed and died in his office on Friday, November 12th. Mr. James was only 49 years of age and had completed nearly 28 years of service with the company. He was an outstanding personality and will be greatly missed by all who knew him. British Institution of Radio Engineers T^HE following meetings oT the A above institution will be held during January : London Section. — Wednesday, January 26th, 6.30 p.m., at the London School of Hygiene and Tropical Medicine, Keppel Street, Gower Street, W.C.I. 44 A Survey of Tuner Designs for Multi- channel Television Reception." D. J. Fewings, B.Sc, and S. L. Fife, A. M.Brit. I.R.E. North-eastern Section. — Wednes- day, January 12th, 6 p.m., at Neville Hail, Westgate Road, Newcastle. Address by the presi- dent, Rear-Admiral (L) Sir Philip Clarke, K.B.E., C.B., D.S.O. South Wales Section. — Wednes- day, January 12th, 6.30 p.m., at the Glamorgan Technical College, Treforest. " Electronic Counting Devices." F. H. Gage, D.Sc. Scottish Section. — Thursday, January 20th, 7 p.m., at the Depart- ment of Natural Philosophy, the University, Edinburgh. " Modern Ship-to-Shore Communication." G. Macdonald. Marconi Staff Appointments ]yjR. E. GREEN, M.Sc, having . ivl reac h e d the normal retiring age, has relinquished his position as head of the Transmitter Ad- vanced Development Group of Marconi's Wireless Telegraph Co., Ltd. He is, however, continuing his The transmitter is designed for the transmission of radio-telephonic or telegraphic communications over long distances and under heavy traffic conditions. Communications for Pakistan pYE LIMITED are to supply and A install £150,000 worth of equip- ment for the Sui Gas Transmission Company's pipeline communica- tions system in Western Pakistan. The system will include an extensive V.H.F. multi-channel Two members of the Pye radio unit prior to the start of their survey for a communications system in Western Pakistan. work for the company on a full- time basis as a consultant engineer. Mr. V. J. Cooper, B.Sc, has been appointed chief engineer, Advanced Development. Radiogram Gift A COSSOR radiogram was recently presented to the Islington Branch of the Infantile Paralysis Fellowship. This pre- sentation was made on behalf of A. C. Cossor, Ltd. by its medical officer, Dr. L. B. Bourne, in the presence of the Mayor, Councillor Mrs. J. M. Barnes. Transmitter for Belgium A HIGH-POWER (30 kW) H.F. communication transmitter type HS.51 has been shipped to Belgium from the Chelmsford works of Marconi's Wireless Tele- graph Co., Ltd. This transmitter has been manufactured for the Belgian R.T. and T., the equivalent of the British G.P.O., and is to be installed by Marconi engineers at Ruisselede, Belgium. route, providing speech and tele- printer channels between all the major points ; a V.H.F./A.M. Fixed-to-Mobile scheme, giving complete coverage of the pipeline route for maintenance purposes ; and a duplex H.F. radio-telephone or teleprinter system between Karachi, Sui and the main inter- mediate point. Cable and Wireless Limited. 15,000- Mile Cable Chain A 15,000-mile cable chain, the longest unbroken cable and landline circuit ever formed, is being set up to link Sydney with London direct, via Canada. The chain is intended as a permanent improvement in communications between the United Kingdom and Australia. Savings Scheme W/'OLF dealers throughout the country are now operating an " easy-stage " savings scheme for the benefit of prospective pur- chasers of Wolf Cub equipment. 74 PRACTICAL WIRELESS February, 1955 A New Technique for Sound Recording A COMBINATION OF TAPE-RECORDING TECHNIQUE WITH ORDINARY SOUND-ON-FILM By A. G. Thomson COST has been the principal factor limiting the use^of films with sound effects and recorded commentaries for scientific, educational and commercial purposes. This limitation has been over- come by the development of a powder which, when skilfully bonded to film, provides an easier and far more economical means of adding sound to motion pictures. Sound may be recorded on film by means of light or by varying the magnetic field, on the principle of the magnetic tape recorder. Magnetic sound tracks are applied in the form of a narrow " stripe " down one edge of film in approximately a similar position to the photographic track. A stripe which occupies the whole area normally used for the optical track is O.lOin. wide and is termed full width stripe or full striping. Since the stripe is opaque any optical track which it covers can no longer be played. It is also possible to apply a half stripe, 0.05in. wide, which may be located on either side of the centre line of the optical track, so that the latter can still be played at a slightly reduced volume level. Half striping produces a slightly lower volume of sound than full striping, but is otherwise equally effective. It is also possible to add a narrow stripe 0.03in. wide to the outside edge of silent (double perforated) film. This is known as edge-striping and has an even lower output than that of half striping, though in other respects it is equally effective. The use of magnetic striping has led to the develop- ment of equipment specially designed to take maximum advantage of the opportunities presented by this very flexible system of sound recording. The scope for this new technique has been greatly extended by the introduction of a magnetic-optical projector, which makes it possible not only to project 16 mm. films of all types, but also to record material from practically any source. Manufactured by Kelvin Hughes for Simplex-Ampro, Ltd., it is claimed to be the only equipment in the world which records from an optical track and has a three-channel mixing unit operating from a remote control panel. The set has been designed to give the fullest possible facilities, so that recording can be carried out in a truly professional manner. It includes a separate remote control unit, which is provided with twin channel input for recording and monitoring by meter as well as headphones. The complete equip- ment is supplied in three cases. One case houses the projector proper, its lower half being in the form of a tray in which the projector is mounted. The loudspeaker carrying case forms the loudspeaker baffle and holds a spare 1,600ft. film reel, together with loudspeaker cable. The third carrying case, besides housing the remote control unit, contains a Reslo ribbon microphone with a very wide frequency range, monitor head set, control cable, head cable, and power cable. The Sound-head The projector is provided with a footage counter- meter and has an improved sound-head with no guideways. The film passes from the bottom to the top of the sound drum which rotates in an anti- clockwise direction. Since the stripe is on the outside of the drum, the magnetic head is able to follow the film and conform to its minute fluctuations. The magnetic head is adjustable in all three planes to give perfect contact with the film. The greater frequency ranges available from magnetic recording made it desirable to design an improved speaker unit, which has been backloaded and baffled to lower the low- frequency response by a full octave. The amplifier is also of a new and improved design and uses valves of standard makes. The projector operates on 105-125 volts alternating current, but can be operated on A.C. supplies outside this range by using an Ampro transformer. It will accommodate reels from 400ft. to 2,000ft. capacity and may be used to project sound films at a constant speed of 24 frames/sec, or silent films at a constant speed of 16 frames/sec. This projector has been specially designed to play edge stripe on double perforated film, half track, or full track. It will play optical and magnetic tracks on colour or black and white film either separately or simultaneously, and can change from one half track to the other with no mechanical switchover whatsoever. It will record from two sources separately or mixed (e.g., micro- phone and gramophone), and it will record from existing optical track with the addition or insertion of extra material from other sources in a single operation. One of the main advantages of the equipment is that the remote control unit can be placed well away from the projector, so that even the minute amount of noise made by the recording instrument A view of the new projector, February, 1955 PRACTICAL WIRELESS 75 can be completely eliminated. In fact, for fully pro- fessional results it is preferable to have the com- mentary read from another room. This can be easily achieved with the remote control unit which allows the projector to be operated and the recording to be fully monitored. Operation When recording from a magnetic track, the striped film is threaded through an erase head and into the The complete equipment, with an operator making a magnetic recording. projector in the usual manner. When the recording amplifier is switched to the record position, a red signal lamp comes on. The operator then switches on a turntable and checks the level of the opening music, which should peak at the full zero level on the programme meter. He next checks his voice on the second channel of the three-channel mixing unit. His voice should peak at full zero, but if the music is a background to speech it should be peaking at minus 3, and this will be clearly indicated on the programme meter. In the monitor 'phones the operat6r can hear both the level of the music and his voice. Switching on the projector he records his message. Should he make a mistake, he has merely to reverse the projector to a point prior to the error and re-record the passage. The offending section is automatically removed by a special erase head, which operates continuously during recording to ensure that no previously recorded sound track still exists. To record from the optical track of a film the operator plugs the output from the speaker line into the 15 ohms position on channel 2 of the remote- control unit. The optical volume on the projector is turned to three-quarters and the tone-control to the » desired position. The operator can then switch the channel selector switch into the 15 ohms position and record the message from the optical track, adding music from a gramophone disc or any other source, or. making additional comments at the appropriate times. The recordings are played back in the conventional manner, using the volume-control marked microphone for the magnetic volume and the normal volume- control for the optical volume where required on half-track recordings. There are virtually no limits to the facilities offered by this remarkably versatile equipment. From the remote-control position the operator can switch the projector on and off and monitor on his head-set by following the visual indications of the programme meter. He can mix both music and speech and record from the optical track of the film at will, or he can use the 15 ohms input from another projector or tape- recorder or sound effects. If the output of a gramophone is too small for recording purposes, he can take out the speaker plug and plug it into the sound amplifier position, controlling the tone from the recorder itself. An inexperienced amateur doubt- ful of his ability to mix sound and speech together can first record the music throughout the length of the required section of a film. He can then rewind the film, by-passing the erase head, and run it through again to record whatever comment- ary is desired.- By this means the original music recording is auto- matically dropped to background level behind the speech. In order to prevent assistants on fieltl work from altering the original commentator's carefully chosen words, the projector has been so designed that the erase head can be unplugged, thereby ensuring that the recording will not be changed. It is evident that there are four ways in which recordings made by means of magnetic striping can be played back. It is possible to play either the original optical track or the track containing the original optical recording and the new magnetic recording. Again, by careful thought and the use of the two volume-controls selected sequence can be played back from either track or both tracks can be played together. The practical significance of these novel facilities is at once apparent. If a film is out of date it can be half striped and the material or policy, or any important passages that should be retained, can be recorded on to the magnetic striping at the same time as new speech or new background music is being added to the magnetic track. When a documentary film is made for use in schools several different commentaries for the various age groups are required. Formerly it was necessary to make each of these commentaries on a separate optical track. Now it is a simple matter to make a copy for the oldest group, half stripe it, and record the commentary for the youngest group. The two recordings can each be played back separately, or the sequences from both play-backs can be mixed to provide the recording for the middle group. Training Most hospital groups cdnsider that films of opera- tions play a valuable part in the training of medical students and nurses, because the camera can show 76 PRACTICAL WIRELESS February, 1955 close-ups that could not be seen by the majority of those present in the theatre. Before the development of magnetic optical projectors, the surgeon had to be in attendance whenever the film was shown in order to provide the necessary commentary. Now the film can be half striped and a suitable commentary can be recorded, enabling the film to be shown at any time without the surgeon being present. Moreover, by varying the recorded commentary the same film can be adapted to the requirements of any type of audience. Nowadays, there are many industrial users who have a 16 mm. silent camera and have never attempted to reproduce sound or speech. At a very modest cost they can now have their silent films copied and striped and record suitable commentaries with effects. There are many potential applications in factories for staff training and the training of service engineers. A particularly important advantage of half stripes is that they enable the language difficulty to be over- come. For example, a film illustrating the production, operation or uses of any particular equipment can be made in Britain, copied, striped, and sent all over the world, commentaries in the appropriate language being added in each country. The method also offers an ideal solution to the language problems when films are shown in such vessels as oil tankers, whose crews may be drawn from a number of different countries. Magnetic optical recordings of the type described are also likely to be extensively used in the motion- picture industry itself, since they offer full studio facilities at a very much lower cost. In fact, the opinion has been expressed that because of its greater efficiency and economy, magnetic recording will eventually be universally employed in place of optical tracks. SJuApluM Xp and Jfowai TJtuc/wphonM By E. G. Bulley THE lip and throat microphones were originally developed for use by the armed forces, and are both similar in construction. The former type is worn on the upper lip and does to a certain extent prevent or limit unwanted background noise. Likewise, the throat type is placed against the throat and, by so doing, background noise is reduced. Basically, these microphones consist of one or two carbon buttons, the former being known as the single button, whereas the latter is the double button type. The single button type consists of a metal diaphragm which rests against an insulated dish containing carbon granules, the granules being commonly known as the button. One electrical connection is taken from a metal backplate, and the other is taken from the diaphragm. The double button type, however, can be said to consist of two single button types connected in a push-pull arrangement. This type must not be confused with the lip-type differential microphone, the latter type of microphone having two carbon buttons, one arranged on either side of the diaphragm but having only one electrically connected. It is assumed that the purpose of the other button is to assist in physical balance between the two sides of the diaphragm. Furthermore, should the reader purchase one of these microphones, Diaphragm he will notice that there are holes on both sides of the outer casing, these holes being located at the base of the instrument. The purpose of the holes is to allow the sound waves to enter both sides of the micro- phone and create equal and opposite pressures on the diaphragm. Operation The operation of the carbon microphone is simple, and only a brief explanation will therefore be necessary for the reader to appreciate the types that are available on the surplus market. As previously mentioned, the electrical connections are taken from the diaphragm and the case, current being fed from a suitable supply which flows through the loosely- packed carbon granules, and as the diaphragm vibrates, the pressure on the granules increases and decreases. These phenomena causes a fluctuation of current in the circuit in which it is connected. Typical circuits utilising the single and double types are shown in Figs. 2 and 3. Many of the lip and throat microphones are of American manufacture, the units being moulded in rubber with an elastic neck- or head-band. Never- theless, the moulded rubber casing can be removed and the microphone used for various experiments. Driver Fig. 1. (left) — Circuit using the single carbon button type of microphone. the double-button type. . Circuit employing February, !Vb5 PRACTICAL WIRELESS 'ESTIHSTRUMi Part 2 of a New Series of Articles Dealing with the Practical Application of Standard Test Equipment By Gordon J. King, A.M.I. P.R.E. {Continued from page 1 4 January issue) LET us look again at the simple circuit in Fig. 4 ; we know, by calculation, that 200 volts are developed between points B and A — or between points B and C for that matter. Yet when we endeavour to prove this by making a voltage measure- ment the meter indication is nothing like it should be. If we refer back to the voltmeter circuit of Fig. 2, we shall immediately realise that by putting a volt- meter across either of the two points we are in effect shunting either Rl or R2 by the internal resistance of the instrument. Instrument Sensitivity (3) The magnitude of this resistance is, of course, dependent on how much current is required to provide f.s.d. on the meter. Clearly, then, this is governed by the sensitivity of the meter itself, which in turn determines the overall sensitivity of the instrument proper. This characteristic is revealed in the term " ohms per volt." Let us consider a meter that requires 1 milliampere for f.s.d. If we adapt this to read 1 volt f.s.d., then we find, by using Ohms law, that the meter circuit must possess a total resistance of 1,000 ohms (1x1,000/1 mA). We know, of course, that the meter itself has resistance, but this figure is included in the total resistance value, so that a 1,000 ohms of resistance exists between the actual terminals of such a 1-volt meter. Thus, we now know that an instrument using a 0-1 milliampere movement has a sensitivity of a 1,000 ohms per volt — bearing in mind that the current to be considered is that necessary for f.s.d. A meter which has a f.s.d. of, say, 0.5 milliampere would, of course, go to make a voltmeter of 2,000 ohms per volt sensitivity — 20,000 ohms per volt sensitivity would be realised by using a meter of 50 micro- amperes (0.05 milliampere) f.s.d., arid so on. We shall now have a much better idea what happens when we connect our, say, 1,000 ohms per volt meter — set on the 200 volts range — across points B and A of Fig. 4. The circuit will most certainly be disturbed by the total voltmeter resistance — now 200,000 ohms (1,000 x200 ohms) — shunting Rl. This r-- is illustrated by Fig. 5, where a little simple cal- f Vj culation will soon reveal ->r the error of our measure- L -- ment. We know that the total resistance of two resistors connected in parallel is equal to Rl x R2/R1 +R2. Working this out for Rl =//v/n *IM£l 400V and the total resistance of our 1,000 ohms per volt voltmeter set on the 200 volts range, we quickly dis- cover that in place of the 1 megohm resistor Rl we now have an effective resistance of 166,666 ohms. This, being still in series with R2 across the voltage source, incites a rise in circuit current from 0.2 milliampere to a little over 0.34 milliampere, whilst the voltage across Rl and the instrument resistance falls to something like 56 volts. And this is precisely what our voltmeter will register — quite a difference between this and the actual 200 volts ! From this reasoning it will be evident that if we increase the voltage setting on the range switch, say. turning it to the 500 volts position, we shall obtain a more accurate reading as then the terminal resistance of the instrument is that much larger. For this reason, when testing voltages in high-resistance , circuits it is desirable to employ the highest range possible, consistent^ of course, with useful scale indication. If, on the other hand, we reduce the voltage setting, the terminal resistance of the instrument reduces accordingly, the circuit under test becomes even more heavily shunted, and as the result it is very unlikely that an increase in needle deflection will be achieved. Engineers and experimenters who are in possession of commercial receiver service data will have noticed that circuit voltage figures are given with respect to a specific type instrument. This applies particularly to the valve electrode voltage figures. In cases where a specific instrument is not quoted, or if a meter sensitivity figure is not given, the voltage readings should be taken only indicatively. An experimenter using a 1,000 ohms per volt instrument, for example, will get an entirely different set of readings from an experimenter using a 20,000 ohms per volt instrument. For the same reason it is desirable also to know the actual voltage range to which the instrument was adjusted when the test readings were taken. Where possible, then, one should always aim at acquiring a multi-range meter of high sensitivity. This applies more particularly to the television and radio Voltmeter resistance • 200.000CI 0-2 m A with voltmeter disconnected Effective resistance between A and B = 166,666 ohms Fig. 4 {left). — The voltmeter V will not register the true voltage across R\. Fig. 5 {right). — Showing how the cir- cuit at Fig. 4 is effectively altered by the voltmeter. RI^IMO. R 2=1 MCI 400V 0-J4/7) A with voltmeter connected 78 PRACTICAL WIRELESS February, 1955 experimenter. For the electrical engineer an accurate instrument of mediocre sensitivity is all that will normally be required, for such an operator's work will generally be on low-resistance circuits where the shunting effect of the instrument will be negligible. At this point it will be interesting to note that by using a 20,000 ohms per volt instrument, set on the 200 volts range, approximately 175 volts will be measured across Rl (Fig. 4). The accuracy would be even greater by using the instrument set on the 500 volts range. Measuring Valve Electrode Voltages (4) When we measure valve electrode voltages the above reasoning holds good in a large number of cases. Let us look at the circuit at Fig. 6. This shows a pentode valve connected as a typical voltage actual voltmeter connection is made to the control grid a measurement of G.B. voltage is not possible. If we wish to measure the voltage on the screen of the valve, things become a little more involved. Let us assume that we are using an instrument having a sensitivity of 1 ,000 ohms per volt set on the 200 volts range, and that the value of the screen feed resistor Rs is 100,000 ohms. If, without the instrument connected, the screen current is 0.5 milliampere, application of Ohms law will reveal that Rs will drop 50 volts and that 150 volts will exist between the screen and earth. The circuit is perfectly happy until we connect our voltmeter between the screen electrode and earth. In effect, this is the same as connecting a 200,000 ohms resistor between screen and earth. Fig. 6 {left). — A typical voltage amplifier circuit. Fig. 1 {right). — Since there is no current in Rg under normal conditions, both sides of Rg are negative with respect to cathode. amplifier. We shall observe that the stage embodies a cathode biasing arrangement, which means that the resistor Rc in the cathode circuit causes the cathode to rise to a positive potential relative to the control grid (grid 1) or earth, by reason of the voltage drop across it. Clearly, then, if we connect a voltmeter across Rc we are effectively measuring the bias voltage of the valve. Moreover, an accurate voltage reading will be obtained at this point owing to the relatively high ratio between the resistance of the meter and the resistance of Rc. It would be quite a different matter, however, if we were silly enough to make a voltage measurement between grid 1 and cathode. If we used a very sensitive instrument we should probably get a reasonable indication. But when we realise that we are obtaining such a reading by putting Rg in the meter circuit we should well expect a considerable error. Most of us know that the value of Rg might well range between 0.25 megohms and 10 megohms, and, even though the valve itself may not be taking grid current to provoke a voltage drop across Rg, the meter must pass current to take energy from the circuit to move the pointer. This fact is brought out better in Fig. 7, which clearly shows both sides of Rg negative with respect to cathode— this is because there is no current in Rg under normal conditions. The voltage drop is VRc across the cathode resistor, so why measure this through Rg ? It seems from our " Query Service " that a number of experimenters feel that unless an Vo It meter Fig. 8. — A method of accurately assessing the elec- trode voltage of a valve. The current in Ra is measured under normal conditions and then the electrode is energised from a low-resistance H.T. source, being measured by V. The voltage is adjusted so that A indicates the normal current in Ra. This resistor in conjunction with Rs thus forms a potential divider circuit from which the screen is fed. The result is that the current in Rs increases, the screen voltage falls, and as a direct consequence the valve takes less screen current. This is indeed a complex state of affairs, and one that cannot be readily analysed, for the reduced screen current does not incite a rise in screen voltage equal to the voltage drop caused by the resistance of the instrument. If the two voltage deviations were equal and out- balancing, measurement of the screen voltage would be a relatively simple matter. Of if we could look at the screen cathode path of the valve as a pure resistance, a resistance connected in series with Rs could be substituted for the valve and analysis made as for the simple potential divider circuit shown in Fig. 4. Unfortunately this is not possible if great accuracy is required, since the voltage-current relationship of the valve does not follow Ohms law. This reasoning applies equally, of course, to measurement of the anode potential, or any high- resistance valve electrode taking current. If it is essential to take a very accurate measurement the following procedure should be adopted. First, take careful note of the electrode current, and then disconnect the electrode from the H.T. line. Next connect a H.T. battery or a low resistance H.T. source to the H.T. negative line of the set, and feed the valve electrode from this through a milliammeter. {To be continued) February, 1955 PRACTICAL WIRELESS ) ) J ) ^ ) j The International Short Wave League SEVERAL of my readers, in view of my comments on clubs which claim national or international status, have written to me regarding the International Short Wave League which operates from 86, Barrenger Road, London, N.10. One of my readers says that there is an honorary secretary and honorary editors, but no mention of a treasurer. He also tells me that there is no indication of annual area elections nor do members receive a balance-sheet accounting for receipts and expenditure. In the journal issued by this League it has been stated that the league membership stands at several thousands. One reader says that his membership number is approaching 6,000. The annual subscrip- tion is 10s. a year, so there is a considerable sum received by the League in subscriptions alone. I should have thought that under such circumstances, with a comparatively large sum of money involved, there would have been an annual general meeting and balance-sheet distributed together with a list of members. After all, a man who joins an inter- national organisation wants to know who his fellow members are. Personally, I have no knowledge of the League, nor have I had any complaints concerning it. I gather, however, that no one has received a balance-sheet, although when 1 wrote -to the hon. secretary I was informed that they would be " pleased to show me all our books and records, which are in \ any case always open to inspection and scrutiny by members." I can only comment here that it is quite unreasonable to expect thousands of members to call at the headquarters of the League in London in order to inspect the books. The secretary does not state in his letter to me whether these books and records have been independently audited. It is said by some readers that all they receive is a membership card and copies of a roneoed journal. I understand, however, that no officer receives any emoluments, although it is not clear whether they receive expenses. My letter to them invited answers to the following questions : Is an annual general meeting held and are the officers elected annually ? Is the secretary a permanent secretary who cannot be removed from office ? As you claim to have some thousands of members at 10s. a year, this represents a considerable sum of money. Is a balance-sheet published to members showing how the money is expended? Is this balance-sheet independently audited ? What benefits do the members receive ? Is a list of members published as would, of course, be necessary in the event of a balance-sheet, so that members could assess revenue ? The reply was : M In view of your request for ' suit- able ' answers to a series of searching questions, and that the form of our answers is apparently intended to represent the other side of some ' story,' we should be pleased to show you all our books and records, which are in any case always open to inspec- tion and scrutiny by members. Furthermore, we should be very pleased and proud to explain the whole set-up of the I.S.W.L. to you or your representative at your earliest convenience." Accordingly, the secretary was invited to call at the office of this journal, but countered with a request for us to visit them. Surely a visit should be quite unnecessary, and it should have been easy for the secretary to have answered in writing the questions set forth above? There is no suggestion, of course, that the club is not properly run, but in order to remove doubts in the minds of readers who have written to me I invite the League to reply to those questions so that 1 may publish them for the benefit of all concerned. I belong to many clubs myself, and am always provided with a list of members, notice of annual general meetings and a balance-sheet, accounting for the club's revenue and expenditure. After all, members are entitled to know what the financial position of a particular club is, since they are jointly and severally responsible in N law for the club's affairs; being unincorporated they are not a legal entity and members themselves become liable. The hon. secretary and treasurer are now invited to give answers to the questions enumerated above. I also invite correspondence on the subject from members of the League. The Radio Amateur Invalid and Bedfast Club T HAVE been in correspondence with the secretary and others concerned with the Radio Amateur Invalid and Bedfast Club. I do not like that term " bedfast," which is a slang term anyway. My concern is that this club appeals for donations for radio amateurs who are incapacitated and confined to bed. The club also appeals for wireless components and books, and I asked whether I could have further details such as the number of members, whether annual general meetings are held and the officers elected annually, whether a balance-sheet is produced, showing how the donations are being disbursed, together with a statement from the bank and details of all expenditure. If so, could I have a copy ? I also wanted to know whether such a balance-sheet, if it exists, has been independently audited. I required to know when the club was founded, how many people have responded to the appeal for donations, and how much has been donated from the com- mencement. Failing this information, it was pointed out, notices of this club could not appear in this journal. Readers will remember the World Friendship Society of Radio Amateurs which I criticised very severely some time ago and which now seems to have gone out of existence. Apparently the membership of R.A.I.B.C. at the end of November was 20 with seven local representa- tives, five of whom are licensed amateurs. Until I receive satisfactory replies to my questions notices, 1 understand, will not appear in this journal. 80 PRACTICAL WIRELESS February, 1955 An Economical Quality Receiver EIGHT STAGES IN FOUR VALVES • VARIABLE SELECTIVITY \ NEGATIVE FEEDBACK By W. N. Stevens THE problem facing the writer was the construc- tion of a broadcast band receiver which would give good quality and yet have simple circuitry. Primary consideration was good reproduction of the local stations, with a good selection of Continental stations. The trouble is of course, that, generally speaking, the higher the quality desired the more complex the circuitry becomes. Yet, for various reasons, components and cost had, in this case, to be kept to a minimum. Obviously, as in much radio work, the answer was a compromise. The arrange- ment finally chosen is shown in Fig. 1. The Circuit In the interests of gain, selectivity and general convenience, a superhet circuit was chosen. Ahead of the frequency changer an R.F. amplifier seemed to be desirable, for the usual reasons. On medium waves the stage gain of a tuned R.F. amplifier is considerable, but in the receiver being discussed gain was not the main aim of such a stage. Some extra gain at R.F. was desirable, but the object of the amplifier was primarily to obtain a better signal-to- noise ratio and to prevent cross-modulation distor- tion. In view of all this, it was decided to use an untuned amplifier, thus obviating the need for extra coils and an extra gang on the variable tuning capacitor. Here it might be mentioned that one of the con- siderations involved in building the receiver was to use as many components as possible that were already on hand ; hence the unusual valve complement. The R.F. stage uses a 6AC7 high slope pentode, but this is not critical. Alternative types which come readily to mind include 6SG7, 6AG7, 6SK7 or, in the double- ended types, valves such as the 6K7. The R.F. stage is an orthodox untuned amplifier, except perhaps for the cathode bias arrangement, which will be described, later. The grid resistor Rl is returned to the AGC line, with the object of preventing overloading on strong signals. The output of the R.F. stage is capacitance coupled to the mixer stage through a variable capacitor C4, which is pre-set to the best value on completion. Since best results are obtained when the output is coupled to a high impedance, the capacitor is returned to the junction of the mixer grid, tuning capacitor and coil. In the original model the coil is the secondary winding (grid) of a standard medium-wave inductor with the primary winding left unconnected. If greater selectivity is required, normal transformer coupling can be used between the R.F. stage and the mixer. It will be seen from the circuit that AGC control is applied to the mixer stage. Frequency Changer This is a normal shunt-fed circuit, using standard coils. There are, however, a few points of interest. flg t i._ Theoretical circuit of the receiver, February, 1955 PRACTICAL WIRELESS 81 Since the maximum conversion is obtained when the oscillator anode and mixer screen are at the same potential (in this case 100 volts for the 6K8), they are fed from a common point, R6 being the common dropping resistor. This arrangement simplifies wir- ing and does not have any deleterious effects through interaction. The switching system in the mixer anode circuit is to enable a choice of two degrees of selectivity- Normal and Wideband. In position " 1," the 5 pF capacitor C12 is, in efTect, shunted across the primary of IFT-1. The receiver is lined up in this position. By switching to position " 2," the capacitor is taken local stations (and, possibly, the more powerful Continental stations) one may enjoy the benefits of a wide bandwidth in the I.F. circuits since there will be no side-band cutting and, at the same time, the reduc- tion in R.F. gain reduces the possibility of interference from any nearby stations. Reflex Amplifier The third valve V3 (a 6B8 double-diode-pentode) performs four separate functions and thus enables considerable economies to be made. In the first place it accepts I.F. signals from the frequency changer and operates as a normal pentode I.F. amplifier. At the off the pnmary c.r- same CUlt and ^hnnt^H : ............................ cuit and shunted across the secondary circuit, this change- over providing wide-band charac- teristics. Another pole on the rotary switch performs a similar operation in the anode circuit of the I.F. amplifier. At the same time, a third pole on the switch affects the gain of the R.F. amplifier. In posi- tion " 1" (Normal) the resistor R4 (220 ohms) is switched in circuit so that the amplifier is functioning at normal efficiency. But in position " 2 " (wide-band) the normal bias resistor R4 is switched out of circuit, leaving in its place a high-value resistor (R5), thereby reducing the gain considerably. The switching system enables the receiver to be operated in the normal manner when tuning to distant stations, especially where the weaker signals require maximum gain and selectivity. When listening to Resistors Rl,12. 470,000^. R2, 11, 13. 100,000 R3, 8. 47,000 Q. R4. 220 Q. 10,000^ (see text). 27,000 Q, 1 watt. 330 Q. 2.2 M O. 27,000 O. 470,000 P (see text). 150*2 (see text). 1,000 a, 5 watt. LIST OF COMPONENTS R5. R6. R7. R9. R10. R14. R15. R16. Capacitors : CI. 40 pF. C2, 3, 7, 16, 17, 19, 21. 0.01 w. C4, 5, 9. 70 pF, trimmer. C6, 10. 470 pF swing, twin- gang. C8, 13. 100 pF. Cll,14, 15. 470 pF. C12,18. 5pF. C20. 0.001 //F. C22. 25 //F,25v. wkg. C23, 24. 8 + 16 //F, 350 v. wkg. C25. 0.1 v¥, 1,000 v. wkg. time, the signals are rectified (demodulated) by the two diodes — which are strapped for convenience. The resultant audio voltages are picked off the diode load, fed back to the grid of the pentode and amplified at audio frequency. TheA.F. output is taken off the screen - grid which is used as a triode ki anode. •* Additionally, the audio voltages appearing across the load resistor are fed back to the grid circuits of VI and V2, through the filter R9/C16, to provide AGC control. It should be noted that no AGC is applied to V3, because, in addition to being an I.F. amplifier, it also functions as an A.F. amplifier and must therefore operate at a fixed gain. Rl 1 , C14 and CI 5 comprise the usual R.F. filter to . prevent any residual R.F. from being fed into the Fig. 2. — Layout and principal measurements.. 82 PRACTICAL WIRELESS February, 1955 audio stages and thus causing instability. The by-pass capacitors are smaller than usual so that they do not by-pass the audio voltages as well as the R.F. voltages. The reflex stage is provided with individual anode decoupling (RIO, CI 7) as an aid to stability, although this may not be necessary in many cases (see later). Output Stage The final stage in the receiver is a conventional output amplifier. - In the original model the valve is an EL33, simply because one happened to be avail- able. The choice is wide and any of the usual types may, of course, be used as convenient, although the value of t?»e bias resistor (R15) should be noted. Of the more popular types we might mention the 6V6 (cathode resistor 240 ohms), 6F6 (410 ohms), KT63 (420 ohms\ KT33C (190 ohms), etc., using a main H. T. rail of 250 volts. In the interests of quality simple voltage feedback is provided, R14 being the negative feedback resistor. Shunted across this resistor is a tone-control system comprising C20 and VR1, the latter being the manual tone control. This simple top-cut control provides a considerable range of variation to suit most tastes and conditions. VR2 in the grid circuit is the usual manual gain control. Power Supply The total H.T. current consumption of the receiver is under 75 mA at 250 volts. The simplest and cheapest H.T. supply to provide this power, using components already to hand, consisted of a metal rectifier and simple smoothing filter of an 8-1-16 /iF electrolytic and a 5-watt 2,000 ohms resistor in place of the conventional smoothing choke. The L.T. requirements are 6.3 volts at 2 amps (or I. 5 amps if a 6V6 is used as the output valve). This is supplied by a suitable heater transformer across the mains supply. Construction The receiver was built into a spare cabinet which had been earmarked for such a purpose for some time previously. The chassis measured Bin. x6in. x2in., but it is possible to construct the receiver on a much smaller chassis, as a little ki juggling " with the main components will show. Fig. 3 shows the main drilling dimensions and placing of above-chassis components. There is little point in discussing at length the actual wiring-up of the receiver as even the beginner should have no difficulty providing that the mains parts are mounted as shown and care is taken in the usual directions. It is a good point to fit solder tags to each of the valveholder mounting bolts to provide quick earth returns. Most of the components can be mounted around their respective valveholders or from the tags of major components. In the original model it was not found necessary to fit any tag strips at all. Earth returns for each stage should be taken to a common point to prevent earthing at different R.F. points and thus inviting insta- bility. Particular care should be taken in wiring up 1 1) e V3 circuit, but provided leads are kept reason- ably short and direct there should be no troubles arising from the rather complicated action of this stage. The matter o screened leads is best tackled in the following manner : Wire up the receiver without any screened leads at all except the two from (a) the aerial terminal to the VI signal grid and (b) gain control VR2. Then, should any of the usual troubles be experienced, .vital* leads can be replaced by leads in screened cable until the trouble is cured. The most vulnerable leads are those from the secondary of IhT-1 to the grid of V3, from the secondary of IFT-2 and any leads which pass through the chassis from LI, C6 or C10. In the original receiver only the two leads mentioned above required screening. The use of screened cable should, of course, be avoided where possible to avoid undue losses. Final Adjustments The first job on completion is, of course, to line up the I.F. transformers in the usual way. This should be done with SI in position " 1 " (normal). Then the R.F. circuits may be aligned. In addition, there may be a few other adjustments required to ensure good results. For instance, the coupling capacitor C4 should be adjusted so that adequate coupling takes place with the lowest possible capacitance. If the coupling is too tight the tuned circuit in the mixer grid will be damped and selectivity will suffer to some extent. C4 should be slackened off so that sufficient gain is obtained consistent with selectivity. When making this adjust- ment, remember that one side of the capacitor is at high potential. Also, the resistor R5 may need attention. In the London area, in which the original receiver is used, anything up to 10,000 ohms will be suitable. In some areas', however, it may be necessary to reduce this value to obtain adequate gain ; shunting temporary resistors across R5 in situ will soon determine how {Concluded on page 110) PRACTICAL WIRELESS 83 D.C. Voltage 0—75 millivolts 0—5 volts 0—25 0—100 „ 0—250 „ 0—500 „ D.C. Current 0 — 2.5 milliamps 0—5 0—25 0—100 0—500 9$. /JlJ2j2AJS4J7n I E1ECTRICA1 / TEiTIWC A.C. Voltage 0—5 volts 0—25 M 0—100 „ 0—250 „ 0—500 „ Resistance 0—20,000 ohms 0—100,000 „ 0—500,000 „ 0 — 2 megohms 0—5 0—10 ,. GUARANTEE The registered Trade Mark u Avo " is in itself a guarantee of high accuracy and superi- ority of design and craftsmanship. Every new AvoMinor is guaranteed by the Manufacturers against the remote oossibility of defective materials or workmanship. / A dependably accurate instrument for testing and fault location is indispensable to the amateur who builds or services his own set. The UNIVERSAL AVOMINOR (as illustrated) is a highly accurate moving-coil instrument, conveniently compact, for measuring A.C. and D.C. voltage, D.C. current, and also resistance : 22 ranges of readings on a 3-inch scale. Total resistance 200,000 ohms. . Size : 4$ ins. x 3lins. x ] litis. Complete with leads, inter- Net t weight : 1 8 ozs. changeable prods and croco- * . gi. . dile clips, and instruction Price: £10 : 10 : 0 book. The D.C. AvoMinor Is a 2|-inch moving coil meter providing 14 ranges of readings of D.C. voltage, current and resistance up to 600 volts, 120 milliamps, and 3 megohms respectively. Total resistance 100,000 ohms. >■ *.« Size : 4&ins. x 3 1 ins. x Hins. Complete as above Nett weight : 1 2 ozs. Price : £5:5:0 Sole Proprietors and Manufacturers : — AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO., LTD. Winder House, Douglas Street, London, S.W.I. 'Phone : Victoria 3404-9 a modern high quality amplifier & reproducer for the home constructor BUILD THIS HIGH QUALITY AMPLIFIER Every part specified and readily obtainable from your Radio dealer. Every stage in construction clearly explained with step-by-step wiring, in instruction book. Rock-like stability in performance — no hum, no microphony, wide frequency response (9 octaves), low harmonic distortion at full power (12 watts) and ensured reliability, with Osram valves specified. Full control on frequency charac- teristic to introduce " art " into listening. Unparalleled clarity of both speech and music when used in conjunction with G.E.C. Metal Cone Loud- speaker in octagonal loaded-port cabinet. Overall frequency response of the complete equipment, comprising LP. record, specified pick-up, Osram 912 amplifier and G.E.C. Metal Cone Loudspeaker in octagonal loaded-port cabinet. How to build the Osram 912 A book giving full constructional details is obtainable from your PRICE dealer, or by post (3d. extra) from The Osram Valve and Electronics Dept. 3/6 THE GENERAL ELECTRIC CO. LTD. MAGNET HOUSE, KINGSWAY. LONDON, W.C.2. 84 PRACTICAL WIRELESS February, 1955 1B4 1C6 1G4 1LN5 1LA4 1P5 1R4 1R5 1S4 1S5 1T4 1T5 1U5 IV 2A6 2A7 2X2 3D6 3S4 5U4 5X4 5Z3 5Z4 6A6 6A8 6AC5 6 ACT 6AG5 6AL5 6AK6 8/( 6AM6 11/ 6AQ5 9/6 6AT6 107- 6B4 9/- 6B7 11/- 6B3 10/6 6BA6 11/6 6BE6 8/9 6BH6 11/ 9/6 9/6 8/6 6/6 9/6 9/6 9/6 10/- 10/- 10/- 10/- 10/6 10/ 8/' 9/6 11 6/6 9/- 10/- 10/6 12/ 11/ 10/6 9/- 12/6 9- 8/6 9/6 9/6 -Qreat Britain's Valve Mail-Order House SALE 6BJ6 - 11/- 6BW6 10/6 8/6 9/- 11/- 9/- 10/- 9/- 11/- 9/- 9/6 5/- 7/6 12/- 8/6 10/6 8/6 6C4 6C6 6C8 6D6 6F6 6F7 6F8 6G6 6G8 6H6 6J5 6J6 6J7 6K6 6K7 , 6K7/M 9/6 6K8 11/6 6L5 < 7/6 6L6 11/6 6N7 . 11/ 6Q7 12/6 6R7 7/9 6R7/M 7/9 6SA7 9/6 6SG7 6SH7 6SJ7 6SK7 6SL7 6SN7 6SS7 6TH8 6U7 6V6 6ZY5 6W5 6X4 9/6 8/- 9/6 9/6 11/6 11/6 8/- 9/6 10/ 10/- 11A 10/- 6X5 7A7 7B7 7C5 7C6 7H7 7N7 7Q7 7R7 7S7 7V7 7Y4 8A1 8D2 9D2 12AL5 12AT7 10/6 12H6 6/6 12J5 12SA7 10/6 12SH7 8/6 12SJ7 9/6 12J7 11/6 12K7 11/6 12SK7 10/- 12SL7 12/- 12SQ7 10/- 12SR7 8/- 12Y4 9/6 12AX7 9/6 14 F6 10/ 25AC5 6/6 25Z4 25Z5 19 32 33 35 35L6 DEMOBBED VALVES MANUAL Giving equivalents of British and American Service and Cross Reference of Commer- cial Types with an Appendix of B.V.A. Equivalents and Comprehensive Price List. We have still some Valves left at very old Budget Rates. (33* %) which are actually sold at the old price. (1951 rate.) TUBE CRT 516 Magnetic v. heater. 4 to 5 kV. E.H.T. Int. Octal Base. BARGAIN, 57/6 P 1 F C O All - in - one Radio - meter i A.C./D.C. Tests* everything in* Radio. Com- plQte with Test prods. 29/6. Post 1/6. Carr. and Crate. 4/6. TAYLOR METERS EASY TERMS Ask for Catalogue on B.T.H. GERMANIUM CRYS- TAL DIODE. Complete with 7/- 1 Blueprint and operating in 9/6 structio ns. 2/-. . 4/ " ROD ANTENNAS. 1ft. sec- tions interlocking and extend- ing, copper plated steel. BARGAIN. Dozen 2/6. MSPen 5/- l OM4 5/- P2 3/- P215 21- PM12M 5/- PM24A 7/6 SP2 5/6 SP42 5/- SP61 2, VP13C 7/- VP23 3/ Q-MAX * Chassis^ Cutters with Keys The easiest and $ quickest way of ^cutting holes [I, sheet metal. The cutter consists of three parts: A die, a punch and an Allen screw. The operation is quite simple. Prices incl. key : tin., 12/4 ; iin.. 12/4; iin.. 13/4 lin., liin. and l{in., 16/- 9*** llin. and 1 Jin.. 18/- each ; liin.. 19/9; 23/82tov, 31/9; 24 in.. 36/9 ; lin. square, 24/3. Post 1/-. TYANA TRIPLE TURK! Small Soldering Iron. Latest development. Complete with detachable bench stand, 17/6. Post II-. Continued here w. British Types next month Post 9d. SERVICE SHEETS The one you require enclosed if available in a dozen assorted of our best choice. 10/6. 7/6 MICROPHONE 7/6 This U.S.A. Throat Mike Made by Universal Microphone Com- pany, in California, is highly sensitive. Complete with neck piece clip and illustrated 4 page specification and original carton. Bargain Price Only fraction of original cost. Limited quantity. Post 9d EXPERIMENTAL KITS in Radio, T. V. etc. LEARN THE PRACTICAL WAY Specially prepared sets of radio parts (which you receive upon enrolment) with which we teach you, in your own home, the working of fundamental electronic circuits and bring you easily to the point when you can construct and service radio sets. Whether you are a student for an examination; starting a new hobby; intent upon a career in industry; or running your own business — these Practical Courses are intended for YOU — and may be yours at very moderate cost. All lessons and equipment supplied immediately and become your own property. EASY TERMS FROM 15/- A MONTH POST THIS COUPON TO DA Y Please send me your FREE book on Practical Courses: To: E.M.I. INSTITUTES, Dept. 32X, Grove Park Road, London, W.4. NAME ADDRESS SUBJECT(S) EXPERIMENTAL OUTFITS: RADIO (basic) — A course in bask principles^ „ RADIO— Instruction and equipment from which you, build a Radio Receiver. TELEVISION — Instruction and equipment for building a Television Receiver. - . Also for Mechanics, Electricity, Chemistry, Photography, Carpentry, Draughtsmanship, Commercial Art, Amateur S.W. Radio, Languages. The only Postal College which is part of a world-wide Industrial Organisation E.M.I. INSTITUTES February, 1955 BUILDING COMMUNICATIONS RECEIVERS 3.— DOUBLE SUPERHETS AND TUNING INDICATORS {Continued from page 23 January issue) Double Superhets SOME ex-service and other communications-type receivers employ two frequency changers and two sets of I.F. amplifier stages. Such a circuit might employ conventional R.F. and F.C. stages, followed by I.F. stages such as shown last month. The output from this section of the receiver would not, however, be taken to the detector stage, but to the second F.C. stage, a typical circuit for which is shown in Fig. 2. This stage changes the frequency to a different one from that already employed and further I.F. amplifier stages, all operating on this new frequency, then follow. A number of advantages arise from such a method of operation, which increases selectivity and sensi- tivity greatly. With a large number of I.F. stages operating on one frequency, instability may easily arise, and this is avoided by a number of the stages operating upon one frequency, and the remainder upon a different frequency, as with the double super- net. The rejection of adjacent-channel interference is also increased. The circuit in Fig. 2 is typical, and A.V.C. could be applied. The tuning of the oscillator coil is not variable, as with the first F.C. stage, but pre-set by means of C2. CI is the usual coupling condenser. The selection of suitable I.F. transformers and oscillator coil is not difficult, and many standard and ex-service transformers can be used, while an ordinary " unit " coil, possibly dust cored, can be used for the oscillator circuit. If 465 kc/s trans- formers are used in the initial I.F. stages, 110 kc/s transformers may be used in the stages following the second F.C. stage. If, however, ex-service trans- formers of rather higher frequency are employed in the initial stages, then 465 kc/s transformers can be used in the later stages. Alternatively, numbers of small long-wave ex-service coils exist which may readily be used in the later stages, 465 kc/s trans- formers being used in the first section of the I.F. amplifier so that standard signal-frequency and oscillator coils can be used in the R.F. and first F.C. stages. The frequency of the oscillator coil may readily be determined by adding together the first I.F. and second I.F. transformer frequencies. With 465 and 110 kc/s components, this would be 575 kc/s, which is approximately 520 metres. A coil tunable to this wavelength would be obtainable without difficulty. Fairly high initial intermediate frequencies are more usual, though not essential, and may be of about 1 to 7 Mc/s. Typical Valve Sequences The use of the maximum number of stages is by no means the most desirable feature, but the provision of adequate selectivity and sensitivity, with low HX+ HX+' Fig. 1. — A crystal I.F. circuit. Fig. 2. — A Double Superhet circuit. 86 PRACTICAL WIRELESS February, 1955 noise ratio. A study of commercial communications receivers will show that three LF. stages are seldom exceeded, even in very expensive equipment. More than one tuned R.F. stage may be found, but is not very usual, and is best avoided in home-built equip- ment. Some double-superhet receivers employ no I.F. stage at the initial frequency, the first F.C. stage, with transformer or transformers, being followed immediately by the second F.C. stage, which may feed a single LF. stage to be followed by the detector. Where two or more I.F. stages are used, the double-superhet is not very often employed, except when extremely high standards of selectivity and sensitivity are required. It will therefore be seen that the construction of such equipment is hot necessarily one of great cost, and about six to ten valves, in all, will be found a convenient number to employ. (Usually, of course, no great difficulty should arise in adding further stages later, if wanted.) Where simplicity is important, the FC/2-I.F./DDT/ Output type of circuit can give exceedingly good results, further improved by the addition of an R.F. stage. If both R.F. stages and a further I.F. stage be used, a standard of performance adequate for most purposes can be obtained. Usually, a very high audio output is not required, and a single stage, delivering 3 to 5 watts, is ample. Quality of repro- duction need be by no means poor, and negative feedback, combined with 44 fiat " tuning may be used for local-station listening, resulting in a circuit with the widest possible usefulness. Fig. 3. — Tuning meter in LF. anode circuit. Tuning and Signal Strength Meters Many of the simpler communications type receiver have no signal strength or tuning meter. With other receivers, such a meter is built in, or can be purchased as a plug-in, separate unit, usually in a small desk-type housing, with flexible lead. A tuning meter may be added to any superhet, and will then indicate the correct tuning point for any R.T. signal, or show the relative strength of the carrier wave of such a transmitter. It is not suitable for I.C.W. Morse transmissions, since in the intervals between letters, or the symbols of a letter, there is no carrier wave to deflect the meter. The meter thus endeavours to follow the keying. When employed for its usual purpose, no such difficulty arises, and a steady indication is obtained, irrespective of speech, music, or other carrier-wave content. A signal strength device is much more sensitive to small changes than the human ear, and also provides a definite reading which may be noted from day to day, or compared with readings made from other transmissions. Any modification which increases signal strength at the receiver (such as improvements at the transmitter, or an improved aerial, etc.,, at the receiver) will be shown at once by an increased reading. A tuning meter is primarily concerned with showing the point of exact tuning for any station, and may consist of a Magic Eye or moving-coil meter. For this type of indicator, the Magic Eye has the advantage of low cost, small panel size, and robustness. The meter is, however, better when comparisons of signal strength are required, and is almost essential for the signal strength type of circuit. Meter Tuning Circuit A simple and effective method of using a meter for tuning indication is shown in Fig. 3. The meter may be connected in the anode circuit of the frequency- changer, mixer, or I.F. valve, provided the valve chosen is under the control of the A.V.C. system. It is also feasible to group together the anode circuits of two or more valves, so that the meter reads the combined anode current. In operation, the anode current depends upon the A.V.C. voltage, which, in turn, depends upon signal strength. Anode current is at maximum with zero signal, and at minimum with maximum signal. Resonance is thus the lowest obtainable meter reading, while the greater the signal strength of the transmitter, the lower will the reading be. Such a meter thus provides comparison of signal strengths, and accurate tuning. 5- 7mA — >- IkJX Fig. 4. — A typical signal strength meter. The anode current change of an average I.F. valve type will be about 2 to 3 mA. In order that a zero signal strength indication may be obtained on the meter scale, the meter should be of a type having a full-scale deflection under the normal anode current rating of the valve or valves to which it is connected. The "Zero" control shown consists of a wire- wound variable shunt. In use, the receiver is tuned to a point whjre no signal is obtained, and the control adjusted until the meter indicates full-scale. The control is then left untouched. When a station is tuned in, the pointer will fall back to an extent depending upon the signal strength of the station. A 1 mA. or 2 mA. meter, suitably shunted, is satis- factory. The .1 /zF condenser is for decoupling. Such a meter may be added to any commercial or amateur-built superhet. The " S " Meter The 44 S " or 44 R " (Signal strength or Readibility) meter employed in communications equipment February, 1955 PRACTICAL WIRELESS 87 usually has a circuit such as that shown in Fig. 4. Though frequently fitted in the F.C. or mixer anode circuit, any A.V.C. operated valve may be employed. Operation is rather different from that of the circuit in Fig. 6. Assuming, as example, that the anode current of the valve is 5 mA. at maximum signal, and 7 mA. at minimum signal, the voltage drop in the 1 K. resistor will be 5 v. for maximum signal, and 7 v. for minimum signal. The voltage at one meter terminal would thus be approximately 243 for minimum signal, and 245 v. for maximum signal. If the meter is able to read from 0 to 2 v., and has its second- terminal maintained at 243 v. by the potential divider, it will read zero for minimum signal, the reading rising to full-scale for maximum signal. With this circuit, the receiver is thus tuned for maximum deflection. The circuit may initially be set up in such a way that zero and maximum meter readings are obtained with certain minimum and maximum signal strength levels. In practice, zero reading will be obtained with zero signal. Initially, the 100 K. potentiometer requires to be set with the slider towards the H.T. positive end varies from type to type. This value may be modified within limits by changing the load resistor value. Reduced target and anode voltages reduce the A.V.C. voltage required for zero shadow angle. The control voltage ranges from about 8 to 22 volts for an eye such as the 6U5, down to 3.3 to 8 volts for types such as. the 1629. Fig. 6A shows the usual circuit. The 100 K. resistor and .05/tF condenser may be omitted where no instability arises. Used in this way, the eye will show accurate tuning positions for any transmitter powerful enough to operate the A.V.C. system, and also give some indication of signal strength. An improved circuit is shown in Fig. 6B, and enables the voltage applied to the control grid of the eye to be adjusted. Generally, the .5 megohm control may replace the A.V.C. circuit fixed resistor found in this position. With this circuit, a sensitive eye may be adjusted until its range corresponds to zero and maxi- mum signal strengths. A further method is to employ a certain shadow angle as reference, and have the .5 megohm control panel operated, with a scale. When the signal strength of a station is to be read, the control is then adjusted until the exact shadow angle lOOka fi gt S,—An alternative method of connecting the meter of Fig. 4. of the circuit, or a heavy current may flow as the rectifier warms up. This should be watched. When the receiver has reached a stable operating tempera- ture, the control may be adjusted. If the meter moves the wrong way on tuning in a signal, connections to it require to be reversed. To avoid possible damage due to turning the potentiometer slider too far towards the negative side of the circuit, the potential- divider may be made up from one fixed and one variable resistor or potentiometer. A second circuit giving similar results is shown in Fig. 5, the potential-divider circuit being fixed, and the 1 'K. resistor variable. The operation of this circuit is basically the same as that in Fig. 4. The 2 KQ resistor and .1 /iF condenser are included for decoupling. Magic Eye Circuits The magic eye is usually operated directly from the A.V.C. circuit, the A.V.C. voltage controlling the anode current of the triode section of the eye. Many ex-Service magic eye or tuning indicator valves are cheaply obtainable, and will generally be satisfactory. It should be noted that a small number of such eyes have no triode section, and a separate valve would be required, here, if such an eye were used. In addi- tion, the triode section control grid voltage required for zero shadow angle (or sensitivity of the eye) Fig 6 — Magic eye tuning indicator circuit is obtained. The control pointer will then indicate the relative signal strength, against its scale. With the control left in a middle position, the eye can be used for tuning only, in the usual way. Where manual R.F. or LF. gain controls are fitted, the influence of these must not be overlooked. For example, the meter reading would fall back if gain were reduced in any preceding stage, or in a following stage which comes within the A.V.C. net- work. This difficulty may be overcome by employing a switch to cut out any R.F. or LF. cathode or screen- grid gain control, gain being at maximum when the switch is closed. Volume can, if required, be kept down by the A.F. volume control, this having no effect on the meter reading. Such a precaution is only necessary when making definite readings of signal strength. The meter or eye will, of course, continue to function as a tuning meter, irrespective of the setting of a manual control in another stage. Such a manual control should not, however, be used in the stage where the meter is employed. In addition, it should be assured that the beat- frequency oscillator (if present) is not operating, as in some circuits this will change the signal reaching the second detector in such a way that the A.V.C. voltage is modified. 88 PRACTICAL WIRELESS February, 1955 Diode-transistor Loudspeaker Receiver A VALVELESS MIDGET OF INTERESTING DESIGN By Capt. Graham TESTS showed that one transistor is practically useless for broadcast reception. More than one calls for bias resistors, transformers, etc., imposing pre-set loads, and limiting the output of the final transistor to an output which is much less than it could give if it were unhampered. Diodes work on very high frequencies, and so it occurred to me to use a diode to feed a transistor. The result was that the transistor gave out a remarkably higher output. Further tests showed that a pre-set bias is not necessary for broadcast reception. A very simple circuit was therefore evolved with only one diode and one transistor, which works to its full capacity in a., most efficient manner, gives an output sufficient to work a loudspeaker, and has very good fidelity. In fact, it is so good that for the past nine months it was used in preference to four other expensive receivers. Also, recordings on a new Ferrograph 2B are superior to recordings from a commercial Hi-Fi valve tuner Tuned to BBC Home Service, with a*0002/jf in series with atrial, gives 1mA. Type U2 Ceil jjj Fig. 2. — Details of a layout built into a torch case. diode. There is no background noise and negligible crackling during a storm and high-voltage low-current discharges. A first-class horizontal aerial and earth is necessary. High- 4 ' Q " coils and wiring with short leads avoid capacity losses. Care should be taken not to overload the transistor by powerful local stations. In all other respects there are no snags except that the transistor must be correctly and care- fully soldered ; any false connection will destroy it. If not overloaded it should last a lifetime, at negligible cost. The Circuit (Fig. 1) The Brimar GD3 germanium diode is connected with the red end to the coil centre tap, to rectify and supply a modulating negative potential to the transistor. The other end of GD3 is connected to the base of a Mullard OC71 germanium junction transistor of the p-n-p type. When the aerial is disconnected, the transistor base is at the same D.C. potential as the grounded emitter ; the meter reads zero, and then the transistor does not allow any current from the battery to flow through it. But when the aerial is connected and the coils are tuned to a station, the meter shows a reading, and now the transistor does allow the battery current to flow through it to work the loudspeaker. Consider what happens during half of a cycle of a radio-carrier frequency. When the grounded end of the coil is positive and the centre tap is at a negative potential, the coil is supplying current derived from the aerial. Electrons tend to flow from negative to positive just as in the valve theory we all know. The rectifier diode GD3, being connected as it is, accepts the electrons from the negative centre tap. Electrons pass through it to the base of the transistor and because it is p-n-p type the base likewise accepts. Electrons flow through the transistor to the grounded emitter, which is at the same positive potential as the grounded end of the coil attracting these electrons. This com- pletes one circuit and current has passed through the transistor. If instead of the 3-volt battery a micro-amp meter is connected, it shows a reading. This proves that some of the electrons from the coil flow from base to collector, through the transformer, the 5 mA meter, the fiK meter to the earthed end of the coil, completing another circuit, and some of the aerial Current may flow through the transistor in this circuit, since both the emitter and the collector are at the same potential as the grounded end of the coil in so far as these electrons are concerned. From the foregoing it becomes evident that a very pronounced change takes place in a transistor when a current flows through it. Without a current and with base at same potential as the emitter, it does not pass battery current, but with a negative potential at base and current flowing through, a secondary effect takes place, it tloes pass a very considerable current. Furthermore, this greater current is controlled by the transistor to pass through in very definite proportions, depending upon the amount of aerial current flowing at any instant or depending upon D.C. potential at the base and a steady current flowing, and so curves can be plotted. Negative pulses produced by the diode are enormously amplified and are passed through the transformer. Since these half-cycle pulses are modulated by music, etc., sounds are reproduced in the speaker. *** February, 1955 PRACTICAL WIRELESS 89 Battery Effect There are two good reasons why a very much greater current passes from the battery. Firstly, the three-volt battery has a higher voltage than the small fractions of a volt at the base, and it can give a current far in excess of requirements, whereas the diode can only supply the few microamps available from the aerial. And secondly, because the junction transistor can pass an excessive current which will destroy it, even with a small voltage. It is constructed with fair contact surfaces and thin junctions having little resistance and high efficiency when working as stated. If the battery connections were to be reversed with the negative terminal connected to earth, the transistor would allow such a high current to pass through it from the battery that it would be destroyed and the milliamp meter would also be damaged. Hence the warning : be very careful to connect the transistor and the battery correctly. Test with a high-resistance voltmeter having one milliamp full-scale deflection in place of the mA meter. If the transistor or battery is wrongly connected, the voltmeter will show almost the full battery voltage, but if correctly connected it will show a very small reading near zero. Do not use an ohmmeter to test a transistor ; some such meters will pass a damaging current. Use a voltmeter in series with a battery, then you know that the maximum current will be less than full-scale deflection current, and the meter reading will show what proportion of the current is passing through. Half of full-scale deflection represents \ milliamp, etc. During the positive half-cycle the diode naturally rejects and current does not pass through with wrong polarity to interfere with the proper working of the transistor which now has a period time of half a cycle to completely dispose of the negative pulse it received. The .002 condenser is to by-pass any stray R.F. from interfering, and it retains some of the negative potential and acts as a bias device. The meter reads a shade higher when this condenser is connected from base to ground. If the diode is shorted or removed, and the base connected to a tapped coil, the reception is not much better than with the diode alone ; apparently some sort of a mixture of various conditions takes place in the transistor which as yet cannot adequately deal with R.F. all on its own ; still thinner ones or composite ones may be evolved in time even for R.F. amplifica- tion, and short aerials. This receiver works like a D.C. valve amplifier. If the coil is disconnected and a small D.C. negative . potential is applied to the diode (p) pr the transistor Vbase, the meter shows a mA reading in definite proportion and curves can be plotted as for a valve. j These curves show a remarkably - £ '0003 7 high slope which works out to over 60 mA/v. There is no receiver valve which can give such high efficiency, mostly not a tenth of it. Long high horizontal aerial 0005 7" 75 Q O o o o o o . GD3 red } *002/jF mica O-SmA. Meter D.C. RDKR. A Twin Track 2 speed recorder . . will g^v ttenew P£ A gg°5g£ K> £ n assembled AMPLIFIER, • ard reels up to 1.200 ft. Constructors can build it for £40 including the ff™^ CASE • MICROPHONE, i,200 ft. REEL OF TAPE, and an attractive PORTABLE CARRY CAb*. • HATOO SETS , , v,„c.oio vicivin"- nrovision for the attachment of an S (a) THE NEW ARMSTRONG F.C.43 CHASSIS ... an 8 valve 4 waveband chassis havm 0 prov^iu • (b) f VALVE^SUPERHET PORTABLE in small Attache Case ... for Battery or Mains operation. • (c) «> or 3 VALVE BATTERY PORTABLE all dry T.R.F. design, (d) CAR RADIO. A Superhet design for 6 or 12 volt supplies „ . m . r _„ 1R/fi ex tra if required S u ^y^.,,r-^ m r, tt. ntiAQCTc r<™, «i o*-oi w assembled for £6.9.6. Bakelite case iwg J?*cf& m eg follow (e) 4 VALVE T.R.F. CHASSIS. Completely __ DENCO F.M. Unit. Consisting of 5 Valve Superhet design incorporating Discriminator. STERN'S MAINS IN IT KITS. Two types available : (a) For R.F and F/C Stages followed by two I.F.s and Ratio volts and 1.4 voltsjJb^j^r^oUs^ L4 volts : . TWO COMPLETE Hi-Fi AMPLIFIER KITS A HIGH QUALITY 8-10 watt AMPLIFIER Th3 Ideal Amplifier for General Home Use and for Small Halls, etc. PRICE OF COMPLETE KIT. IN- CLUDING VALVES and DRILLED CHASSIS £7.10.0. (Plus 2/6 carr. and ins.) WE WILL SUPPLY IT COMPLETELY BUILT FOR £9.10.0.XPlus 5/- carr. and ins.) Designed for high quality reproduction up to an output level of 10 watts, having 6V6s in Push- Pull and incorporating negative feed-back. Suitable for use with all types of Pick-ups and most A 12-Watt "HIGH FIDELITY ' Push-Pull AMPLIFIER Comprising a Main Amplifier Chassis and a Remote Control Pre-Amplifier/Tone Con- trol Unit. The remote i control unit measures i only 7in. x4in. x 2m.. and » contains four controls, ' being : Bass -Treble- Volume and a Radio. ► Gram, Microphone ► Switch control. It .- ► incorporates its own feed-back circuit on 1 the Bass Channel. Loop negative feed- THE COM- PLETE KIT IS AVAI LAB L E FOR £14.0.0.(Carr. and ins. 3/- extra.) types of of 3 and *aIs al re e prmmcfio°n VTadto'(where" a 'tuning 1 ' unit Is used) and Record playing. • Separate Bass Boost and Treble controls ffi^^rlSffiti^^™^ 1/- and includes STERN RADIO LTD. 109 & 115 FLEET ST., E.C.4, csl^^m^ • This Amplifier compares well with the Williamson and • similar designs at a fraction of their cost, . The complete • set of assembly instructions are available for 2/ 92 PRACTICAL WIRELESS February, 1955 PRECISION BUILT MATCHED COMPONENTS M.G. GANG mm CONDENSER S L 8 -SPIN WHEEL DRIVE M.G. GANG CONDENSER Available as I, 2 or 3 gang, 490 p.F. nominal capacity, matched and standardised to close limits. Supplied with trimmers if required. Other capacities available — details on request. Cadmium plated steel frame. Aluminium Vanes. Low loss non-hygroscopic, insulation. Spindle ]in. dia. projects I ^in. from front plate. Front area 2|in. x 2-&in. including sweep of vanes. Length excluding spindle : Price I gang — I £ in. 9/3d. 2 gang — 2 ,^ in. 3 gang — 3|in. l4/-d. l8/3d. S.L.8 SPIN WHEEL DRIVE A precision slide rule drive. Complete with 3-band glass scale, 9in. x 4f in. Printed — short, medium and long wave bands with station names. Scale length 7in. The spin wheel drive gives easy control through a ratio of 24- 1 . Fitted with constant velocity coupling, eliminating strain on the Condenser, and providing mechanical and electrical isolation from vibration and noise. Supplied with florentine bronze escutcheon. Price — 27/6d. complete. Write for fully illustrated catalogue. J ACKSO BROS. (LONDON) LTD. KINGSWAY . WADDON SURREY Telegrams : WALFILCO, SOUPHONE, LONDON Telephone : CROYDON 2754-5 Car, £13.19.6 packing 7/6 Burgoyne TWO-SPEED TAPE DECK if & 7i-/sec. Here is the heart of your Tape Recorder. If you are building or modernising your own equipment you MUST have the latest BURGOYNE Tape Deck giving 2 speeds, designed for building into complete recorders. EASY H.P. TECHNICAL DATA ★ 2 HOURS' PLAYING TIME. * Fully automatic speed change. * Instantaneous and positive braking. * Powered by 3 high-grade motors. Twin track heads. Minimum wow and flutter. ★ Frequency range 50/10,000 c/s at 7i"/sec. ★ 200/250 v. A.C. mains. FACILITIES Send only 10% deposit, balance over any period up to 18 months. Made exclusively for : e. & g. MAIL ORDER SUPPLY CO. The Radio Centre 33, Tottenham Court Road, London, W.I. MUSeum 6667 WE SPECIALISE ONLY IN RADIO COMPONENTS— LET US QUOTE YOU BY RETURN FOR ANY RADIO VALVES OR PARTS YOU REQUIRE HIRE PURCHASE TERMS ON KITS, INSTRUMENTS, ETC. Osram " 912 " and Milliard " 510 M Amplifier Parts in Stock. NEW CATALOGUE WITH H.P. TERMS 6d. QUOTES S.A.E. RADIO SERVICING COMPANY 82 SOUTH EALING ROAD LONDON W.S EAL. 5737 NEXT DOOR SOUTH EALING TUBE STATION (TURN LEFT) February, 1955 PRACTICAL WIRELESS 93 The Twenty-second Article of a Series Explaining the Fundamentals of Radio Transmission and Reception. This Month Rectifiers are Dealt With IT must be remembered that Ohm s law for alter- nating current, or rather the formula relating to it differs from that relating to direct current We have seen that in an alternating current circuit, Ohm s law expresses a relation between the E.M.r., ot tne circuit, the current flowing and the circuit impedance IE 1 and Z respectively) from which E equals 1Z This brings us to the question of what is known as tne RMS value, or the root mean square value ot alternating current or voltage. It is the square root of the meL value of the squares of the instantaneous values taken over a complete cycle and when an alternating current or voltage is Bgecifed, IMS almost invariably the root mean square value that is useci It is sometimes referred as the effective vflfctf.. W brief, this means that 230 volts A.C. is not the same thing as 230 volts D.C. Rectifiers It is important to remember in mains transformers m after transforming the voltage it is still alternating current and must therefore be converted into D.C. This is achieved by either half-nave or full-wave rec%catU and this achieved by a particular type of rectifier. Even after rectification, the current is still too "rough'' as it is a pulsating D.C. supply, and smoothing must be introduced. Rectifiers are of two types, the valve rectifier and the metal rectifier, although there are other types, such as chemical rectifiers, the Tungar rectifier, the vibrating reed rectifier, as used in car radio, the rotary convenor, the mercury vapour rectifier and the copper-oxide rectifier. For radio purposes, a selenium metal rectifier is mostly used today. Valve and metal rectifiers are available in half- wave or full-wave types ; the symbols for these are illustrated in Fig. 96. Now a rectifier suppresses the flow of current n one direction and it therefore follows that a halt- By F. J. CAMM wave rectifier gives pulses at half frequency, and a full-wave rectifier at double frequency. Neither, however, actually delivers direct current but a pulsating current of fixed polarity. Rectifiers are used for rectification of the H.F. signal, or to put it another way, for high-frequency rectification as well as for providing power supplies for high-tension purposes. , * Signal rectifiers, whether of the valve or metal type are much smaller than those required for power supplies since the requirements are smaller. Typical signal rectifiers are the Westector and germanium diodes. It is important to note that half-wave rectifiers for power supplies are only used where the current supplies are very small. If a current greater than that of an individual valve, whether employed for full-wave or half-wave rectification is called for, then two or more valves may be connected in parallel. it., Metal rectifiers are also used in voltage doubler or bridge circuits, as shown in Figs. 97 and 98. The voltage doubler as shown in Fig. 97 uses a metal rectifier, and it is connected together with fixed condensers to provide a bridge circuit, resulting in a step-up in voltage. The condensers used in the voltage doubler circuit are of critical value, and the makers' recommendations must be adhered to. In the circuit shown, the bare rectifier circuits are illustrated ; the A.C. input may consist of a mains supply or,' as is more usual, a mains transformer. The D.C. output will, of course, be fed into a suitable smoothing circuit to be dealt with later. Smoothing Chokes Reference has been made to the necessity for smoothing and for this purpose a choke is employed. A choke is a simple component consisting essentially of a length of wire wound on a former built up from 4 K fig 96.— A half-wave valve rectifier, a metal rectifier and a full-wave valve rectifier. c. t 1: D.C f ig 97. — ^4 m eta .rectifier voltage doubler circuit. 94 a number of iron laminations by the method already described for low-frequency transformers. The simplest type of iron-core choke is one intended for coupling together two valves on the choke capacity principle, or for connecting a loudspeaker to an output valve. The essential design data is an induc- PRACTICAL WIRELESS -OAC. O Fig. 98. — A bridge rectifier circuit. tance of not less than 50 henries at the norma 1 working current, a D.C. resistance of 2,000 ohms or less, and a safe current carrying capacity of not less than 20 mA. When dealing with currents above, say, 50 mA, it is advisable to employ a smoothing choke of greater dimensions and having a lower D.C. resistance. It is also an advantage for the choke to be of the constant inductance type, so that its s inductance does not vary measurably when the current passing through the winding is varied. In order that a choke should show such characteristics, there must be an air gap in the core. That is to say, the T- and the U-pieces should not touch each other, but should be arranged with a small gap between them. The iron-cored choke can be used for any purpose where a choke is required, but it is specially suitable for use in powerful mains receivers for smoothing or feeding the loudspeaker. It can also be used successfully as a loudspeaker field replace- ment choke. Smoothing chokes generally should have an inductance of not less than 30 henries, when carrying the maximum D.C. current. In some instances it is found convenient to employ the field coil of an energised moving-coil loudspeaker as a smoothing choke, but in that case it is essential HT.+90V. 2500 D 250 V. 60mA. 7*SV05A. /soon =3 c/ Mains h —ve Fig. 99. — A half -wave rectifier with resistance smoothing. that there should be a fairly considerable surplus H.T. voltage, since the resistance of the field winding is generally about 2,500 ohms. Resistance for Smoothing A resistance may be used for smoothing where the current requirements of the set are low, but it must be remembered that whereas a choke has a low February, 1955 resistance, a resistance will cause a much greater vo tage drop and necessitate adjustment of other values in the circuit. The circuits in Figs. 99 and 100 show choke and resistance smoothing respectively. It is important to remember that any coil may be considered as a choke, even a tuning coil, for the correct definition of a choke is that it is an inductance, and all coils have inductance. Inductance is a term often misunderstood, but an idea of its meaning can be grasped by considering it as providing resistance to alternating or H.F. current. No matter how a length of wire is coiled, its resistance to D.C, which is governed by Ohm's law, does not change, the resistance of a wire being proportional to its length. That is to say, the resistance of 2 yds. of wire of a given gauge will be twice that of 1 yd. of wire of the same gauge. The impedance or reactance of the coil, however, to alternating current varies greatly. For example, the length of wire used for the average H.F. choke has a resistance to D.C. of about 300 ohms, whilst its inductance may be approxi- mately 250,000 micro henries. This means that its impedance to low-frequency current has a frequency of 1,000 cycles per second (equivalent to the highest note of the human voice), is 1,500 ohms, whilst the impedance to a current 100,000 times per second (the frequency equivalent to a wavelength of 300 metres) is 1,500,000 ohms. Low-frequency Chokes A similar state of affairs exists with regard to low- frequency chokes, for one which is rated at, say, 20 henries would have an approximate D.C. resist- ance of 250 ohms. But the impedance of the choke to an alternating current of 50 cycles would be about 6,000 ohms, or 12,000 ohms if the frequency were doubled. These examples show one important use of inductance. An inductance coil or choke can be designed to provide an easy path for direct current whilst offering a considerable resistance to alternating current, or by using a smaller inductance value to HT+3 Fig. 1 00 .— A similar circuit, but with choke smoothing. offer a comparatively low-impedance to low-frequency alternating current, and a high-impedance to high- frequency current. This, in brief, means that the . impedance of a choke increased with inductance and frequency, and vice versa. (To be continued.) EVERYMAN'S WIRELESS BOOK By F. J. CAMM Ith Edition From 12/6, by post 13/- GEORGE NEWNES, LTD., Tower House, Southampton Street, Strand, W.C.2. February, 1955 PRACTICAL WIRELESS _ 95 A Valve -Voltmeter A TEST-SET OPERATING DIRECT FROM A.C MAINS By " Radio Engineer " AFTER building several valve voltmeters, some bad, and some better, I ultimately built one operated directly from the mains ; that is to say, I omitted the H.T. winding, using a filament transformer at slightly reduced voltage. The object was to keep the cost down as much as possible, at the same time seeing that the efficiency was not impaired. Simple to use and not too big was the thing I was after. The accompanying illustrations will give an idea of the resultant build-up. The first question was ... to what extent the mains variation would affect the accuracy of the meter. I made a check on two commercial valve voltmeters and one which I had already built and which had P.P. with primary 325-0-325 volts, F.W. rectification, and the results in all cases were the same. When in use with the mains voltage reduced by 10 per cent, on 250 volt mains, the accuracy of reading on the 100-volt scale was 5 volts lower ; exactly the same result as with one directly fed from the mains, so, unless one buys or makes one's own with stabilised P.P. and multivalve balancing arrangement, there is not much hope in expecting accuracy of two per cent, or less from the average valve voltmeter. Design The main emphasis around which I have built the latest valve voltmeter is as follows : Input resistance 10 meg. on D.C. and A.C. up to 1,000 volts. The A.C. is rectified by two separate diodes built in the probes. One reads up to 250 volts, 50 c/s. to 1 50 Mc/s— the second can be used on 10 volts to 1,000 volts or more, depending on the conden- ser rating selected. The first valve is a 6AL5 double- diode. The 1,000-volt probe has an EY51 valve View of rear of panel. On the right is a view of the finished instrument. rated at 5 k/w. R.M.S. so it is quite capable of hand- ling 1,000 volts A.C. The valve being wire-ended and with small dimensions makes it very suitable for fitting into a probe, but the frequency is limited to about 10 Mc/s— this one is primarily built for ranges between 500 to 1,000 volts A.C. As will be seen from the circuit, a double-triode is bridge connected, and a 0.5 mA. meter is used as balance indicator. P.P. as mentioned ;L.T. transformer; 2 RM1 ; two8//F electrolytics ; 10 K. smoothing resistor, and neon parallel with D.C. output is fed to 10 K. potentiometer (zero set) via 2.2 K. to the anodes of 12 AU7 and the meter. The positive input terminal is connected to the earth side, that is to say, to H.T. negative, and the H.T. negative is positive from the P.P., so that if the negative lead is connected first, some deflection may be obtained— ignore this— the final reading with both leads connected will be correct. The input voltage is fed through a 10 meg. chain of resistors assembled around a ceramic switch, having three wafers ; two of them are one-pole five-way, and the third one is for switching in and out the calibration controls. The original switch used was three-wafer, one-pole five- vvay _the third was altered accordingly. Case The case is made of .024in. thick tin-plate, which is very easy to bend and the brackets holding all the components can be easily soldered to it. It \s composed of four parts, the top and bottom bent round 1/I6in. to 3/32in. to form the lids. The centre piece is a strip 3in. v/ide and 28in. long bent on to fin. round steel to make rounded corners, joint soldered, and bottom lid soldered to it. Ventilation holes drilled on both sides with two holes opened up 3/16in. or I'm. to be able to reach with screwdriver the cali- bration controls (see Fig. 3), with- out opening the instrument. The top lid of the case is made in a similar manner to the bottom, ex- cept that dural or aluminium panel is added to strengthen the lid. The lid serves as a chassis to hold all the components, which are fixed on the brackets. The brackets are made of 5/16in. round brass drilled and tapped both ends and the ends 96 PRACTICAL WIRELESS February, 1955 of the strip are soldered to the back of the lid to act as the pillars. Five will be needed — two for the transformer, two for the bracket which holds the calibration controls, and one short pillar to hold the 'in diameter View of the two probe s. Top, the 250- volt and below the 1 ,000-volt probe, valve- j | To valve voltmeter 4 long I 'inside dia. IOOOV C PROBE + F 4MC1 I F FK EYSI 01 OIjjF IOOOV AC. H Polystyrene plug /q dia. made o f two halves . Ph. bronze spring ■ Brass sleeve -Polystyrene bush - Earth (e) O E holder is also fixed on a strip of tin-plate and soldered to the lid. This also should be well made and preferably shock- proof with rubber mounting, LIST OF COM 12AU7 6.3 rectifiers. All the necessary holes should be drilled for switch, zero set, fuse, neon, mains in, and two terminals, and one to take A.C. socket for probes. As can be seem^rom the photograph, the A.C. socket is four points, but three are quite satis- factory and this is shown inthediagram. The sockets and plugs must be of very high in- s u 1 a t i n g quality (not from H.T. battery). In the prototype, they were made from polystyrene of Valves 6AL5 EY51 1 ceramic switch, 1 p. 5j 3 -pin socket, 2 plugs. 2 terminal sockets and pi? 1 10K W.W. potentiometer. 2 5K W.W. preset (calibratii Chain of resistors 10 meg. tc £ w. (see diagram). Remaining resistors 2-5 per 1 neon G.E.C. and holder (n 1 fuseholder 250-500 mA. 2 RM1 150 v. each. 1 filament transformer 6.3 v, 2 0.75 ohm 10 w., W.W. (se 1 M.C. meter 0.5 mA full d< 28//F 450 v. D.C. electrol} 3 0.01 / 4S/6 : WB114, 2.3/6 i WB15. 42/- : WBllb and WB117, 7 6 each : WB118, 22'6 ; WB119, 26/9 ; WB121 3/6 1 ; WB124, 10 6 ; Westinghouse^/EHT/30, 17/-. Plus condensers and resistors as per our general list. ^ ln ^^ NGLI 'Lf OMPOIVK!STTS - ALLEN. Teleking Chassis. : ££ llsets (TK & Super-visor), 44/6 ; LO.308, 40'- ; FO.30. r >. 21 - : DC.300e, 39/e ; FC302, 31/- ! CL.16 & 18, 7/6 each SC.312 21/- : AT.3.10 30/- : pP.117, 9/- ; BT.314, 15/- ; DENCO Chassis Magnaview. 37 6 ; Chassis, Super-visor, 51/6 ; Coilsets. Mcpna view. 41/2 ; WA/DCA1, 43/- ; WA/FCA1. 31/- : WA/LC1 and WOl. 7/3 each ; WA/FMA1, 21/- ; WA/LOT1, 42/- ; WA/FBT1 16'-. Send 6.... 13/6 ea. Post and packing 2/0. CHOKES 20 IT, 250 fi, 60 in A. Clamp construction 6/- ea. 3 OH, 200 fi, !>0 in A. Clamp construction 8/8 ea. ©H, 250 m A, 200 fi. Fully ehroctded.... 18/3 ea. TRANSFORMERS FOR BATTERY CHARGERS 230 v. Input Tapped 6- 12 v. 1 amp 13/8 ea. 230 v. Input Tapped 0- 12 v. 3 amp 18/- ea. Both with tap on Primary for 2.5 v. Pilot light. HEATER TRANSFORMERS 230 y. Input 2 volt .5 amp. 4/6 230 v. Input 2 volt 3.0 amp. 7/8 230 v. Input 4 volt 1.5 amp. 5/- 230 v. Input 4 volt 3.0 amp. 10/- 230 v. Input 5 volt 2.0 amp. 10/- 230 v. Input 6.3 volt .5 amp. 5/- 230 v. Input 6.3 volt 1.5 amp. 6/- 230 v. Input 6.3 volt 3.0 amp. 8/- 230 v. Input 12 volt .75 amp. 5/- SENTERCEL RECTIFIERS KM I, 3/8 ea. ; RM2, 4/2 ea. ; RM3 6/- ea. ; RM4, 16/- ea. METAL RECTIFIERS 12 v. i amp., 1/6 ea. ; 12 v. ] amp. 4/8 ea. ; 2 v. I amp., 3/- ea. ; 250 v. 45 mA., 6/3 ea. ; 250 v. 75 m A., 7/6 ea. ; 300 v. 60 mA., 7/6 ca. GOLDRING PICK-UP HEADS. Pick- up head type. No. 112 (2,000 ohms), complete with lead. Price 17/6 each. TINNED COPPER WIRE— All 4 oz. Keels. Hi S.W.G 2/- 18 S.W.G 2/2 26 S.W.G 2/4 22 S.W.G 2/6 24 S.W.G 2/8 26 S.W.G 2/10 Invite you to build this 4 valve, 2 wave- band, superhet portable for only Oi GNS - Full details, circuit diagram, point to point wiring instruc- tions, and complete list of components. Available 2/6 ea. Case can be supplied separ- ately. Available in the following attrac- tive colours : • Lizard Grey ; • Blue ; • Maroon ; • Brown. All components can be supplied separately. ENAMELLED COPPER WIRE— All 4 oz. Reels. 28 Price S.W.G. 1/11 2/1 2/3 2/5 2/7 2/8 2/11 40 3/1 3/3 3/5 3/7 3/11 4/2 CHASSIS Aluminium Undrilled with Reinforced Corners. Available in t*e following \ sizes. 6in.x4in.x2i in 4/6 ea. 8in.x6in.x2i in 6/3 ea. 10m.x7in.x2£in 7/3 ea . 12m. x8in. x2Jin 8/6 ea. 14in. x 8in. x 2 J in 8/6 ea. J6in. x9Ui. x2Ain 12/- ea. All are four sided— ideal for radio receivers — amplifiers — powerpacks, etc. PERSPEX IMPLOSION GUARDS Incorporating escutcheon and filter. I2in. type, 11/6 ea. ; 16in. tvpe, 14/6 ea. POCKET TEST METER Ex-Govt., volt meter two ranges 0-15 v. ; 0-250 v. D.C. Complete in case, 17/6 ea. IRON ELEMENTS Standard adaptable type, 230 v., 450 w., 1/8 ea. Morphy Richards, replacement type, 3/8 ea. H.M.V. replacement type, 3/- ea. PLIERS, with side cutters, 4/3 pair. NYLON DRIVE CORD 25 yard reel nylon drive cord on wooden reel, 2/8 ea. CRYSTAL DIODES Plastic case, wire ends, 2 for 2/1. BI CONDENSERS Waxed cartons, with flying leads. 4 mfd., 500 v., 1/6 ea; 4x4 mfd., 500 v., 3/4 ea. LOUDSPEAKER UNITS Rola Sin. Speaker with trans- former 16/- ea. Elac 5in 16/8 ea. Plessey 6 i in. lightweight unit 16/6 ea. Rola 6$ in. standard type 17/6 ea. Lectrona 6iin. with trans- former 18/- ea. Truvox 6|in. wafer type 20/- ea. Plessey Sin. lightweight unit 17/6 ea. R. & A. Sin. Standard type... 17/6 ea. Plessey lOin. unit 18/6 ea. Elac lOin 22/6 ea. R. & A. lOin 22/6 ea. Goodmans lOin. unit with transformer 26/6 ea. Elliptical 4in. x7in. unit 18/6 ea. Mains energised Sin. unit, 1,000 fi 21/- ea. Mains energised 6 J in. unit, 600 £2 17/6 ea. BAKELITE CASED BI CONDENSERS 0 mfd., 1,000 v. ; .01 mfd., 4 Kv. ; .001 mfd., 4 Kv. ; .03 mfd., 4 Kv. ; .03 mfd., 500 v. All 1/6 ea. DUBILIER NITROGOL CONDENSER 12 mfd., 350 v., 5/6 ea. OSMOR COIL PACKS Type H.O., 48/- ea. Type LM, 40/- ea. Type T.B., 50/- ea. Type TRF, 40/- ea. SPRAGUE CONDENSERS .05 mfd., 500 v. ; .01 mfd., 1,000 v. ; .1 mfd., 350 v. ; .02 mfd., 750 v. All 8/- doz. TCC VISCONOL CATHODRAY CONDENSERS Type CP57XO, .002 mfd., 18 Kv., 7/6 ea. Type CP55QO, .001 mfd. 6 Kv., 5/- ea. Tvpe CP55TO, 500 pF., 10 Kv., 5/- ea. VIBRATORS, ETC. Vibrator unit for 6 v. operation, 14/6 ea. Post 1/6. 6 v. and 12 v. Vibrators, 4-pin UX types, 6/6 ea. DOUBLE TRIMMERS 250/250 pF ; 100/100 ; 100/50. All 6d. each. YAXLEY SWITCHES 3 pole, 3 way, 1 bank, 1/6 ea. 3 pole, 3 way, 3 bank, 1/6 ea. Octal Plug and Socket (screened), 1/- ea. PRE-SET CONTROLS (CARBON) 50 KB, i Megfi, 1 Megfi, 2 Meg fi, 1/8 ea. GOLDTONE BAKELITE CASED BUZZERS 2/6 ea. PENCIL RECTIFIERS HUNTS CONDENSERS Type W99, 200 pF., 350 v., 7d. ea. Type VViti), .005 mfd., ]50 v., 7d. ea. Type W48, .1. mfd., 400 v., 1/- ea. Type L44, .1 mfd., 500 v., 8d. ea. RESISTORS 2 WATT 2.2 Kfi, 470fi, 7.5 Kfi, 22fi loOfi, 5.6 Kfi, 380 O, 150 KQ i meg.fi, 39 Q, 1.5 meg., 56 K fi. All 4d. ea. ★ **★★★ EX GOVERNMENT AND SURPLUS CONTROLS This popular range is suitable for all Television constructors, etc. Keep your costs down when building the "Argus" or "Simplex" receivers. Available: 500 fi, 600 fi, 1,5001) • double type, 2 Kfi, 5 Kfi, 10 Kfi, 20 Kfi, 25 Kfi, 50 Kfi, 200 Kfi, 100 Kfi, i meg.fi, i meg.fi, 1 meg.fi, 2 meg.fi, 50 Kfi double type. All 1/2 each. COLVERN PRE SET CONTROLS Type CLR 1106/88 1,000 fi ... 1/8 ea. Type CLR 1 126/7S 250 fi ... 1/8 ea. ★ ★★★★★ RESISTORS The following range of Resistors is available in j/-J watt and 1 watt. Prices i/J watt 3d. ea. 1 Matt 6d. ea lOfi 120fi 1.5 Kfi 18 Kfi 1.8KB 2.2 K fi 2.7 K fi 3.3 Kfi 3.9 Kfi 4.7 Kfi 5.6 Kfi 6.8 Kfi 8.2 K fi 10 Kfi Kfi 12 fi 15 fi 18Q 22 fi 33 fi 39 fi 47 fi 56 fi 68 fi 82 fi 100 fi 150 fi 180 fi 220 fi 270 fi 330 fi 390 fi 470 fi 560 fi 680 fi 820 fi 1 Kfi 1.2 Kfi 15 Kfi 22 K fi 27 KO 33 K fi 39 Kfi 47 Kfi 56 K fi ,68 K 82 K fi 100 K n 120 Kfi 150 K fi 1 80 K fi SOLDERING IRONS Solon type 964 fitted oval bit 18/11 ea. Solon type 968 fitted pencil bit 18/11 en. Both types are complete with approx. 6ft. of Henley 3 core cable and are suitable for 230/250 v. SOLDERING PASTE Large tin soldering paste by Tyne Chemical Co., approx. 1 lb 1/8 per tin CONDENSERS The following is a selection of our stocks of manufacturers' suipluB condensers all by well-known makers —DUBILIER, B.I., BEC (EDI- SWAN), SPRAGUE, etc. Aluminium Can Types Clip Fixing: 8x8n K3/25, 5/8; K3/40, 7/6; K3/45, 8/2;K3/50, 8/8; K3/60, 8/8 ; K3/100, 14/8. CONTROL KNOBS in MODERN STYLING Tastefully and clearly engraved in gold. Size A. Diameter 1 gin. Depth fin. Size B. Diameter ljin. Depth |in. These Mouldings are available in two colours : W'alnut and Ivory. They are suitable for use with Jin. spindles and are simply and firmly held by means of a grub screw and locking nut. Prices : Type "A" —1/6 each. Type" B "—1/2 each. Plain Knobs can be supplied in either size or colour : Price 1/- each and 8d. each respectively. Inscriptions available : — RADIO: "Volume," " Vl/On-Off," "Wave-change," " Tuning," " S.M.L. Gram.," " Radio-Gram.," ~ Tone," "On-OfT." TELEVISION: "Contrast," "Brilliance," " Brilliance/On-Off," "Focus," "Brightness." AMPLIFIER : "Treble," "Bass" ("plus any of, those shown above). TAPE RECORDER : " Record-Play." WHEN ORDERING PLEASE QUOTE "DEPT. P.W.' each 4/- 4/- 3/- 3/8 2- 3 - 4 - 3 3 mfd. 450 v 16 mfd. 450 v. . 8 x 24 mfd. 350 v. . 8 x 32 mfd. ¥ib v. . 12 x 4 mfd. 450 v. . 16 mfd. 450 v 16 x 8 mfd. 350 v. . 16 x 16 mfd. 350 v. 16xl6x8mfd.350v. 3/6 20 x 20 mfd. 500 v.... 4/8 24 mfd. 450 v 2/8 24x16 mfd. 350 v. 3/6 32 mfd. 450 v 3/- 32x8 mfd. 350 v.... 3/6 32 x 16 mfd. 350 v. 4/6 32 x 32 mfd. 450 v. 6/11 32 x 32 x 8mf d . 350 v. 5/6 :V2 each 32 mfd. 350 v. 25 mfd. 25 v 5/8 60 mfd. 450 v 2/9 64 mfd. 350 v 2/- Dubilier (B.R. Range) : BR. 850. 8 mfd. 500 v 2/9 BR. 1650. 16 mfd. 500 v 3/3 BR. 2050. 20 mfd. 500 v 3/6 8x8 mfd. 500 v. ... 4/- BR. 501. 50 mfd. 12 v 1/9 16x16 mfd. 500 v. 5/- 16x8 mfd. 500 v.... 4/9 TERMS : Cash with order or C.O.D. Postage and Packing charges extra, as follows : Orders value 10/- add 9d. ; 20/- add I/- ; 40/- add 1/6 ; £5 add 2/- unless otherwise stated. Minimum C.O.D. fee and postage 2/3. MAIL ORDER ONLY 104 PRACTICAL WIRELESS February, 1955 ALFRED PADGETT 40 MEADOW LANE, LEEDS, 11 Established 20 years. 150 ONXY. — Navy Receiver Type M361, coverage 4,500-2.000 kc, 2,250-1,000 kc. 1,080- 460 kc. 510-215 kc, 235-100 kc. Clean condition. Less valves. £1-15-0- Carr. and packing-. 8/-. Best buy we have offered so far. SELENIUM METAL RECTIFIERS. — Small in size, 250 volts at 100 mA. Brand new 6'6, 50/- per doz. Post free. STEP DOWN TRANSFORMERS.— 200- 250 volts, 50 cycles. Sec. windings. All at 2 amps. 30, 24, 20, 18, 15. 14, 12. 10. 9 8 5 4 3 volts. Worth Double. 12 months' guarantee Price 17/-. Post free. SINGLE HEADPHONES. — Low resis- tance. 1/6. Postpaid. MIXED RADIO PANELS. - Full of Resis- tors and Condensers. 4'6 per dozen panels. Post free. Wonderful Value. TYPE 1125 SETS. — Complete with two 8D2 valves. New. 6/6. Post 2/4. LINF CORD. — Point two or point three. Three - or Four-Way. Cut lengths of 1,000 ohms, 3/6, or II- per yard. Post free. CRYSTAL DIODES. — Two for 2/2, 12'- per doz. These are not rejects. DIAL LAMP BULBS. — S. 2 volts at .3 amps. 5/- per doz. Post free. Fully guaranteed. A ' SCOPE ALL BUT MADE.— B.C.929 Tube Unit, complete with 3BP1 tube 2X2 6X5, 6SH7s, 6H6, 6G6. Brand New, £3-12-6 Carriage free. BANKRUPT STOCK of Heavv Chromium Plated U2 Size Focusing torch cases. Price %I< Post 6d. Don't scrap that failing car battery or radio cell Try Renewbat condi- tioner. Works like Simple and safe to nstructions. Car size 3/-, p.p. 6d. ; 'Radio size 1/9. CHAMPION PRODUCTS, 43, Uplands Way, London, N.21. Phone • LAB 4457 | Introducing the : — 1 I 1 Reg. Design A'o.867804 TYANA TRIPLE THREE MAKE SOLDERING A PLEASURE SMALL SOLDERING IRON Complete with detachable BENCH STAND 19/6 The smallest high-power soldering iron. Length only 8}* • adjustable long bit dia. 3/16" ; mains volt- ages 1 00/ NO, 200/220 230/250. The "STANDARD" Popular Soldering Iron now reduced to 14/1 1. Replacement Elements and Bits for both types always available. KENROY LIMITED 152/297 UPPER ST., ISLINGTON, LONDON, N.I. Telephone : Canonbury 4905-4663 RADIO KIT 19/6 Build this hiffh Quality portable radio in 45 mins. Exceptionally sensitive, twin-tri- ode circuit, using unique assembly system, can be built by anyone. Size only 6lin x 5in. x 3in. in handsome black-crackle'steel case with beautiful black and gold dial (stations printed !). Covers all medium and long-waves. Uses one only self-con- tained dry battery, cost less than Id. for 5 hrs. ! Many unsolicited testimonials.— Mr. Xorton, /of Oxtecl, writes : " Yester- day evening on the medium waveband, I counted 32 separate stations ; I am very pleased with the set, which is well worth the money:' Mr. Fraser, of Ipswich, writes : Its performance is almost unbelievable, it gives me stations I've never been able to get on my lo.rge radio.'" Send To-day C heque/* WO COD, 22- (includcs 2/6, post/packing) for Case, Dial, Handle, Plans. Parts Lists, etc. (Cost of Parts to Complete the Radio only 27/6.) Sent by return. {Overseas orders welcomed. ) BRIGHTON RADIO CO. ♦> ♦> ♦> ♦> ♦*•» Fidelia HAND ' M BUILT " | RADIO " ' UNITS ' THE < FIDELIA , Hand built MAJOR IG ■ high quality radiogram chassis at ,♦ economic price, 10 valve, model < V illustrated. £32-8-4, De-luxe 9 valve , ; model £25-5-0 7 valve £21-12-0, ,♦ 8 valve £24-18-4 ^ Technical data sheets free. Electro Acoustic Developments, 2, ♦ Amhurst Road, Telscombe Clifo, Sussex. < ♦♦:♦❖♦:♦♦:♦❖♦:♦♦:♦♦:♦♦:.♦:♦♦:.♦:♦•:♦♦:♦< -Finger Pianists-. Build your own electronic keyboard and play everything ! Send tor free leaflet. Guitar, cello, flute and trumpet are all easy. Write now.?. C & S, 10 Duke St. 7 Darlington, Co. Durham L. J. T. MAIL ORDER SUPPLIES 119, WESTBOURNE GROVE/ LONDON, W.2. VALVES 6X5 EA50 80 6K7 5Z4 6Q7 35Z4 35L6 IT4 IR5 IS5 7/- 1/9 8/6 5/3 8/6 8/9 8/9 8/9 7/6 7/6 7/6 3V4 3S4 IL4 IU5 6V6 6K8 7S7 7B7 7Q7 7H7 7C5 7/6 7/6 7/6 7/6 7/6 8/6 fh »/- »/- 91- 91- 3/- 4/4 2/3 7C6 9/- I2K8 9/- I2K7 9f- EB9I 7/- EF9I 6/- I2BE6 5/6 6AQ5 8/~ 6BW6 8/- 6X4 7/6 35W4 7/6 Mould Seals .05 7.1 d. .04 7d. ELECTRO LYTICS 84-16 450 v. 32+32 450 v. 8.450 200MA 450 v. Resistors. — } watt, l|d. ; \ watt, 2d. ; I watt, 3d. ; minimum, 12 each. 3 Valve Amplifiers. — A.C. only. 4 watts Output, Tone Control, etc. Few onl/ [ suitable all types Pickups), £3 7s. 6d. each. Radio Cabinet Kit. — Complete, size ll^in. x 7in., x 5in., 26/6 each. All goods offered subject to being unsold. Equivalents will be supplied unless otherwise stated. All valves guaranteed. Terms :, Cash with Order or C.O.D. Packing Free. Postage at cost on orders under £5. IESTOSCOPE MoiHsTester^ For high and low voltage test ing :— 1-30 and 100-850 volts A.C. or D.C. Write lor interesting leaflet 29F 16 page manual Chow to make 50 electrical tests) J/- post free. RUNBAKtN • MANCHESTER TAPE RECORDERS P.W. and SOUND MASTER We can supply complete kits or separate components for both the P.W. and Sound Master Tape Recorders. Sound Master Instructions, 6/6. FULL HIRE PURCHASE FACILITIES Only 2/- in the £ deposit. WATTS RADIO, 8, Apple Market, Kingston- on-Thames, Surrey. Phone : KINgston 4099. February, 1955_ PRACTICAL WIRELESS 105 power pack when not excited, the output of the pack will not soar so much as when the PA is cut ott completely in idle periods. This in itself is often enough to prevent chirp on a topband VFO rig. Tuned PA A conventional tuned tank PA is shown in Fig. 3, but a PI network tank circuit may appeal to many. To tank circuit BC RX 3r effects on straight receivers. It will be found in cases of kt second channel " BCI that the frequency of top- band operation is given by adding twice the BC receiver IF to the frequency to which the BC receiver is tuned. These second-channel images tune sharply just like a normal signal, but may, of course, be superimposed on a BBC channel when operating at certain topband frequencies. Thus a BC receiver of 465 IF if tuned to 900 kc/s may receive the image frequency of 900 + 2 x 465 = 1,830 kc/s, if it has poor aerial circuit selectivity. As 1,830 kc/s is a popular topband frequency, trouble may easily arise in this way. The remedy is to supply a wavetrap tuned to the offending topband frequency and fit the trap in the aerial lead of the broadcast receiver. This does not, of course,- solve the problem in the case of receivers with built-in frame aerials, and the simplest solution is to avoid the topband frequencies which give an image frequency that falls on the Home or Light programme frequencies. Generally, however, BCI problems are generally curable on topband, although the closeness to the broadcast band does tend to accentuate some forms of BCI. fig 5 —Traces of TVI may be a lie via ted by a tuned trap in the anode circuit of the PA and oscil- lator stages. A few turns of wire of I'm. diameter are resonated by an air trimmer to the TV channel. f igt 6.— Image type of BCI may be cured by a wave- trap in the broadcast receiver aerial lead. Additional traps may be added if necessary. The traps are tuned to the transmitter frequency causing BCI. For a topband set-up there is little point in making the output capacitor variable, especially if feeding a coaxial line feeding an aerial tuning unit. Therefore, for a rig intended to run at 10 watts input at 300 volts H.T., i.e., say, 33 mA at 300 volts, the constants shown in Fig. 4 will be found quite suitable. A 1,000 pF output capacitor such as a high-grade mica type will be suitable, while the tank coil inductance should be such as to give resonance with a total anode capacitance of around 200 pF. These values should give an optimum operating Q value for the loaded tank circuit. TV Interference It should be noted that in all cases it is assumed that operation will be into an aerial tuning network, and that the rigs described will be link-coupled into such aerial tuning units. In this way harmonic rejection will be obtained and the possibility of TVI lessened. It must be mentioned that while TVI is not such a problem on topband as on the higher bands, it is still a possibility, particularly in weak signal areas on the fringe of TVI service areas. One method of value for suppressing TVI is the use of a tuned TVI trap (Fig. 5) inserted in the PA anode circuit A few turns of wire are resonated with a small air condenser to the TV channel. This pre- caution and the use of a similar trap in the VFO anode may reduce mild TVI to nothing. Where real TVI is experienced, only the full gamut of anti-TVI devices will be effective, but really stringent TVI measures are not always needed on topband. One type of interference occasionally noticed when operating on topband are BCI complaints. These are generally due nowadays to second channel inter- ference on superhet receivers, rather than to swamp Image BCI It will be appreciated that with the usual 465 kc/s IF for broadcast superhets, image BCI may occur when working on any frequency between 1.8 and 2 Mc/s in the topband. This will show up as image interference on signals between 870 kc/s to 1,070 kc/s in the broadcast band. In the London area the region around 1,830 kc/s is one that might cause trouble to BBC listeners. However, in some areas where the BBC programme is received on some other channel, image BCI might be caused when working on some other topband frequency. Differing intermediate frequencies may also cause image signals to appear on frequencies a little outside the range of 870 to 1,070. Thus a 456 IF broadcast receiver might respond to images tuning (apparently) at from 852 kc/s to 1,052 kc/s on the receiver corresponding to actual topband operation in the range 1.8 to 2 Mc/s. Depending on the area of operation and the local BBC frequencies, therefore, it may be necessary to protect two or even three spots on the broadcast receiver corresponding to programme channels. Fortunately, in such cases two or three wavetraps may be inserted in series in the aerial lead of the receiver thus interfered with, and each trap may be tuned to a different spot. By this means even bad or multiple image interference may be cured. With this one point of BCI interference, therefore, topband operation is generally plain sailing, so that a rig for operation on 160 metres adds a further source of enjoyment to amateur activities. PRACTICAL WIRELESS CIRCUITS 16th Edition By F. J. CAMM 10/6, by post 10/1 1 From GEORGE NEWNES, LTD., Tower House, Southampton Street, Strand, W.C.2. 106 PRACTICAL WIRELESS February, 1955 11.— TUNED AMPLIFIERS By R. Hindle (Continued from page 12 January issue) | A Series of Articles Dealing with the Theoretical \ Considerations of Amplifier Design, and Con- j taining at a Later Stage Constructional Details oj Various Types of Amplifier. Mismatch THE tetrode is some- what different with regard to output loading. It will be seen from Fig. 42 that an increase in loading (making the slope of the load line less) will run the valve more into the curved regions at the extremes of the load line and so would give rise to increased distortion but a decrease in load making the line slightly steeper would have less effect on the distortion produced. Generally speaking, however, one has to be more careful to match the tetrode or pentode than the triode. Of course, the same factor of variation in the impedance of a speaker with frequency, as was mentioned previously, applies also to the case of a tetrode and this also tends to be more serious in its results with the tetrode. Converting to Tetrode Working The comparison between triode and tetrode working is easily tried out with the units the construction of which has been described, and it is very likely that the constructor will prefer to leave the tetrode connection in operation for general use in view of the fact that the output is six times as great for sub- stantially the same input. All that has to be done with both versions of the output amplifier is to break the link between pins 5 and 6 of V[ (Fig. 38 and Fig. 41), leaving the output transformer connected to pin 5.' Pin 6 is then connected directly to H.T., conveniently on the electrolytic smoothing capacitor to the tag to which the H.T. end of the output transformer is already connected. The bias resistor, R2, has then to be reduced to 240 ohms either by substitution of one of the correct size or by connecting across the 300-ohm resistor one of 1,000 ohms, the matching r of speaker may need to be altered if the triode was given its correct load of 4,000 ohms but as was explained earlier a tapping giving a load of 5,000 ohms was used for the triode connections as this was the nearest available ratio and in this case no change had to be made for tetrode. When discussing untuned amplifiers it was stated that the frequency of operation of the signal did not alter the basic theory of amplification, and it is true that the resistance-capacitance type of coupling there discussed would work equally well at any frequency if the resistive load at that frequency was of the order mentioned in the design data. The first R.F. amplifiers were, in fact, resistance-coupled triodes, but, unfortunately, the amplification obtained was very much less than was to be expected from the size of load used. The basic reason for this, as a matter of fact, was also dealt with when dealing with audio amplification. The reader will perhaps ^-—Series tunKi circuit. remember that the am- plification of the upper audio frequencies- was limited by the inevitable stray capacitances, and as these were found to be more effective the higher the frequency it is not surprising that they are even more effective at the much higher radio frequencies. These stray capacitances, even with reasonably careful construction, are likely to be about 40 pF, and are effectively in parallel with the load resistor. The load specified for the amplifier (Fig. 14, on page 342 of the June issue) was 100K-Q for the triode case, but the reactance of the strays even on medium wave ranges is only about 4,000*2, and on the short waves will be only a few hundred ohms. The gain on the medium wave range works out then (using the formula (3) on page 233 of the April issue) at only three times. For the pentode case the specified load was 220KX>, and the gain is gm. Req (page 343 of June issue) or about four times on the medium wave range instead of 176 times as was found to apply at the audio frequencies where the effect of the strays was negligible. This is not the whole story, unfortunately, and even the very limited gains quoted above are impracticable for other reasons. The anode load has its effect on the input impedance of the valve. When explaining the Miller Effect it was pointed out that with a resistive load a capacitance is fed back to the input circuit. When the anode load is pre- dominantly capacitive, as in the case now being discussed, the effect is equivalent • to a resistance across the grid circuit. Now if, as would be usual, there were a tuned circuit at the grid the resistance would have the effect of damping it and so reducing its selectivity and lowering the amplitude of signal existing in the tuned circuit so that the gain is still further reduced. In fact, in the average case under these circumstances it is very likely that there will be a reduction instead of an increase in signal. Reasons for R.F. Amplification If R.F. amplification is so hard to achieve why bother about it at all ? Audio amplification, we have seen, is reasonably easy and gains of well over 100 times per stage are practicable, so at first glance the solution would appear to be first to convert the signals into audio by means of a detector and then to amplify. There are many reasons why this is not (Continued on page 109) fmmm^ — 1|- — o Signal 1 Source Signal Source ■-n/WVVWWW- Voltage Amplified Signal Fig. 45. — Use of series circuit amplification. for voltage February, 1955 PRACTICAL WIRELESS 107 MULLARD AMPLIFIER A high qual- ity Ampli- fier designed by Milliard engineers. Robust, high fidelity, with a power out- put exceed- ing 10 watts and a harmonic distortion less than .4% at 10 watts. Its frequency response is extremely wide and level being almost fiat from 10 to 20.000 c.p.s. —three controls are provided and the whole unit is very suitable for use with the Collaro Studio and most other good pick-ups. The price of the unit completely made up and ready to work is £12.10.0 plus 10/- carriage and insur- ance. Alternatively if you wish to make up the unit yourself we shall be glad to supply the components separ- ately. Send for the Mullard Amplifier shoppin? list. ADJUSTABLE THERMOSTAT Ev3 250v. Heavy silver con- tacts can be adjusted to operate between 70 . , fleg.-300 deg. F. These are suitable for Aquarium heaters Electric Blankets, etc., etc. 1 amp. model, 3/6 ; 2 amp. model, 5/6 5 amp. model. 8/6. Post, etc., 6d. extra THE ELPREQ TAPE RECORDER 4ft. long made from heavy gauge sheet steel (galvanised), 1 kW, suitable A.C. or D.C. Price £2, or with thermostat £3.15.0. Note : The thermostat mounts separately and will control up to three heaters. CONNECTING WIRE SNIP P.V.C. insulated 23 s.w.g. copper wire in 100ft. coils, 2/9 each. Colours available : Black, Brown, Red, Orange, Pink, Yel- low, White, Transparent. 4 coils for 10/-. operate, is 35 Gns. 12/6. Hire Pur chase terms if required This instrument com- bines the Mk. IIIU Truvox Tape Deck and the Cleveland Wide Band Amplifier with a special high flux speaker and forms one of the finest tape recorder combinations available today. It will, of course, play pre- recorded tapes as well as make its own record- ings of radio, music, meet- ings, tele- phone conversations, letters, etc.. etc. The price, complete with reel of tape and ready to Carriage and insurance MULTI-METER KIT The Meter trated u r e s volts, m/amps ohms. It has a sensitivity of 200 ohms per volt and is equally suitable for the keen experi- menter, service engineer or student. All the essential parts including 2m. moving coil meter, selected resistors, wire for shunts, 8-point range selector, calibrated scale, stick-on range indica- tor and full instructions for making are available as a kit, price 15/-, plus 9d. post and packing. TERRIFIC NEW CIRCUIT We have evolved a new, T.R.F. circuit which, gives really amazing results — equal in fact to many superhets — you really should make one, up. Mains driven of course. For all the parts including valves (6K7, 6J7, 6F6. 6X5) and bake- lite case with back, we charge only £5. 10. 0., plus 2/6 post and ins. Data is free with parts or available separately at 2/-. -THIS MONTH'S SNIP ATTACHE CASE Complete with leather handle and clasps — suit- able for al- most any portable instru- ment. Limited quantity— price 23/6, carriage and packing. 2/6. A similar model but slightly smaller and in one colour, 19/6, plus 2/6 carriage. lin. MICROMETER Exceptional purchase enables us to offer a lin. precision micrometer at the very low price of 10/-. A micrometer is an essential part of an engin- eer's equipment. You will have found the need for one on many occasions in the past for measuring wire gauge, etc. If you act quickly you can now acquire this precision instrument at the remarkably low price of 10/-, post free THE F.M. FEEDER UNIT SERVICE DATA 100 service sheets, covering British receivers which have been sold in big quantities, and which every service engineer is ultimately bound to meet. The following makers are included : Aerodyne. Alba, Bush Cossor, Ekco, Ever-Ready, Ferguson, Ferranti, G.E.C.. H M V., Kolster Brandes, Lassen, Mc- Michael. Marconi, Mullard Murphy Philco, Philips. Pye, Ultra. Undoubtedly a mine of information invaluable to all who earn their living from radio ser- vicing. Price £1 for the complete folder Our folder No. 2 consists of 100 data sheets covering most of the popular American T.R.F. and superhet receivers " all dry," etc.. which have been impor- ted into this country. Names include Sparton. Emmerson. Admiral. Crossley, R.C.A.. Victor, etc. Each sheet gives circuit diagrams and component values, alignment, procedure, etc., etc. Price lor the folder of 100 sheets is £1. Post free. MAKE A CONVECTOR Almost any metal case can be converted into a useful con vector type if you use our elements. Wound on porcelain at 250 watts, price 2/6 plus 6d. postage. Four or more post free. All the parts necessary to make the Denco F.M. Unit are now available. The unit gives an A.F. output suitable for feeding in at the pick-up sockets of any standard broadcasting receiver and superior results can be expected. The full constructional details as prepared by the Denco technicians are available —price 1/6 post free. Alternatively, they will be given free to those ordering all the parts which come to £6. 7. 6, plus 2/6 post and packing. Note : four valves and everything including a prepared metal chassis is supplied. Approximate chassis measurements are 6 x 6 x 1 J. SNIP FOR CONNOISSEUR Decca crystal pick-up— turnover head— suitable for long-playing or standard records— limited quantity— 29/6, pos t and packing 2/-. ^arminTfor constructors Modern style cabinet in con- trasting ve- neers, with metal chassis, three knobs, coloured scale and pointer. Price 29/6 post, etc., 2/-. All other compo- nents to build 2 -waveband superhet. Price £5. Data. 16 (fre$ with components). BEETHOVEN 5 -VALVE SUPERHET Complete with valves and Rola loudspeaker, ready to work off A.C. mains — three waves (L., M. and S )— large dial, slow motion drive, dust cored coils, etc. £8. 17. 6, or £2 deposit (balance over 12 months), carr. 7/6. Fine walnut veneered and polished cabinet to take the Be ethove n 5 -v a 1 v e Su p er h et with 6Un. loudspeaker, thus making •a really ex- cellent table model— worth £18-£20. Price 49/6, carr. and packing 5/- extra. If bought with the Beethoven chassis, the hire purchase deposit is £3, carr. 10/- ELECTRONIC PRECISION EQUIPMENT, LTD 42-46,Windmill Hill, Ruislip, Middx. Phone: RUISLIP 5780 Half day, Wednesday. Post orders should be addressed to Dept. 7, RUISLIP Personal shoppers, however, can call at : 152-3, Fleet Street, E.C.4. Phone: CENtral 2833 Half day, Saturday. 29, Stroud Green Rd., Finsbury Park, N.4. Phone: ARChway 1049 Half day, Thurdsay. 249»Kilburn High Road, Kilburn. (Now Open) 108 l M VERS AT, i SHUNTS 1% accuracy for any 1 mA. or 500 uA meter. Only one simple adjustment to make, no calibrating meter being required. With instructions. Guaran- ^ d °H,X ear : S505 ( * mA -> covers 1. 5. 25, £K an 1 500 pA- S51 (1 mA.) covers 2, 10, 50, 200, mA and 1 amp. Price 15'-. Shunts for all meters with ranges to your specification. Reasonable prices Please give meter details, ranges required and accuracy. D.C. MULTIMETER KIT for 500 U A meter Complete kit of six 1% High Stability ™i st ° r s. three other resistors. Shunt fc>505, Potentiometer and instructions, 26/-. A.JD./D C. MULTIMETER KIT.-As above. ™ th * 4 - ex £ ra 1/o Higrh sta bility Resistors and Westinghouse Meter Rectifier to give 4 A.C. volts ranges, 45 6. 18-way, Single Pole, Switch, 7/-. Westinghouse Meter Rectifiers, with 1% %?}%8 h % s f ?L fou / A - C - volts ran =es and circuit. For 500 uA or 1 mA meter, 19/6. ^J^TANCE BOX SIANDARDS.- ?^n lv on°o>^ 1 ^ wound Resistors. 1, 2, 2. 5, 10, 20, 20. 50, 100, 200. 200, and 500 6hms giving 1 to 1,110 ohms in 1 ohm steps, 30/-. Sf^^V^T 1 *! ARMS.-Nominal 100 2k ms -n?£ tl0 . 1 t0 1- Ratio accuracy 0.01% 5/6 ; 0.1° o , 4/6 ; 1%, 3/6. i PR o^nnS OIV >, *ESISTORS.-Any value 1 to 1,000 ohms, accuracy 0.5%. Eureka wound on strip, 2/9. ~ CALIBRATION SERVICE FOR RFSIS |ANCE CAPACITY BRIDGES ' and Charged GENERATORS. — Reasonable PRACTICAL WIRELESS CABINETS February, 1955 •///, DL92 ' ^ ™ : 6K8G 8/6|25Z4G 9/-,EL91 1A7GT U/fl 1N50T 11 ' 6 11/6 ANNAKIN'S Great Bargain A HISS FILTER CHOKE which reduces surface noise from badly worn records. Fully screened. Easily installed. A very real improvement. Limited number only At the very special price of only 10/- each, post paid U.K. and Eire. C.W O. only. Money Back Guarantee. Write now for free list of other bargains. 25, ASHFIELD PLACE, OTLEY, YORKS. 1R5 1S4 1S5 1T4 3Q4 3Q5GT 11/6 3S4 7/3 3V4 7/3 5U4G 8/6 5Y3GT 7/9 5Z4G 9/- 6A7 10/6 6AL5 6/9 6AM5 6AM6 6AT6 6BE6 6BH6 6BJ6 6BW6 6BW7 6CH6 6C4 6C9 6F6G 6F12 7/9 7/9 8/6 6/6 6K8CT 9/- 25Z6G 87 6K25 H/6 35L6GT 6LD20 9/6 8/6 6P25 14/6|35Z4GT 6P28 17/ 6Q7GT 9/ 6SN7GT 8/6 6U4GT 6V6G V/e 6V6GT 7/6 6X4 7/6 6X5GT 6/9 8/6 35Z5GT 8/6 50L6CT 8/6 8/6 AZ31 H/6 D77 6/9 DAC32 11/6 „„ DF33 11/6 7/6DH77 8 6 7/6IDK32 11/6 7/6IDK92 7/6 7/6 DL33 11/6 7/6 7H7 7GT. 7C9 7117 7S7 7Y4 8/6 DE35 *9/6 10C1 12/6 EB91 6/9 10F1 10/-EBC33 7/6 10LD11 EBC41 10/- 10/- EBF00 6/6|10P13 10/- 10/6 6/6 10P14 13'6ECC81 9/- 7/6 12AH8 10/- ECH35 9/6 12AT7 9/- 11/9 8/6 12AU6 7/6 ECH42 7/9 12AU7 10/ 8/6l t 10/6ECL80 7/6 12J7GT 10/ 7/9 10/6 EF37A 6F13 13/6 12K7GT 17/. 6F14 13/6 9/-EF39 * 6F15 10/6 12K8GT EF41 £JG 8/6 10/6 EF50 S£5£ T 7/6 12Q7GT EF80 6K7G 6/- 9/6'EF91 6K7GT 6/-I20D1 9/-IEF92 Postage 4d. per valve extra EY51 EZ40 EZ41 7/9 12/- 8/6 9/- FW4/500 12/6 KT63 7/6 MU14 8/6 N78 11/6 Pen45 15/- PL8.1 11'6 PL82 PL83 PY80 PY81 PY82 PZ30 6/6 9/- 7/6 10/- 79 5/6 T41 U14 U22 U24 U25 U50 U78 U404 U301 12/6 9/6 10/- 7/9 17/- 15/- 8/6 7/6 17/6 12/- 7/9 8/6 9/- 14/6 UBC41 9/- UBF80 10/6 UCH42 ii/6 UF41 9/- UF42 13/6 UL41 11/6 UU8 16/- UY41 9/- VP4B 12/6 W77 5/6 X78 13/6 Y63 7/6 Z77 7/9 ST A. 4587. READERS RADIO gjSBftSSj^ Xtal SPOT FREQUENCY SiG. GEM KIT TELETRON SUPER INDUCTOR COILS Originators of Type HAX. selectiv diode coil, as Radio unit in Tape and quality Amplfrs., 3/- ea. Dual wave TRF coils (illus- trated), matched pair. 7/-. FEU RITE ROD Aerials. MW- 8/9 ; Dual wave, 12/9. E.C.O. Coils, IFT's.etc. Stamp for list and circuits. The Telet Co.. Ltd., 266. Nightin- gale Rd„ London, N.9. | HOW. 2527 Fully screened in steel case. 4in. x 4in. x 3in. Six switched Freqs., 3 Med., 3 Long Wave. No external batteries. Easily built, 35/-. OTHER KITS. 465 Ke;<= I.F. Aligner (Similar to Sig. Gen.) 445 to 485 Kc/s, 17/6. Six Range Res/ Cap Bridge, 10 Ohms to 5 meg. & 500pf to 50 mfd., 31/6. Audio Bridge, 50 to 16,000 cycles, 38/6; Inductance Bridge, 42/6. Multi Ohmer, 25/-. Full Instructions' P. & P. 1/6. Stamp for lists. RADIO MAIL, 4& 6, Raleigh Street, Nottingham. THE POCKET LOUDSPEAKER SET Using our clear drawings and instruc- tions you can build a midget loud- speaker receiver small enough to fit in the jacket pocket. This receiver is lully self-contained—no aerial, earth £hp ™Sl??^ er ^ upp]y beingr required.* 1 he midget batteries used are of the new layer type specially produced by a famous British maker for this class of receiver. Only one single-gang midget variable condenser is used for tuning nft£ t ^H eCe . Iver re£ l ulr es no alignment! The midget components chosen are current British production, not surplus or ioreign. Instructions and Drawings, including Theoretical Circuit and Point-to-Point Wiring Diagram. PRICE 3/6 POST FREE This offer applies only f o Ot. ltrif ain, Irish Republic and Northern Ireland. SWIFT RADIO 102, BATH RD., WILLSBRIOGE, Nr. BRISTOL Orders by post only. February, 1955 PRACTICAL WIRELESS 109 a practicable solution. First, a perfect detector cannot be set up. A detector is basically a non- linear conductor and the voltage at which the resistance changes from a high value to a low one is not sharply defined. The characteristic shows a gradual curve, and over this range distortion is produced. Now a small input signal will never move off this curve and so the audio will be distorted before it reaches the audio amplifier, and nothing done there will restore it. For satisfactory results it is essential that the signal fed to the detector should be large, so that the detector is swung well over the curve and into the straight part of the character- istic. Not many people are so favourably placed that all the signals that they wish to listen to are so strong that they will provide a detector directly with a sufficiently large input, and so some form of R.F. amplification is inevitable. Secondly, a high degree of selectivity is required, particularly for modern conditions. This is best achieved for ordinary purposes by means of a number of tuned circuits. Other methods that may provide adequate selectivity with perhaps only one tuned circuit invariably limit the bandwidth to which the equipment will respond (as will be seen later). All the tuned circuits could be put before the detector, but gain will suffer even further and also the detector circuit itself has the effect of damping the tuned circuits at its grid and so preventing the selectivity that is being sought. There are detector circuits that aim at avoiding this damping, but they have their own problems. We have then to face the problem of R.F. amplifica- tion, and there is, in fact, a very convenient way of preventing the ill effects of stray capacitance ; that is to cancel it out with a reactance of the opposite kind. Now inductive reactance is opposite in kind to capacitive reactance, and if an inductance of reactance equal to that of the strays is added to those stray capacitive reactances there will be no residual reactance and we are left only with the resistance of the circuit. This is, of course, what tuning means. The apparent snag is that a capaci- tance and an inductance can have equal reactances only at one frequency j above that frequency capaci- tive reactance decreases and inductive reactance increases. This snag can be changed into a virtue by using an inductance smaller than would be required to cancel out the capacitive reactance, and then adding more capacitance in the form of a variable capacitor. The capacitor can then be adjusted to cancel the inductance at the frequency required at the time ; in other words, the circuit can be tuned so that it will select the wanted transmission and reject those not wanted. The strays now become part of the tuning capacitance, and can have no ill effects apart from reducing the amount of capacitance required of the variable component for a given frequency. of a capacitor is — ' tbe minUS sign in thiS CaSC being used quite arbitrarily to indicate that capacitive reactance is exactly opposite in character to inductive reactance. The circuit has a resonant frequency, being the frequency at which the two reactances are equal numerically. The resonant frequency can thus be easily found. It is the frequency at which InfL =- 27ifC i.e., 47i 2 f 2 LC = 1 f 2 = 1 and the resonant frequency f 2 = 4jz 2 LC 1 Tuned Circuits It is necessary now to give some consideration to the nature of circuits combining inductance and capacitance. A circuit consisting of both inductance and capacitance is a tuned circuit ; it inevitably has resistance as well for neither the metal plates of the capacitor nor the metal wire used to wind the inductance can have zero resistance. The currents flowing in the tuned circuit have to pass via the resistive paths, and so there is a power loss. The reactance of an inductance is 2jifL and the reactance Series Resonance Now if the inductance and capacitance are con- sidered as being in series as in Fig. 44 the reactances, being equal and opposite, will cancel out and a zero reactance is presented by the two together. There is still the resistance of the components, however, and this is shown in Fig. 44 as a separate component so that it will not be forgotten. This resistance, then, is the only opposition to the flow of current and the magnitude of the current is determined entirely by the resistance and not at all by the inductance or the capacitance. This current has to flow through the capacitance, however, and sl voltage is produced across the component according to Ohm's Law ; similarly, a voltage is produced across the inductance and as the current is the same in each case and as the reactances are equal the voltages generated across each component will be the same (so far as the resonant frequency is concerned and for the moment conveniently forgetting the resistance that is, in practice, integral with the components). As the reactances are of opposite sign, however, the voltages across the individual components are of opposite polarity and will cancel each other out, just as two batteries of equal voltage, if connected in series incorrectly by connecting like poles (e.g., positive to positive instead of positive to negative), will cance each other out and give zero voltage. So the total voltage applied by the source in Fig. 44 appears across the resistance (for the time being considered as a separate component) and satisfies in the light of Ohm's Law the original stipulation that the resistance was the only effective opposition to the flow of current. By restricting the resistance in the series tuned circuit there is no reason why quite a large current should not pass through the circuit even with a quite modest applied signal voltage ; if the perfect com- ponents without resistance could be made the current would be infinity with the smallest voltage ! In practice the resistance can be made so low that the resulting current flowing produces across the induc- tance (and also across the capacitance, each com- ponent considered separately) a voltage very much higher than that originally applied from the source and thus a voltage amplification can be obtained in the tuned circuit alone, even without any valve or other device connected to it. The degree of magni- fication obtained in this way depends on the ratio of reactance of either component to the resistance of the circuit. This ratio measures the " goodness of the circuit and is called the circuit " Q." Be careful to note, however, that to make use of the 110 PRACTICAL WIRELESS amplification of a series tuned circuit the output must be taken across only one component of the tuned circuit (either inductance or capacitance) as shown in Fig. 45. Parallel Tuned Circuit A series tuned circuit obviously has a low impe- February, 1955 Resonance Fig. 46 (Top). — A parallel-timed circuit. Fig. 47 (Bottom).— Resonant curve. Parallel impedance plotted against Qx ^iM^SSS^ Resonant frequency' dance, as the resistance has been reduced to a mini- mum to give the best Q, and the two main reactive components cancel out, whereas we were searching ECONOMY QUALITY RECEIVER (Continued from page 82) far this value must be reduced. It is suggested that resistors of 6,800, 4,700 and 3,900 be tried experi- mentally if a reduction is considered necessary. Another component which may need alteration is /R14, which provides the negative feedback path Naturally, the lower the value of this component the greater will be the feedback— and the lower the overall gain. Raising the value of R14 will increase the gain but reduce the amount of feedback. Again it is a matter of compromise ; suggested values for R 14 are 250,000 ohms, 500,000 ohms, 750,000 ohms and one megohm. If possible, use the lowest value mentioned to obtain maximum feedback. During alignment it may be found that the receiver tends to develop self-oscillation as the I.F. trans- formers are brought into alignment. This is not an uncommon occurrence in superhet receivers and is generally due to long, straggling leads, poor solder connections, incorrect operating conditions or a poor physical layout and screening. - Or, of course any combination or permutation of these faults' Quite often this trouble can be cured by wiring in stopper resistors (of around 220 ohms) direct to the signal or screen grids of the I.F. amplifier. Similarly, inadequate screening of vital leads can produce the trouble through inductive coupling However, in the case of a reflexed amplifier all the usual precautions may "still be inadequate. Fig. 1 shows that the amplifier has individual anode circuit decoupling (R10, CI 1). These components may not for a large impedance with which to load an ampli- fier, so the search has to be continued elsewhere In the series tuned case a voltage from the signal source was seen to drive a common current through each component and it was found to produce large voltages across each component. In the parallel tuned case (Fig. 46) the signal voltage is common to (i.e., across) both inductance and capacitance and the current divides between the parallel arms The magnitude of current flowing in each arm is deter- mined, again using Ohm's Law, by dividing the voltage by the reactance of each arm, but as at resonance the two reactances are numerically equal the current flowing in the two arms of the parallel circuit will be equal The reactances are of opposite sign, however I and therefore the currents will be flowing in opposite directions and so will cancel out. If the circuit could be quite without resistance, then no matter how large the signal voltage applied no current would flow into the parallel circuit (except for the initial flow to charge the capacitor) because the current at a given instant in the inductance is supplied by the current flowing in the capacitor at the same instant but in the opposite direction, and none is required from outside. Inevitably, resistance is again present and some current is taken, in fact but only to replace that taken by the resistance ' and transformed into heat. Clearly again, the lower the resistance the less power will be taken from the signal source for a given voltage, and so the higher the Q of the circuit the higher is the apparent im- pedance of the whole circuit ; it can be very much higher than the reactance of one arm of the circuit 1 his is the circuit of use for the present purpose a circuit with a high impedance that can be used as a load for a valve. (To be continued) be necessary and,- indeed, tne receiver may first be tried without them. But if there is any tendency towards oscillation, the filter may help considerably to eliminate the condition. If I.F. instability of a stubborn nature persists check that all earth contacts are sound, that valve screening is satisfactory (earth pin 1 of the 6B8 it a metal type is used. and take all earth returns to this point) and that all other factors previously mentioned are in order. Should the oscillation still p ? r ™ !* ™y be necessary to shunt the secondary otIM-1 with a damping resistor the value of which must be chosen by experiment and may be anything from about 220,000 ohms down to 39,000 ohms The u rge , S i P° ss,b,e va,ue wn »ch will stop the oscillation should be used if selectivity and gain are not to be seriously impaired. The fitting of such a damping resistor should be looked upon as a last resort when it becomes obvious that nothing short of a complete rebuild is likely to cure the trouble ! Another system is to neutralise the amplifier by connecting a very small capacitance (of only a few pF) between the anode and grid. Lest prospective constructors feel that the building of the receiver appears to be beset with troubles perhaps it is advisable to state that I.F. instability is likely to occur in any home-constructed superhet and the above notes should be taken in a general way* applicable to this and most other superhets For there should be no troubles of this nature with the receiver, but a combination of bad layout and wiring could result in instability as it can in other receivers February, 1955 PRACTICAL WIRELESS 111 Volume Controls Midget Ediswan type. Long spindle. Gkutam- teed 1 year. All nine* 10,000 ohms to 2 Meg- ohm ohm CABLE COAX STANDARD Jin. diam. Polythene insulated. GRADE "A" ONLY 8d. yd. SPECIAL. — Semi-air spaced polythene, 80 ohm Coax iin. diam. Stranded core. Losses cut 50% Qd. 3 yd. (just released) No Sw. S.P.Sw. D.P.Sw. 3/- 4/- 4/9 COAX PLUGS ... 1/2 SOCKETS 1/- LINE CONNECTOR 1/2 OUTLET BOXES ... 4/6 BALANCED TWIN FEEDER per yd. 6d. \ 80 TWIN SCREENED FEEDER per yd. 1/- / ohms 50 OHM COAX CABLE, 8d. per yd. lin. dia. TRIMMERS, Ceramic, SO, 70 pf., 9d. 100 pf., 150 pf., 1/3 ; '250 pf., 1/6 ; 600 pf., 1/9. RESISTORS.— All values : 10 ohms to 10 meg., i w., 4d. ; >■ w., 6d. ; 1 w., 8d. ; '2 w., 1/- ; High SUbility, i w. 1%, 2/-. Preferred values 100 ohms to 10 Meg. WIRE-WOUND RESISTORS. — Best Makes Minia- ture Ceramic Type— 5 w., 15 ohm to 4 K., 1/9 ; 10 w., -20 ohm to (5 K., 2/3 : 15 w., .SO ohm to 10 K., 2/9 ; 5 w. Vitreous, 12 K. to '25 K., 3/-. WIRE-WOUND POTS. 3 WATT. FAMOUS MAKE e-Set Min. T.V. Type. Knurled Slotted Krmb. All values 2S ohms to :t0 K,, 3/- ea. 50 K., 4/-. Ditto Carbon Track ,50. K. to -2 Meg., 3/- Standard Size Pots, 21in. Spindle. High Grade. All Values. 100 ohms to 50 K., 5/6 : 100 K., 6/6. W/W EXT. SPEAKER CONTROL 10 Q, 3/-. O/P TRANSFORMERS.— Heavy duty 70 ma., 4/6. Small Tapped pentode, 3/9. L.F. CHOKES 10 h. 05 ma., 5/-. 20/25 h. 100/150 ma,, 12/6. 5 h, 250 ma., 15/-. 15 h. 100 ma., 10/6. LYNX, 3h. 250 ma., 13/6. SIMPLEX, J Oh. 150 ma., 10/6. MAINS TRANS. -Made in our own workshops to high grade specification. Fully inter-leaved and impregnated. Tapped prim. 200 V./250 v., Heater Trans., 0.3 v., li amp., 7/6 ; ditto 6.3 v., 3 amp., 10/8 ; ditto, 12 v., .75 amp., 7/6 ; 350-0-:<50. 80 ma., 6.3 v., 4 a., 5 v., 2 a., ditto :<00-0-3O<>. ditto •250-O--25O, 21/-. AMPLIFIER TRANS. 250 v. 50 ma. i wave, 6.3 v. 2 a. Fully Shrouded, 17/6. View- master Auto Type, 35/-. Teleking, 30/-. Lynx. 30/-. Coronet, 30/-. Simplex, 35/-. Rewinds and Specials to requirements. SOUNDMASTER SPECIALS. — Maine Trans., 35/-. L.F. Choke, 10/6. O/P Trans., 5/6. Envelope, 6/6. Specified Wafer Switches, 22/6 per set of 3. TAPE RECORDING BARGAIN LIGHTWEIGHT XTAL HAND MIKES. Chrome finish— Quality and sensitivity for only 25/-. ELECTRODYNAMIC MIKE INSERT.— U.S.A. make, precision engineered. Size only 1 in. diam. by Jin. Bargain Price 3/9. Matching Trans. 3/9. WOODEN WALNUT CABINET-— 12in. x 7in. X 5in. TRF or superhet., comp punched chassis, dial, hack-plate, drive, pointer, etc., 28/6, plus post 2/-. TYANA.— Midget Soldering Iron. '200/250 v. or 280/*250 v., 14/11. TYANA TRIPLE THREE.— Complete with detachable bench stand, 19/6. NEW SOLON MIDGET IRON. - 25 w., 19/6. 10HA L FOR RADIO CONSTRUCTORS. C.R.T. HEATER ISOLATION TRANSFORMER.— Low leakage winding with 25% sec. boost. Ratio 1 • J '25, 2 v., 10/6 : 4 v., 10/6 ; 6,3 v., 10/6 ; 12 v., 10/6. MAINS PRIMARIES and Specials to Order, from 10/6 each. MIKE TRANSF.— Ratio 50:1, 3/9 ea., new and bo: ■d. ALL WAVE RADIOGRAM CHASSIS THREE WAVEBANDS FIVE VALVES S.W. 10 m.— 50 m. LATEST OSR AM M.W. 200 m.— 550 m. X79, W77, DH77, L.W. 800 m.— 2,000 in. N78, U78. Brand New and Guaranteed, with lOin. P.M. Speaker, A.C. 200/250 v. Four position Wave- change Switch. Short-Medium-Long-Cram. Slow Motion Tuning. Speaker and Pick-up connections. High Q iron-dust cored coils, 465 kc/s I.F. Latest circuit technique delayed. A.V.C. and Negative feedback. Output 4.2 watts. .3 ohms output trans- former on chassis. Chassis size 134 x 54 x 2 \ in., Glass Dial — 10in. x 4£in., horizontal or vertical type available, lit by 2 Pilot Lamps. Colour Black Sta- tion names, L.W. Green, M.W. Red, S.W. White. Four Knobs supplied. Walnut or Ivory to choice, aligned and calibrated. Chassis isolated from mains. PRICE £10/15/0. Carriage and Insurance, 4/0. (Without lOin. Speaker, £9/15/0. Carr. & Ins., 4/6.) BARGAIN OFFERS RECOMMENDED FOR ABOVE CHASSIS GREAT REDUCTIONS Brand New Plessey 3-speed Autochanger Mixer Unit lor 7, 10 and 12in. Records. Twin Hi-Fi Xtal Head with Duopoint sapphire stylus. Plays 4,000 records. Sprung mounting. Superb Quality. Bargain Price 91 gns., post free. NEW BOXED 1R5 185 1S4 1R5 3S4 3V4 3D6 5U4 5Z4 6X4 6AC7 6 AG 5 6AM6 6AT0 6B8 0B AO 0BE6 6BW6 6C4 6F6 VALVES 8/-6K7G 6/6 V'HOLDERS. — Pax : Tnt. Oct., 4d. ; EF50, BA50, 6d. ; BI.2A CRT, 1/3. Moulded : Tnt. Oct. 6d ; K7G, 9d. ; with screening can, 1/6 ; B8A, BH<; B!)A, 1/-; VCR07, 2/6. Ceramic: EF50, B7c! 1/-. ENG. and AMER. 5-, 7- and fl-pin, etc., 1/-. TAG STRIPS.— 2- or 3-way, 2d. ; .4- or 5- way, 3d. ; 6-way, 4d. ; 9- or 10-way, 6d., etc. T/V PRE-AMP. — Channel 1. Easily modified for other Channels or Converter use. Midget Chassis, 4'i x 2i x li. Complete with EF4*2 valve, coax, lead and plug. Ready for use. Brand New Mfrs. Surplus. Listed £3 15s. Special Clearance Price, 27/6 ; p. & p., 1/6. TOGGLE SWITCHES EX-GOVT.— " On-off," 9d. Krsin M'eore solder 00/40. 16 g. or 18 g., 5/6 4 lb., 4d. yd. T.C. wire, 18 to 2'2 s.w.g., per yd., 2d. PVC (Connecting wire, 10 colours. Single or Stranded, 2d. yd. '2 K. 5w. H.D. w/w Pots, 6/6. 10 K., '25 K., Colvern w/w Pot. Iin. spindle, 3/6. SCREENED GRID CAPS I. Oct., or Mazda, 6d. ea. BULGIN HIGH VOLTAGE VALVE CAPS, I. Oct., 1/-. FUSES.— 1 iin. all values 60 ma. to 10 a., 6d. ALADDIN FORMERS and cores, lin., 8d. ; I in., lOd. SLOW MOTION DRIVES, lipicyclic ratio 4 : 1, 2/3. INT. OCTAL CABLE PLUG (8-pin), w ith cover, 1/3. 200-250 Volt SELECTOR SOCKET (2in. x lin.) with Plug, 1/-. PDLOT LAMPS.-6.3 v. .3a., 8d. SPEAKER FRET. -Expanded anodised metal, 14 in. by 9in., 3/-.. EXT. L.S.— Switched Socket, on-off and parallel switching, complete with plug, 2/-. MAINS DROPPERS. 3in. x llin. Adj. Sliders. .3 amp, 750 ohms, 4/9. .2 amp, 1,000 ohms, 4/3- LINE CORD. .3 amp, 60 ohms per foot, 12 amp, 103 ohms per foot, 2 way, 1/8 a yard ; 3-way, 1/9 a yard. 6K8 9/- VT127 " Pen46 BF80 6L6 6P25 6Q7 6SA7 0SH7 0SK7 6SL7 0SN7 liU.KV 6V6 6X5 0AK5 7S7 9D2 12 A6 }*- 10/6 10/6 15/- 9/6 8/6 6/6 8/6 91- 11 ;:;«8, 8/ iO/6 9/6 5/6 7 6 ALL GUARANTEED 12SH7 5/6|EC91 7/6 35 W 10/6 OZ4 8/6 807 10/6 ECH42 12/6 6K6 609 6BG6 UF41 EF41 8D3 CV6 DK91 DF91 DAF91 DL92 DL94 E1148 EA50 6AL5 EB34 EF55 EB91 KRC:;:: EF36 7/6 EF39 91- EF50 Equip 5/6 British 7/6 Sylvania Red 10/6 11/6JEF91 8/- 11/6 EF92 9/6 9/- 7C6 9/6 3/6EY51 12/6 HVR2a 7/6 PY80 11/6 PY82 10/6 PEN25 8/6 SP61 8/6 U22 9/6 U25 12/6 MU14 9/6 ECL80 12/6 I2AH8 10/6 PL81 12/6 12AX7 10/6 12AT7 10/6 6J7 8/6 12BA6 6K7M 8/6 12Q7 10/6 EBC33 9/6 180 9/- Huge Stock B.V.A. Valves at 1951 low tax prices. SPECIAL PRICE PER SET I R5, 1T4, IS5 and 3S4 or 3V4 30/- 6K8, 6K7, 6Q7, 6V6, 5Z4 or 6X5 37/6 SUB MINIATURE VALVES WIRE ENDS + R.F. Pent. .625 v. Fil. • L.F. Pent. 1.25 v. Fil. Brand New. Ex Deaf Aid Apparatus, by Mullard, Hivac and American. Types * XFW 10 • XFY11 4, XFW .>o • XFY32 all at * x*w -o h * 507 AX • DL72 7/0 -fc DF66 « 503AX post free * DF70 • 505AX VCR97 £2 TESTED FULL PICTURE P. & P. 21- PYE Aerial Plug and Socket, 1/6 pr. 5in. RADIO SCREWDRIVERS. — Sheffield made blade, 2£in. x iin. Ins. handle, 5JB00 v., 4id. each. CONDENSERS. — New Stock .001 mfd. 6 kV. T.C.C., 5/6. Ditto, 12.5 kV., 9/6 ; 2 pf. to 500 pf. Mica, 6d. ; .001, Mica or Tub. T.C.C. 500 v., .01 Sprague 500 v., .02 N.S.F. 500 v., .1 mfd. 350 v. tfiemmould Tub., 9d. ; Hunts Moldseal 500 v. .005, .01 mfd., 9d. ; .05 mfd. and .1 mfd., I/-. .25 mfd., 1/6 ; .1 mfd., 600 v., 1/3 ; Tubular .5 mfd. 350 v., 1/9. SILVER MICA CONDENSERS.— 10%. 5 pf. to 500 pf., 1/-. 600 pf. to 3,000 pf., 1/3. DITTO 1% (ex stock). 1.5 pf. to 500 pf., 1/9. 515 pf. to 1,000 pf., 2/-. ELECTROLYTICS ALL TYPES NEW STOCK. Tubular Wire ends 8 + 16/500 v. Dubilier 5/6 16 + 16/500 v. Dub. 6/- 32 + 32/500 v. Dub. 1/275 v. B.E.C. 2/450 v. B.E.C. 4/350 v. Dub. 4/500 v. Htmts 8/450 v. B.E.C. 8/450 v. T.C.C. 8/500 v. Dubilier 10/500 v. Dubilier 16/500 v. Dubilier 8 -\ 8/500 v. Dubilier 4/6 10/350 v. B.E.C. 3/- 32/350 v. Dubilier 4/- 32/500 v. Dubilier 5/- 32 + 32/350 v. Dub. 5/6 32 + 32/500 v. Dab. 7/6 25/25 v. Dubilier 1/9 SPECIALS.— Can Types. 7/6 21- 2/3 1/6 2/- 2/3 2/9 2/9 2/6 4/- 50/25 v. Plessey 1/9 50/50 v. Plessey 2/- Can Types, Clips, 3d. ea. 32 + 32/350 v. B.E.C. 4/6, 32 + 32/'275 v.-B.E.C. 4/6 16/450 v. T.C.C. 3/8 32/350 v. Dubilier 4/- 60/350 v. T.C.C. 6/6 250/350 v. B.E.C. 8/6 8 + 16/450 v. B.E.C. 5/- 8 + 16/500 v. Dubilier 5/6 16 + 16/45(1 v. B.E.C. 5/6 16+16/500 v. Dub. 9h 16 + 32/350 v. T.C.C. 4/6 32 + 32/450 v. B.E.C. 6/6 32 + 32/350 v. + 25/25 v. in same can B.E.C. 6/6 60 + 100/350v.Huntsll/6 100+200/275 v. B.E.C 12/6 500 mfd. 12 v., 3/-; 1,000 mfd. + 1,000 mfd., 6 v., 6/6 ; 6,000 mfd. 6 v., 5/6 ; 1,500 mfd. 6 v., 4/6 ; .1 mfd. 1.5 kV., 3/6 ; SENTERCEL RECTIFIERS. E.H.T. TYPE FLY- BACK VOLTAGES. - K 3/25 2 kV., 4/3 ; K3/40 3.2 kV., 6/- ; K3/45, 3.6 kV., 6/6 ; K3/50 4 kV., 7/3 ; K3/I00 8 kV., 12/6 ; K3/160 14 kV., IS/-. MAINS TYPE. — RMJ, 125 v., 60 ma., 4/- ; RM2, 100 ma., 4/9 ; RM3, 120 ma., 5/9 ; RM4 250 v. 275 ma,, 16/-. KNOBS, GOLD ENGRAVED. — Walnut or Ivory, 1J in. diam., 1/8 each. "Focus," "Contrast," "Brilliant," " Brilliance— On-Off," " On-Off," "Volume," " Vol.— On-Off," "Tone," "Tuning," " Treble," " Bass," " Wavechange," " Radio- Gram," " S., M., L., Gram.," " Record-Play," " Brightness," Ditto not engraved, 1/- each. POINTER KNOBS. — Brown with white marking line, small, 9d., large, 1/-. CODLS.— Wearite " P " type, 2/6 each. Osmor " Q " Type, adj. dust core, 3/6 each. All ranges. REACTION COND.— .0001, .0003, .0005 mfd., 3/6 ea. BANKRUPT STOCK PEN TORCHES.— Dry dex, chromium plated with vest pocket clip. Complete. New battery and bulb. Listed 3/4. Our Price 2/6. HANDILITE. — Drydex, pocket or handbag flashlight. c plete. Battery and lens bulb. Listed 3/5. Our Price 2/6. ALUMINIUM CHASSIS.— 18 s.w.g. Flain, undrilled, folded 4 sides and riveted corners lattice fixing holes. Strong and soundly con- structed with 21in. sides. 7in. x 4in., 4/6 ; llin. x Tin., 6/9 ; 13in. x 9in., 8/6 ; 14in. x llin., 10/6 ; and 18in. x 16in. x 3in., 16/6. T;R.S. CRYSTAL DIODE.— Very sensitive. G.E.C., 3/6. H.R. PHONES.— (Hi-grade Amer.), 15/6 pr. S. G. BROWN'S, 4,000 ohms, 15/6 pr. RADIO COMPONENT SPECIALISTS THO 1685. Buses 68 and 133 pass door. 307, WHITEHORSE RD., WEST S^rday CROYDON, SURREY. JP. Jb P., 2/6. Suitable Battery Portable Sets. LOUDSPEAKERS P.M.. 3 OHM. 3in. Plessey, 12/6. Goodmans 4in. square, 15/8, 5in. 14/6, 6£in., 16/-. 7in. Elliptical, 18/6. 8in. R. & A. 17/6. lOin. Plessey, 25/-. 6 \ in. with trans., 7,000 ohms to 3, 19/6. I.F. TRANSFORMERS 485 Kc/s Sing tuning Miniature Circular Can, 2iin. by llin. diam. Fits octal V-holder cut out. High Q and good bandwidth. By Pye Radio. Two mounting feet. BRAND NEW, 6/9 PAIR 112 PRACTICAL WIRELESS February, 1955 FIRST-CLASS RADIO COURSES . . GET A CERTIFICATE! QUALIFY AT HOME— IN SPARE TIME After brief, intensely interesting study — undertaken at home in your spare time — YOU can secure your pro- fessional qualification. Prepare for YOUR share in the post-war boom in Radio. Let us show you how ! | FREE GUIDE j ■ The New Free Guide contains 132 j I pages of information of the greatest | I importance to those seeking such i j success-compelling qualifications as ■ I A.M.Brit.l.R.E., City and Guilds I 1 Final Radio, P.M.G. Radio i j Amateurs, Exams., Gen. Cert. I I of Educ, London B.Sc. (Eng.), I I A. M.I. P. E., A.M.I.Mech.E., Draughtsmanship (all branches), I etc., together with particulars of I our remarkable Guarantee of SUCCESS OR NO FEE ! I I' Write now ' for your copy oj invaluable publication. It may well I prove to be the turning point in your > career. „ I FOUNDED 1885— OVER | } I50,000jSUCCESSES ~ — J NATIONALS INSTITUTE OF ENGINEERING (Dept. 461), 148, HOLBORN, LONDON, E.C.I 'Bi ENSON' S ETTER ARGAINS BRAND NEW. ORIGINAL CARTONS. R.F. UNITS. TYPES 26 or 27, 27/6, 24, 15/- (Postage 2/6). CONDENSERS, Variable min. spindled, 15, 25, 50, 75 pfs., 1/3. DYNA- MOTORS, D.C. (approx. 250 v. 80 mA, at 6 v.), 8/6. Filters for these, 2/6. I.F.T's, new, canned, 10/13 Mc/s., 1/6. POWER UNIT 285. 230 v. 50 c. input. Outputs D.C. 2 kV 5 mA, 350 v. 150 mA. A.C. 6.3 v. 15 a. 3 valves. New 75/-, carr. paid in- land. TRANSFORMERS, new, std. mains input ; 250-0-250 v. 70 mA, tapped 200 v. , 6.3 v. 3 a. , 4 v. 2 a. , 10/6 ; 600 v. H. W. 6.3 v. 3 a., 4 v. 2 a.. 10/6 ; 230 v. to 6.3 v. 5 a. and 10 a., 17/6 ; 2 kV 5 mA, 2 v. 2 a., 22/6 ; 350-0-350 v. 150 mA, 5 v. 3 a., 25/- ; 55 v. 30 mA (twice). 6.3 v. 3.2 a. , 9/- ; 250 v. H. W. , 30 mA, 6.3 v. 1.5 a., 8/6 ; 190 v. 8 mA. 6.3 v. 1.5 a., 8/6 ; 740-0-740 v. 165 vA, 470- 0-470 v. 220 vA, 4 v. 8 a., C.T. (twice), 30/- (carr. 5/-). 350-0-350 v. 120 mA, 6.3 v. 4 a, 4 v. 2 a, 16/- (postage 2'- each). POTENTIOMETERS, carbon, 10/6 doz. asstd, R1115 Coilpacks, new, 12/6 ; used, 9/6 ; twin-knob S.M." drives, 7/6 ; LP. Filters, 2/6. METAL RECS., H.W., 270 v. 80 mA, 6/- ; 240 v. 30 mA, 4/- ; 480 v. 80 mA, 6/6 ; 600 v. 33 mA, 6/- ; 200 v. 150 mA, 9/- ; 12 v. i a., C.T., 1/6. RECORD PLAYERS, brand new, xtal. P.U. (fam- ous maker), 33J/78 r.p.m., 79/6. CRY- STALS, 100 kc/s 2-pin, English, 15/-. RELAYS, Siemens H.S., lk + lk coils, 10/6. HEADPHONES, new 3 k, 11/6 ; L.F. CHOKES, 15 H 200 mA, 8/6 ; 9 H 100 mA, 7/6 ; 8 H 80 mA, 4/6. VALVE B A RG A INS. — at 2/- : VR65A, VR66. 7193, VR54, 4D1, 991, VR78, VR92, 8D2, RK34, 954, CV54 ; at 4/- : EF50, EF54, EC52, 6AG5. 6C4, 9001, VR116, KTZ41. 2C26, MS/Pen, MH41, 12SH7 ; at 5/- : 12SK7, ARP12, 6SH7, NGT1, CV66, TT 11, 6J5G, EF36, 6AC7, 6AB7, 6G6, 12SC7. Caller.; and Post : W. A. BENSON (PW), 303, Rathbone Road, Liverpool 13. STO 1604 Callers only : SUPE RADIO (W chapel), LTD. 116 Whitechapel, Liverpooll. ROY 1130 ° ^ i Si S a 3 = ^ n 3 H o • 2 i or 09 7Q 1 > to ^ £ % >"1 o G ? a: 3 s B do 5 o c g X g * c z 5 1 05 ^\<-l 3 to a g to o O Qu to to a*05 zh ' ~ >. ^ 05 3 "3 crq ="•3 35 n> CD CL to fa of CD EL V- CD g % c : to§-.| w; Sg"<§2§:£ n sp cd A ac O c/> *» = ^ W P g ST^ 3"^ ^3 g.~03 ° 5 O ft C jL o Q < cd v*75<3 X EL^QO ET O ° < |.f gt 5'^5'ci Q — 3 i-j *7 05 CD 05 3 My 3 3 ^EJ^g^ o c o | s£ cro CD 3 2 G.3 ft O H O Just Published RADIO AND TELEVISION ENGINEERS' REFERENCE BOOK Edited by E. Molloy and W. E. Pannett. Broadcasting - ' Communications ' Navi- gation " Servicing, etc. 36 specialist contributors. Over 1,600 pages. Over 1,860 diagrams 70s. Post Free. Practical TV Aerial Manual for Hands I and III, by R. Laidlaw. 4s. 6d., postage 4d. Practical Wireless Eiieyelopaedia, NEW 13th EDN., by F. J. Caram. 21s., postage 9d. Electronic Measuring Instruments, by E. H. W. Banner, 45s., postage Is. Handbook of Line Communieal ion. Vol. I, by The Royal Signals, 30s., postage Is. Radio Control of Models, by G. Sommerhoff. 5s., postage 3d. Radio Valve Data, 4th edn., compiled by " Wireless World," 3s. 6d., postage 3d. Please write or call for our Catalogue. THE MODERN BOOK CO., BRITAIN'S LARGEST STOCKISTS OF BRITISH & AMERICAN TECHNICAL BOOKS 19-23 PRAED STREET, (Dept. W.I.), LONDON, W.2. "Phone : PADdington 4185. Open all day Saturday. COPPER WIRE COTTON COVERED 4 ozs. V- 2/1 2/2 2/3 2/4 2/5 2/6 2/7 2/8 2/9 2/11 3/1 3/2 3/4 3/6 3/7 3/8 3/M 4/2 4/5 4/8 5/6 6/2 S.W.G. 2 ozs 16 1/4 17 1/4 18 1/4 19 1/5 20 1/5 21 1/5 22 1/6 23 1/7 24 1/7 25 1/8 26 1/9 27 1/10 28 1/10 29 l/ll 30 2/- 31 2/1 32 2/1 33 2/3 34 2/4 35 2/6 36 2/7 37 3/- 38 3/4 39 40 4/6 8/- SILK COVERED 2 ozs. 4 ozs. 1/4 2/ 1/4 1/4 1/6 1/7 1/8 1/9 1/10 1/ 10 l/ll 2/- 2/1 2/2 2/3 2/4 2/5 2/7 2/10 2/11 3/1 3/3 3/5 3/7 3/10 4/1 2/1 2/2 2/5 2/8 2/10 3/- 3/2 3/2 3/4 3/6 3/8 3/10 4/- 4/2 4/4 4/8 5/2 5/4 5/8 6/-. 6/4 6/8 7/2 7/8 POSTAGE EXTRA. POST ORDERS ONLY PLEASE. Send stamp for comprehensive lists. CRYSTAL SET INCORPORATING THE SILICON CRYSTAL VALVE Adjustable Iron Cored Coil. RECEPTION GUARANTEED Polishqd wood cabinet, 15/-, post \l- A REAL CRYSTAL SET, NOT A TOY POST RADIO SUPPLIES 33 Bourne Gardens, London, E.4 February, 1955 PRACTICAL WIRELESS 113 Oscillatory Circuits AN EXPLANATION OF THE WORKING OF THE VALVE AS AN OSCILLATOR By E. E. Apps (Continued from page 50 January issue) REFERRING back to the simple feedback oscillator as described in Part I, it is of interest to note that this type of oscillatory circuit is the basis of the reaction receiver as origin- ally used. The feedback of negative-resistance or energy tended to neutralise the damping of the positive resistance of the circuit, and thus the circuit amplified up to a point where oscillations were set up. The Reinartz Circuit (Fig. 1) This was a circuit that had one coil only for aerial and grid reaction circuits. The aerial and grid tappings should be noted. The reaction is applied by means of a variable condenser connected between anode and the bottom end of the coil. This circuit was, a few years ago, a favourite with short-wave enthusiasts, and gave very good reception if care was taken with the setting of the reaction condenser. Power from the Oscillatory Circuit In transmitters and radio-frequency heating apparatus it is clesirous to draw large amounts of power from the oscillatory circuit (Fig 2). Here the total current flowing through the valve is made up of Io plus an oscillating component whose instantaneous value is represented by io. The current in the coil L is^lso composed of Io-HL. The current in the capacity C consists only of an oscillating component whose instantaneous value is ic. When the circuit is self-oscillating, the peak value of the oscillatory anode current is limited by the magnitude of Io. The oscillatory anode-filament PD is similarly limited by the H.T. voltage. Thus, it will be seen, that the larger the value of the mean anode-current and the D.C. voltage applied to the anode, the jarger will be the power in the oscilla- tory circuit. The Meissner Oscillator (Fig. 3) This circuit, as will be seen from diagram, has no conductive coupling. The oscillatory circuit is coupled inductively to both grid and anode coils. Where it is inadvisable to tap the coils, this circuit is advantageous. Resistance-coupled Oscillator (Fig. 4) This uncommon type of oscillator makes use of a resistance, as a coupling between anode and grid circuits. When the two circuits are tuned so that Li CI=L 2 C 2 , the E.M.F. generated by the valve, the potential across the anode circuit, and the potential across the coupling resistance, are all nearly in phase, and the phase of the potential applied to the grid differs by 180 deg. from that of the E.M.F. generated in the valve. Thus conditions for oscillation are set up and, in this case, frequency stability is high. Relaxation Oscillators (Fig. 5) In a circuit in which the frequency is governed by the capacitance and resistance, and there is a swing from one state of quasi-equilibrium to another, the name of relaxation oscillator is given. As there is no inductance in the circuit, the type of oscillation produced differs considerably from a tuned circuit oscillator. The wave-form is generally non-sinusoidal, and there are harmonics of considerable amplitude. The frequency generated can be varied by the capacitance or resistance, between very wide limits. Fundamental frequencies can be as high as 1 Mc/s or as low as 1 cycle per minute. Fig. 5 shows a simple type of glow discharge oscillator. When a D.C. voltage of a magnitude greater than the striking voltage of the tube is applied, current will flow through R and the voltage across C will rise until the striking voltage is reached. The current through R then increases rapidly and the condenser discharges until the potential across it equals the extinction voltage of the tube. The condenser is now charged again until the striking voltage is again reached. The period of the tube flashes can be varied by adjustment of R and C. When C is large, flashes at a very low rate can be produced. The amplitude is comparatively small, but can be increased by a circuit tuned to the operating frequency inserted in the resistance arm. Multivibrator Oscillator This is a relaxation oscillator of a much better type. It is essentially a two-stage resistance-coupled amplifier, in which the voltage developed by the output of V2 is applied to the input of VI. As each valve produces a phase shift of 180 deg., thus causing the output of V2 to supply to VI an HT-h Grid T -HT+ T' ~LT+ Fig. 1—The Reinartz circuit. Fig. 2. — Tuned-anode oscillator showing current flowing in different parts of the circuit. 114 PRACTICAL WIRELESS input voltage of the correct phase relationshio, oscillations can be maintained. Action of Multivibrator If a minute voltage of a positive potential appears at the grid of VI, this voltage is amplified by both 'WW* February, 1955 6(A) shows that the wave-form of the current in the anode circuits is rich in harmonics, so that, if a standard frequency of known value is used to control the frequency of the multivibrator, it impossible to obtain a large number of frequencies all related to the known frequency. It can be thus seen that the multivibrator is a useful means of generating known fre- quencies for measuring purposes. If, for in- stance, a standard tun- ing-fork of a frequency of 1 kc/s were employed to generate a standard frequency, it would be possible to use the harmonics generated either directly or with a Fig. 3. — Me issuer oscillator. Fig. 4. — Resistance- coupled oscillator. Fig. 5. — Glow discharge oscillator. higher frequency vibrator synchronised by a harmonic of the 1 kc/s. The frequency at which this circuit oscillates is governed, of course, by the values of C and R, and it may be changed by changing the values of these. This forms a very convenient Way of making a variable-frequency oscillator, and will be familiar {Grid TL volts VI Anode [current "> J I S&te^Mf *Z ^ 1 S^tJ^-m ™™-"-equency oscillator, and will be familiar S^l^iffiSs^M BTJ &ffi& , S3&*' te,evision field> teing used point is reached when the anode operation. current of V2 is reduced to zero by' i 1 / the falling grid potential, and the R anode current of VI is large. How- ever, the leakage through the grid circuit resistances gradually restores the grid potentials to normal. When this leakage reaches a point when anode current commences in V2 the process is repeated in the reverse direction. The frequency is deter- mined by the grid resistance R and the grid capacitance CI, and is given approximately by f = — jf all tour resistances are equal to R. Fig. [current ■Simple multivibrator and current and voltage curves. A VALVE VOLTMETER (Cont. from page 98) are terminated in one end). Having earth end pushed by spring fin. out, it gives extra length when it is not easy to reach with the top terminal. 1,000 Volts A.C. Probe (Fig. 5) This probe was built for 500 volts and 1,000 volts A.C. It is important to note that it is not suitable for high frequencies. The probe is made of bakelite tube of lin. dia. inside, l/16in. thick, 4in. long; the covers are of ebonite, the terminal socket of polvsterene. It is much safer than a metal probe. All connections are insulated with plastic sleeving and the joints are brushed over with polystyrene cement. The three outgoing wires are all plastic covered and two of them must be capable of carrying 1,000 volts DC Also, condenser must be at least 1,200 volts D.C. if it is to be used for A.C. 1,000 volts. When dealing with mains voltage always make sure that the earth potential is connected first. As you can see from the photograph, the segment below the volts scale is left uncalibrated. In my own case, I shall calibrate it to read decibels. The ohms range can be added in the form of external adaptor which would consist of switch, H.S. resistors, 1 £-volt battery, four terminals as in Fig. 6. It will read 0-100 ohms to 100 megs. Resistors on the switch can be connected in series or parallei. The meter will read from left to right so that the highest unknown value will be near the full deflection end. Only slight modification will be necessary on the valve voltmeter. One more contact must be added to the D.C. wafer. This contact is connected to the negative terminal. It will then give full deflection of the voltmeter with 1 volt only. If you use the 5-volt position on D.C. you will need 5 volts for full deflection, and the adaptor unit will have to be much bigger to take 5 J volt cells. To save the cost of two filament resistors, the L.T. transformer can be dismantled and about six turns taken off to drop the voltage to about 6 volts on no load. Reduced F voltage gives greater stability, but if reduced too much, it will take too long to warm up. When calibrating the instrument,, start with D.C. side, check on all ranges, adjust calibrating controi in such a way that error on all ranges will be no more than 2 per cent. When calibrating, always check zero set. With the A.C. range, proceed as with D.C. and the result should be as good as with D.C. Always allow about two to five minutes to warm up. February, 1955^ PRACTICAL WIRELESS 115 R.S.C. A4 HI -FIDELITY 25 WATT AMPLIFIER A new design for 1955. " Push-Pull " output. ''Built-in'' Tone Control Pre- amp stages. Increased sensitivity. Even further improved performance figures Includes 7 valves, specially designed sectionally wound output transformer block paper reservoir condenser and reliable small condensers of current manufacture. TWO SEPARATE INPUTS CONTROLLED BY SEPARATE VOLUME- CONTROLS allow simultaneous use oi " Mike " and Gram., or Tape and Radio, etc. etc. INDIVIDUAL CONTROLS FOR BASS AND TREBLE " Lift "and Cut Freauency response +9 ab. ju-ju.wu c/c. Six negative feedback loops. Hum level 66 db down ONLY 20 millivolts INPUT Squired [ for FULL OUTPUT. Certified harmonic distortion only 0.36% measured at 10 watts. Comparable with the very best designs. ENTIRELY SUITABLE FOR SMALL HOMES OR LAIKiM HALLS ^ CLUBS, GARDEN PARTIES, DANCE HALLS, etc. Size approx. 12-9-7in. For A.C. mains 200- 230-250v. 50 c/cs. Outputs for 3- and 15-ohm speakers. Kit is complete to last nut. Chassis is fully punched. Full instruc- tions and point-to-point wiring diagrams supplied. Unapproachable value at 90ns., or ready for use, 50/- extra. If required, cover as illustrated can be supplied for 17/6. H.P. TERMS AVAILABLE. etc. For ELECTRONIC ORGAN or GUITAR. For STANDARD or LONG-PLAYING RECORDS. FOR ANY "MIKE " or PICK-UP. 9 GNS. R.S.C. 10 WATT " PUSH-PULL" HIGH - FIDELITY AMPLIFIER A3 Complete with integral Pre-amp. Tone control stage (as A4 amplifier), using negative feedback, giving humproof individual bass and treble lift and cut tone control. Six Negative Feedback Loops. Completely negligible hum and distortion. Frequency response ± 3 db. 30-20,000 c.p.s. Two independently con- trolled inputs. Six B.V.A. valves. A.C. mains 200-230-250 v. input only. Outputs for 3 or 15 ohm speakers. Kit of parts complete in every detail. £7/19/6, plus 5/- carriage or ready for use, 45/- extra. Illustrated leaflet, 5d. . . II MV LONG PLAYING RECORD TIJI&TABI i: VVI 1 II CRYSTAL PICK- {jP (Sapphire Stylus). Speed 33 * r.p.m. For A.C mains 200-250 v. Limited supply. Brand New Cartoned Perfect Only £3/19/6. Plus carr. 5/- (Normal price £8 approx.). i BATTERY SET CONVERTER KIT. All parts for converting any type ot Battery receiver to All Mains. A.L. 200-250 v. 50 c/cs. Kit will supply fully smoothed H.T. of 120 v.. 90 v or 60 v at up to 40 mA. and fully smoothed L T of 2 v . at 0.4 to 1 a. Price, complete with circuit wiring diagrams and instructions, only 48/9. Or ready to use, 8/9 extra. PERSONAL SET BATTERY SUPER- SEDER KIT. A complete set of parts for construction of a Unit (housed m metal case) to replace Batteries where A C Mains supply is available. Input 200-250 v. 50 c/s Outputs 90 v. 10 mA. and L4 v 250 mA.. fully smoothed. For 4-valve receivers. .Price complete with circuit Only 35/9. Or ready for use, 42/6. Size of unit, 51 x 4 x ljin. A PUSH-PULL 3-4 watt HIGH-GAIN AMPLIFIER FOR £2/19/6. For mains input 200-250 v. 50 c/s. Complete kit of parts including circuit, point to point wiring diagram, and instructions. Ampli- fier can be used with any type of Feeder Unit or Pick-up. This is not A.C./D.C. with " live " chassis, but A.C. only with 400-0-400 v. trans. (Output is for 2-3 ohm speaker.) Supplied ready for use for 25/- extra. Carr. 2/6. Desc riptive leaflet, 7d. P.M. SPEAKERS. All 2-3 ohms, 6 Jim Plessey, 16/9. 8in. Plessey, 15/9. 10m. Plessey, 18/6. lOin. R.A., 26/9. lOin. Rola with trans., 29/6. lOin. W.B. "Stentorian 3 or 15 ohm type HF1012 10 watts. Highly recommended for use with any of our amplifiers, £3/13/6. BATTERY CHARGER KITS For mains 200-250 v. 50 c/s. To charge 6 v. acc. at 2 a.. 25/6. To charge 6 or 12 v. acc. at 2 a., 31/6. To charge 6 or 12 v. acc. at 4 a., 49/9. Above consist of transformer, full wave rectifier, fuses, fuseholders and steel case. Any type assembled and tested, 6/9 extra. THE SKY CHIEF T.R.F. RECEIVER. A design of a 4-stage, 3-valve. 200-250 v. AC Mains receiver with selenium recti- fier ' It consists of a variable Mu high gain H F stage followed by a low distortion crid detector triode. The next stage is a ; further triode amplifier with tone correc- tion by negative feedback. Finally comes the output stage consisting of a Parallel connected double triode giving ample output at an extraordinarily low level of distortion. Point to point wiring dia- grams instructions, and parts list, 2/6. This receiver can be built for a maximum of £4/19/6 including attractive Brown or Cream Bakelite or Walnut veneered wood cabinet 12 x 6 J x 5 J in. PLESSEY 3-SPEED MIXER AUTO- CHANGERS. With crystal pick-up. Duo point alloy stylus (switched) for standard or long playing records for Mams 200-250 v. 50 c/cs. Brand New £10/10/-. Car r. 5/-. MICROPHONES. Crystal type, good quality. Recommended for use with our amplifiers. Hand type, 59/6 ; Stand type, with adjustable stand, £6/19/6. FOUR STAGE RADIO FEEDER UNIT. Design of a High Fidelity Tuner Unit. L. & M. Wave. Full decoupling. Self-contained heater supply. Detailed wiring diagrams, parts list, and illustra- tion, 2/6. Total building cost, £3/15/-. COAXIAL CABLE, 75 ohms, I in., 7d. yard. Twin Screened Feeder, 9d. yard. M.E. SPEAKERS. All 2-3 ohms. 8in. R.A. Field 600 ohms, 12/9.- lOin. R.A. Field 1,000 or 1,500 ohms, 23/9. VOLUME CONTROLS with long (Jin.) spindles, all values, less switch, 2/9, with S.P. switch, 3/9. D.P. sw.. 4/9. SELENIUM RECTIFIERS L.T. Types 2/6 v. £ a. H.W. 1/9 6/12 v. i a. H.W. 2/9 F.W. Bridge Typo 6/12 v. 1 a. 4/9 6/12 v. 1.5 a. 7/6 6/12 v. 2 a. 8/9 6/12 v. 3 a. 12/3 6/12 v. 4 a. 14/9 6/12 v. 6 a. 6/12 v. 10 a. 24 v. 10 19/9 35/- 59/6 H.T. Types H.W. 150 w. 40 mA 3/9 250 v. 50 mA 5/9 250 v. 80 mA 7/9 RM4 250 V. 250 mA 11/9 R.S.C. MAINS TRANSFORMERS (< _ _ _ . a — -m in.,:... ....i.^. OAA.OOn.ORA v Kft FULLY \ .GUARANTEED / Interleaved and impregnated. Primaries 200-230-250 y _50_°Vs s c^ened TOP SHROUDED, DROP THROUGH 250-0-250 v. 70 mA, 6.3 v. 2.5 a. " ... 12/11 260-0-260 v. 70 mA, 6.3 v. 2 a, 5 v. 2 a. .. 14 11 350-0-350 v. 80 mA, 6.3 v. 2 a, 5 v. 2 a 17/9 275-0-275 v. 80 mA, 6.3 v. 2 a, 4 y. 2.5 a 14/11 250-0-250 v. 100 mA, 6.3 v, 4 a, 5 v. 3 a 21/9 300-0-300 v. 100 mA, 6.3 v. 4 a, 5 v. 3 a 21 9 350-O-350 v. 100 mA, 6.3 v. 4 a, 5 v. 3 a 21/9 350-0-350 v. 100 mA, 6.3 v. 4 a C.T., 0-4-5 v 3 a ••• 22/9 350-0-350 v. 150 mA, 6.3 v. 4 a, 5 v. 3 a 27/9 350-0-350 v. 150 mA, 6.3 v. 2 a, 6.3 v. 2 a 5 v. 3 a 29/11 FILAMENT TRANSFORMERS All with 200-250 v. 50 c/s primaries 6.3 v. 1.5 a, 5/9 ; 6.3 v. 2 a, 7/6 ; 0-4-6.3 v. 2 a, 7/9 ; 12 v. 1 a,' 7/11 ; 6.3 v. 3 a, 8/11 ; 6.3 v. 6 a, 17/6 ; 12 v. 3 a or 2.4 v. 1.5 a, 17/6. ELECTROLYTICS (Current production Not ex-Govt.). Tubular Types 8uF 350 v. 8uF 450 v. 8uF 500 V. 16 uF 350 V. 16 uF 450 v. 24 uF 350 V. 32 uF 350 V. 25uF 25 V. 50 uF 12 v. 50 uF 50 V. Can Types 8 mfd. 350 v. 13 8uF 450 v. 2/3 1/9 1/11 2/11 2/3 2/9 3/6 3/6 1/3 1/3 2/3 8uF500v. 2/6 16uF 450v. 2/9 24uF 350 v. 2/11 32 uF 350 v. 2/11 32 mfd. 450 v. 4/9 40 uF 450 v. 4/11 64uF 450v. 3/9 8-8uF&50v. 3/9 8-8 uF 450 v. 3/9 8-16 mfd. 450 v. 2/11 8-16 uF 450 v. 3/11 16-16uF450v. 4/11 16-32 uF 350 v. 5/3 32-32uF450v. 5/11 FULLY SHROUDED UPRIGHT 250-0-250 v. 60 mA, 6.3 v. 2 a, 5 v. 2 a, Midget type 2 i-3-3in. ... ... "16 9 350-0-350 v. 70 mA, 6.3 v. 2 a. 5 v. 2 a 18; 9 250-0-250 v. 100 mA, 6.3 v.-4 V. 4 a, C.T. 0-4-5 v. 3 a ... ... 22/9 250-0-250 v. 100mA, 6.3v. 6 a, 5 v. 3 a, for R1355 conversion ... ... 29/9 300-0-300 v. 100 mA, 6.3 v.-4 v. 4 a, C T. 0-4-5 v. 3 a • ••• 22/9 350-0-350 v. 100 mA, 6.3 v. 4 a, 5 v. 3 a 22/9 350-0-350 v. 100 mA, 6.3 v. -4 v. 4 a, C.T. 0-4-5 v. 3 a ... „ ••• . - 23/9 350-0-350 v. 150 mA, 6.3 v. 4 a, 0-4-5 v 3 a ... ••• ••• ••• 425-0-425' v. 200 mA, 6.3 v. 4 a, C.T. 6.3 v. 4 a, C.T., 5 v. 3 a. Suitable Williamson Amplifier, etc. ... 47/9 450-0-450 v. 250 mA, 6.3 v. 6 a, 6.3 v. 6 a. 5 v. 3 a cww CHARGER TRANSFORMERS All with 200-230-250 v. 50 c/s Primaries ; 0-9-15 v. U a, 11/9 ; 0-9-15 v. 3 a, 16/9 ; 0-9-15 v. 4 a, 18/9 ; 0-9-15 v. 6 a, 22/9. SMOOTHING CHOKES 250 mA 3-5 H 50 ohms 150 mA 7-10 H 250 ohms 100 mA 10 H 175 ohms Potted 80 mA 10 H 350 ohms " mA 10 H 400 ohms 11/9 11/9 8/9 5/6 4/11 E.H.T. TRANSFORMERS 2,500 v. 5 mA, 2-0-2 v. 1.1 a, 2-0-2 v. 1.1 a, for VCR97, VCR517, etc. ... 36/6 Terms : FT IMINATOR TRANSFORMERS P^maJies 200-250 v. 50 c/s, 120 v. 40 mA 7/9 130 v. 50 mA, 6 v. 3 a 14/9 OUTPUT TRANSFORMERS Midget Battery Pentode 66 : 1 for 3S4 etc Smali Pentode 5,000 0 to 30' ... Standard Pentode, 5,000 O to 30 ... Standard Pentode, 7/8,0000 to30 ... Standard Pentode, 10,000 0 to 3o ... Multi-ratio 40 mA, 30 : 1, 45 : 1. 60 • 1,90 : L Class B Push-Pull ... Push-Pull 10-12 watts 6V6 to 30 or Push-Pull 10-12 watts to match 6V6 to 3-5-8 or 15 O ... ... ••• Push-Pull 20 watts, sectionally wound, 6L6, KT66, etc. , to 3 or 15 O Williamson type exact to spec. 3/9 3/9 4/9 4/9 4/9 5/6 15/9 16/9 47/9 85/- ; _ m (LEEDS) LTD. RADIO SUPPLY CO. 32, THE CALLS, LEEDS, 116 Best Buy at Britain's COMMUNICATIONS REC EIVER R1155. --World-wide reception is ensured by the Sf' nfl^? I.F. stages. Five wavebands S L.W. M.W. and 2 S.W.). Magic-eye, large dial and vernier make tuning simple. Con- tained in attractive black crackled cabinet its handsome appearance does justice to its P^ f ormance. supplied with FREE BOOKLET Sivmg circuit data and details of the power pack required for A.C. mains operation. Fully aerial-tested before des- • R^9 n - Gladly demonstrated to callers. BRAND NEW, "MINT" CONDITION in ORIGINAL MAKER'S TRANSIT CASES, %k\'k 9 A 6 ' a Shop-soiled models, as new, *r ,19 ;£;„ A few only used models, £7.19.6, plus 10/6 carriage. w ' POWER PACK & OUTPUT MA^E.— Tnese enable the R1155 to be E?n d AS™T e £l ains WITHOUT ANY MODI- FICATION Three types available, £4.10.0. »cin°k an( l de " luxe model with 8in. speaker, £6.10.0. Carriage 3/6. All Power Packs guaranteed six months. SAVE £££?. DE- ?SS T J^S E NPURCHASING RECEIVER AND POWER PACK TOGETHER. Send S.A.E. for full details of Power Packs and Receivers or 1/3 for booklet. RECEIVER TYPE CR100.-A super com- munications receiver covering 60 kc's to 30 mc/s in six bands. Built-in A.C. mains power pack. 2 R.F. stages, 3 I.F. stages variable selectivity Xtal Filter. B.F.O etc' Good condition, complete with new valves f ? .™H Sted - A bar> eain at £27.10.0, plus *i carnage. 5?' 1 . 1 , 96 RECEIVERS (25/73).-This is a six-valve superhet receiver with 465 kc/s. i'TV™ ^P-SiSJ 6 * 6 wlth a11 valves — 2 EF39 JJ5%i? EF36, 1 EB 9 3 - In Good Conditio plu?2/6 LpoSt Versl0n data ' 0NLY each, SPECIAL BARGAIN OFFER. -Admir- alty Receiver A2074 contains standard A C mains power pack, output 315-0-315 v. 70 mA, ^ nd 5 v ' 2 a - Admiralty rating), ffff Paper smoothing condensers, J? c Svf h S? e ' tv ?° - 0003 mF variables, 34 re- sistors, 34 condensers and stacks of useful components, including output transformer, 6 1 transformer, etc. In grey metal case OMTv^o/a 6l , n - ^i 8in - Good condition. ONLY 19/6, plus 5/6 carriage. PRACTICAL WIRELESS THE VALVE SPECIALISTS Bentley Acoustic Corp. Ltd. 38, Chalcot Road, N.W.I. PRImrose 9090 February, 1955 0Z4 1A3 1A5 IDG 1E7 1G6 1L4) 1LD5 1LN5 1M1 ]R5 1*4 185 1T4 1U5 2C26 2C34 2D2 2D4B 2D13C 2D21 2X2 3B7 3D6 3Q5 4D1 5R4 SATURDAY BARGAINS iilnZ Saturday we dispose of various items of which we have too few to at R ? AL CLEARANCE PRICES. Come and get YOUR bargain. No reasonable offers refused. ?™? IA T# RI L m O»EL MOTORS.-2in. long x I#n. diam., 3/16in. diam. spindle weight 5 ozs. Will work from 6-volt drv 10/6ea y ch reversible - Bal1 bearings. S ^ COIL PACKS.-With circuit diagram and connections. Gram, position 2S.S?2, tch - 14/ 6 each. ^ THREE-GANG CONDENSERS.-390 pF dev pp wiS,°J 1?n dFive t nd anti-backlash Sf* tSSiJfi^ tr I mme rs. Brand new, boxed, 3/6, plus 1/- postage. E.M.I OUTPUT METER. — Desk Type consists of a 2*in. 1 mA. meter with full w«; / t,^ 1 H d ?i e , rec E fier - Ran ^ es 0-500 milli- hmt v 0-5 u at ^ s - Brand Ne w and Boxed. ONLY 35/- each, plus 1/6 post METAL RECTIFIERS. — Heavy duty Se- lenium 10|in. long with 3i'in. square plates Two are required for a full wave bridle Bffi£&£% at 10am P s - Brand new Sad 9/fi^i OU »^ e 2P,, at only 57/ 6 Per pair, plus I A?°o St ^/?- M -t 250 X olts 250 mA - Only 12/6. ?SfiS ac 6 ° r ! for Selenium! 300 v i r vr nl'i,^ n A^ ew ^ ex - w - D - 6/9 eacn - RT^ kL^^P * — Brand New boxed So^o hea X^ d '^ t L out P ut Pentode. 6.3 v 8.8 watts, at 7/6. 7193, same as 6J5 with top grid *J°*» a J 6 ^ 10/6, Post paid. C ° P 7S&Vfr.£E?K twm circ ular polythene cable for extension mams leads, etc. Price 39%? Plu d s 0 3 Z f6 n cL a r rdS ° f 100 yardS f ° r ° nly ROTARY POWER UNIT TYPE 104.- S P 9 U f Ji«' 8 U SP ti I,?° voIt 60 mA - and 6 -3 v. at 2.5 amp. Fully filtered and smoothed and Onlyl5/ P ?S! d ' Meal f ° r Car radi °' 6tC - | CHARLES BRITAIN (RADIO) LTD. - II, Upper Saint Martin's Lane, London, W.C.2. TEM 0545 Shop hours, 9-6 p.m. (9-1 p.m. Thursday) OPEN ALL DAY SATURDAY 5Z3 5Z4 6A3 6AB7 6 AC 7 6 AG 5 6 AG 7 6AJ5 6AK5 6AL5 6AM5 6AM6 6AQ.-) 6ATG 6A7 6B4 6B7 6B8G 6B8M 6BA6 (5BE(i 6/ 8/1 6/6 6/6 6/. 4/6 4/6 12/6 7/- 7 6 7/- 7/- 7/6 4/- 2- 7/6 10 6 4'- 8 6 4/6 8/8 2/6 10/6 3/- 9/6 8/ 8/6 7/6 8/6 8/6 106 U 6/6 6/6 15/. 7/6 7/6 6SL7 6SN7 6SQ7 6SS7 6ST7 6U5(UX) 7/- 6U5(IO) 7/6 7] 9:s 8/6 7/. 7 6 7k 8- 15/- 10 8 8 6 7 6 7 6 6/- 8 6 7/6 8/8 8 6 2 '6 6 6 3 - 4/6 10LD11 10/- 12A6 6/8 12AH7 " 12AT6 12AT7 12AU7 12AX7 12BA(i 12BE6 12C: 6U7G 6V6 6X4 6X5 6Y6 6Y7 6Z4 ?B5A6 25 L6 25Y5 30 35LG 35W4 35Z4 . 'Z5 39/44 42MPT 15/. DL72 DL93 DPT EA50 EA76 EAC91 E1148 EB4 EB34 EB41 EB91. EBC:: EBC33 EBC41 10/- EC91 7/- 5/6 EC53 EC54 EC70 ECC31 ECf'33 ECC35 BCC81 . ECC82 10/6 ECC91 7/6 15/6 2/- 9/6 9/- 2/- 10/6 1/8 11/- 8/- 10/6 7/6 KBC3S K F:{5 KT2 ET68 KT71 KT7t KT7G KTW6J KTZ41 KTZG:i LP2 MH4 ML4 ML8 MSP4 N77 N78 NR54 OC3 OD3 OM4 OM5 OM5B P41 PG1 PCC84 PCF80 PeH25 Pen2G Pen4() Pen 1340 6/- PL81 10/- PL82 10/- PL83 11/6 PM12 4/- PM12M 7/6 PM24A 19'6 PM256 5/- PY81 10/- PY82 8/6 QP21 7/6 QP25 6/6 QV04/7 9 6 RK.;4 2- 7/6 9/- 5/- 12/6 12- 12/- 12/- ! 5/- 8 6 6 6 4/6 5/6 6 6 6/6 7/6 7/6 11/6 7/6 9/- 8/6 7/6 7/6 10 '6 3/- 3/- 12/6 12/6 6 '6 16/- 7/6 COMM UXIC A TIOXS IUX E IY ER K-1155. Another purchase from the Air Ministry enables us to once again offer this superlative Communications Receiver at prices to suit every pocket. A World Station Getter, this covers o wave ranges ; 18.5-7.5 mc/s.. 7.5-3.0 mc/s 1.500-600 kc/s.. 50C-200 kc's.. <«JJ-75 kc/s., and is easily and simply adapted for mains use. full details, with circuits of receiver being suDpIied. New in Maker's Cases and aerial tested, £11.19.6. Ditto but slightly used for demonstra- tion purposes, £9.19.6. Fi 5 ^' 8°od condition, aerial tested. £7.19.6. ^RSi POWER PACK OUT- PUi SI AGE. — In black metal case enabling the receiver to be operated immediately without any modifiration. can be supplied as follows :— Less Speaker, £4/10/- ; with 61 in. P.M. Speaker. £5/10/-. DEDUCT 10/- IF PURCHASING RECEIVER & POWER PACK TO- GETHER. Please add carriage costs of 10/6 for Receiver and 5/- for Power Pack. 7/6 7/6 9/6 7 6 9 9 9 10/6 10/6 10/6 4/-I 8y-L~. 12/61 EC H42 10/ 10/-JECL80 10/ 50/- EF8 10/. 12/6 EF9 11/6 EF22 12/6 8/6 8 8 8 6 7 6 8'- 10 6 9'- 9- 7/6 EF3G EF37 EF39 EF41 EF42 EF70 EF80 43 50Co 50 Y6 85A2 150A2 8/6 10/6 8/6 8/6 4/6 7/- 8/6 8/- 10 6 10 6 J10DDT 8/6 210 LF 3/ 215SU 717A 807 813 832 8GGA 933 95G 1201 1G22 ■41- 8/6 7/6 70/- 25/- 15/- 51- 3/6 7/6 10/ 10 - 9 4 6 10 6 6 6 9 - 10- EF50(E) 5/- EF50(A) 8/ EF54 5/- EF93 EK32 EL32 EL50 EL91 EM34 EY31 EY91 EZ40 EZ41 FC13 GZ32 H30 H42 HG3 HL2 HL13C HL23 RM4 8130 SDG SP2 SP4B SP41 ■SP61 12 5/6 7/6 8/6 11/- 2/6 2/6 SP220(T) 7/6 TDD2 8/6 TH21C 10/6 TP22 8/6 TP2620 17/6 UAF42 12/6 UB41 9/- UBC41 10/- UCH42 10/- UF41 9/- UU7 UU9 U16 U19 U22 U25 U50 U78 !29 VLS49 VP2 VP4 126 10/- 6/6 5/6 7/6 7/6 6/8 YP4B 11/- 9/- 12/8 12/- 20/- 8/- 13/6 7/6 8/6 10/6 ! 80/- 8/6 20/ 6/6 10/6 11/- 7/1 9- 9 - 9- 14- 7 6 7 6 7 8 3- 7 6 5 6 VU120A 3/- YP13K VP23 YP41 VR53 VT50L W61 W77 XG-. XGG X79 XP(1 8/8 5/6 10/6 7/6 13/- 5) 4/- *H{1.5J 4/- HL23DD6/6 Y63 7/6 HL41 7/6 Z77 6/6 HL1320 6/-IZ719 10/6 Special offer of miniatures : CK303AX. CK505AX, CK523AX, CK525AX. XFW10, XPW20, XFY10, XFYJ2, XFY23, XH1 5 XSG1.5, all at 6/6 each. All boxed with refund guarantee. Post Gd. each. Immediate despatch. C.W.O. or C.O.D. TRANSFORMERS. — Manufactured to our specifications and fully guaranteed Normal primaries. 425v.-0-425 v. 250 ma H v - I a -' 6 - 3 v - 4 a - 5 v - 3 a - ONLY 50/- : 3o0 v. -0-350 v. 160 ma.. 6.3 v. 6 a., 6.3 v. 3 a. 5 v. 3 a. ONLY 42/6 ; 250 V.-0-250 v. 100 ma.. 6.3 v. 6 a., 5 v. 3 a. ONLY 32/6. 350 v.-O- of>0 v. 150 m.a., 6.3 v. 5 a.. 0-4-5 v. 3 a. ONLY 32/6. The above are fully shrouded upright mounting. 5.5 kV. E.H.T. with 2 windings oi 2 v. 1 a., ONLY 72/6 ; 7 kV. E.H.T.. with 4 v. 1 a.. ONLY 82/6. PLEASE ADD 2'- POSTAGE FOR EACH TRANSFORMER. CRYSTALS. — British Standard 2-pin 500 kc/s, 15/-. Miniature 200 kc/s and 465 kc/s. 10/- each. 8 VOLT VIBRATOR PACKS.-Made by H.R.O. of America. Output 165 v. 80 ma . 6.3 v. 3 a. Contains 6X5 rectifier and full smoothing. Self contained in black crackled cabinet size 7in. x 7Jin. x 6in. ONLY 29/6 (postage, etc., 2/-). AMERICAN ROTARY TRANSFORM- BRS.— 12 v. D.C. input, output 255 v. 65 m.a.. Useful for car radio, or running electric shaver from car battery, etc. ONLY 22/6 VACUUM PUMPS.-These are ex-R.A.F! rotary vane type, and are ideal for handy- men and model makers, etc. New and Unused. ONLY 22/6 (postage, etc.. 2/-). EWICATOR UMTS, TYPE 6.-Contain VCR97 Tube with mu-metal screen. 4 valves - EF50 and 2 of EB34. NEW CONDITION" ONLY 59/6 (carriage, etc.. 7/6). PVE ".—45 Mcs. I.F. Strips. The strip that is ready made for the London Vision Channel. Complete with 6 valves. EF50 and 1 of EA50. BRAND NEW. ONLY 70/-. R.F UNITS TYPE 28 & 27.-For use with the R1355 or any receiver with a 6.3 v. supply These are the variable tuning units which use 2 valves EF54, and 1 of EC52. Type 26 covers 65-50 Mcs. (5-6 metres) and Type 27 covers 85-65 Mcs. (3.5-5.0 metres). Com- plete with valves and BRAND NEW IN MAKER'S CARTONS. ONLY 35/- each. MODEL MAKER'S MOTOR.-Only 2in. long and ljin. diameter with Hn. long spindle. Reversible poles. Will operate on 4, 6. 12 or 24 volts D.C. ONLY 10/6. CHASSIS OF INDICATOR 233.-Con tains VCR97 C.R.T. holder. 11 valve-holders, resistors, condensers, etc., etc. BRAND NEW ONLY 10/- (carriage, etc.. 5/-.) METERS F.S.D. SIZE AND TYPE PRICE 1 m.a. D.C. 2}in. Flush square 15/- 1 ,, D.C. 2{in. Flush circular ... 22/6 1 .. D.C. ?4in. Desk type 25/- 5 ,, D.C. 2in. Flush square 7/6 100 .. D.C. 2?in. Flush circular 12/6 150 „ D.C. 2in. Flush square 7/6 500 ,, thermo 2in. Flush square 5/- 500 ,, thermo 2in. Proj. circular 5/- 20 amps D.C. 2in. Proj. circular 7/6 40 amps D.C. 2in. Proj. circular ...... 7/6 30-0-30 amp D.C. Car type moving iron 5/- 15 volts A.C. 2Un. Flush, circ, mov. iron 8/6 All meters Brand Now in Maker's Cartons Amounts ghen for carnage refer to inland only U.E.I. CORPORATION, 138, Gray's Inn Road, London. W.C.1 (Phone : TERminus 7937) (Open until 1 n.m. Saturdays. We are 2 mins. from High Holborn (Chancery Lane Station) and 5 mins. by bus from king* Cross.) February, 1955 PRACTICAL WIRELESS 117 Programme Pointers Somerset Maugham Festival THE most notable event since my last contribu- tion has been the Somerset Maugham Festival, in honour of the famous author's eightieth birthday. I heard the following plays :— 44 Lady Frederick," " The Circle," " For Services Rendered and "Sheppey." There were also adaptations ot some of his novels and short stories and, as I write, there is more to come. It can be said, without qualification, that every- thing gave unalloyed pleasure. Mr. Maugham boasts of being an entertainer first, last and all the time and a right royal one he is. He never philoso- phises or tries to 44 teach " his listeners in the Shavian manner. Nor does he boggle over sex, which rides triumphantly through much of his best work. But he can point a moral and underline it, too, as in the terrific anti-patriotic finale to 44 For Services Rendered," which created such a furore when originally produced in the West End, or the anti- gambling and other high-toned moralities ot S Sheppey." Everything in short makes for perfect radio fare ; not better than Shaw and some others, of course, but different and all that could be asked of an author. There was much fine acting during the Festival and the productions seemed uniformly good. I par- ticularly liked almost everyone in 44 The Circle — Ronald Squire is always Ronald Squire. Monica Gray and Belle Chrystall in 44 For Services Rendered and Sonia Dresdel in 44 Sheppey " stood out. Charles Leno in the latter title role was rather too much on one note, whilst Rachel Gurney in 44 Lady Frederick was not quite the 44 femme fatale " conveyed by the dialogue. By the way, surely the dressing-room scene in this is one of the best things of its kind in English comedy since the screen scene in 44 The School for Scandal " which it slightly resembles. Other Plays Among other good plays were two American ones 44 The Wooden Dish " and 44 Winter Journey. The American drama always has great vitality, gusto and charm whatever else it may either possess or lack. These two pieces made no exception. 44 The Wooden Dish," by Edmund Morris, was one of those plays I mentioned in connection with 44 Return to Tyassi ' which fail in the West End in spite of glowing tributes from the Press. It was a dramatic tale of the sad conflict between age and youth and the demands of the latter on the former. Joan Miller, Bessie .Love, Finlay Currie and many others wrung our withers and our hearts according, doubtless, to the age group of each one of us listening. " Winter Journey," by Clifford Odets, was a " back- stage " play posing ,no problems concerning society and its welfare and containing those two powerful and popular puppets Sam Wanamaker and Constance Cummings, as well as Guy Kingsley Pointer, Reginald Tate, etc. I enjoyed both. 1 cannot stay on radio drama any longer except Our Critic, Maurice Reeve Reviews Some Recent Programmes to mention that 44 The English Captain " also had excellent qualities, but that 44 Mr. Mysterious " afforded an exception to an otherwise exceptionally good month. Ted Ray for Humour 44 Ted Ray Time " seems to have a fair share of humour whenever I have heard it. Ted himself, with Harold Berens, should assure this. Audrey Jeans effectively takes Kitty Bluett's place. Saturday night's 44 Variety Playhouse " always seems a hundred per cent, better when Vic Oliver is the host and master of ceremonies. There have been some irritating changes in the hitherto fixed times for the presentation of 44 Music Magazine," 44 The Critics" and Alistair Cooke's 44 Letter from America," either of the original trans- mission or the repeat. Furthermore, the repeat times are distinctly variable and cannot be relied upon. Although doubtless plenty of listeners are glad of the changes, I do hope there are many who, like myself, have been annoyed and inconvenienced. 44 The Adventures of Sherlock Holmes " have long been a classic in their special genre. Like Gilbert and Sullivan, Oscar Wilde and Dickens, they carry the Victorian flavour in its most attractive guise into our more sophisticated age irrespective of the current fashion in greenback 44 whodunit." Their charm is perennial proof of their quality. The present series has afforded great pleasure to the older listener who remembers their original appearances in the lamented Strand, as well as to those unacquainted with them. The casting of Sir John Gielgud and Sir Ralph Richardson as Holmes and Watson was sufficient proof of the importance that was, I submit, correctly attached to the series. Val Gielgud, John Cazabon ' and William Fox complete the almost perfect cast. Music Music, too, has been in full spate, as it ever is. I liked an original programme of French and Russian numbers conducted by Jean Martinon, whilst the following week he included that most famous of modern — 40 years old now — masterpieces, Stra- vinsky's 44 Petruschka," just as lovely in the concert hall as the theatre. Ariur Rubinstein was his ever brilliant self in a Beethoven concerto and a Liszt programme. There can be few finer Liszt players to-day. Moiseiwitsch gave a Schumann recital and included the fascinating but seldom heard 44 Kreis- leriana." Here, too, is a master in a certain type of music. 118 PRACTICAL WIRELESS News from the Trade February, 1955 The Brenell Tape Deck THE new Brenell Tape Deck is introduced as a high-quality unit at a moderate price Of particular interest is Vie new toggle-action grin between capstan and pinch wheel which ensures a tape drive free from slip and which eliminates one of the major causes of "wow" and nutter The capstan is directly coupled to a heavy flywheel whfch power" 6 " by a rubber belt from a motor of adequate Three independent motors are employed to ensure reliable operation with simple Switching BrS is arranged mechanically and gives an effective quick stop action with complete freedom from Ipe RD Junior Mk II pre-ampliner respectively They are both virtually entirely new designs but we have used the present models as standards of C 7S°er t0 — n ' Se the T" -'Pavements. 0 ' equire'd "ffi allt0 ™ tiCal , ly f,Ued 110 S0 'S control fitted hS t, C ° mro1 : ° ptional P resenc e The new Brenell Tape Deck showing underside with switch interlock. % ^S^y&^ C ° rreCt,y Set —ate ^Three speeds are available with a playing time 2 hours at 3fin. a second. 1 hour at 7£in. a second. I hour at 15in. a second. The record/playback head is a high-impedance l r r ■ W1 - a h r eavy gau ^ e Mumetal screen to ensure elimination of 50-cycle hum due to magnetic fields from motors, transformers, etc. The erase head is a low-impedance type which SSS&^JS fr ° m 3 '<"^-e fflSS The tape drive is from left to right, using the upper track to conform to British Standards. With overall dimensions of J5in by Him;, with 3iin. below deck, this instrument costs £15 10s complete. — Brenell Engineering Co. Ltd., 2, Northington Street London, W.C.I. The RD Junior Amplifier and Con- trol Unit Control Unit : Treble control now continuously variable. In the circuit used this results in a more satisfactory characteristic, lacking the "blare" associated with some forms of control when boost is applied. A switched bass control has been retained as here the more satisfactory characteristic is obtained with a circuit employing switching. Tape Jacks- Previously only included on the Senior model are now fitted to the front panel. ' UnTt hC £^ a! \n^ lifi n C ? tS £ ' 6 and the Contr °l unit, ty—Rogers Developments Co. Rodevco VEAQ. ' BIackheath Road > Greenwich, London! ^pHE RD Junior Amplifier and Control Unit are being intro- duced Jo replace the present RD Baby de-luxe Mk. II amplifier and The RD Junior Amplifier and Control Unit, February, 1955 PRACTICAL WIRELESS 119 LASKY'S PAGE OF MONEY* AV/NG OFFERS 2 a. 4 a. METAL RECTIFIERS 6 and 12 volt F.W. 9/- ... 12/- ... 19/6 6 Volt iamp. ... 2/6 lamp. ... 4/6 R.M.I ... 3/10 R.M.2 R.M.3 R.M.4 4/3 51- 16/- 12 Volt I amp. ... 3/11 1 amp. I.F. TRANSFORMERS MINIATURE. J x } x 11 Ins. PRICE 10/6 pair. WEARITE TYPE 550. 445-520 kc/s. 8/6 per pair. WEARITE TYPE 500. 450-470 kc/s. 8/6 per pair. * CRYSTAL, DIODES Wire ends. Glass 1/6 Type WX.6. Wire ends... 1/6 TELESCOP IC PORTA HI. I : AERIAL MASTS Made of lightweight but ex- tremely strong alloy. Extends to 15ft. Guyed at top and centre. Supplied complete with all guy lines. LASKY'S PRICE, 25/- Carriage free. 25 UNITS. Complete 3 valves. New. 19/6. Secondhand. 15/-. Carriage, 2/6 each extra. SCPERIIET COIL PACKS With Circuit. No. 1. L.M.S.G. Size : 42 x 5 x 24 ins. With Jin. spindle... 19/6 No. 2. M.S.S. Size : 4 x 4 x 3ins. With iin. spindle 16/- Both for use with 465 kc/s I.F. LOUDSPEAKERS First Quality. All 3 ohms speech COMPLETE 5 VALVE RADIO CHASSIS Brand New and Unused. A.C. D.C. Mains. 200/250 volts. *I.F. 465 kc/s. *A.V.C. * 4 Watts output. *3 Station Pre Set. * Frame aerial. * Fully aligned. * Chassis size only lOins. x 5Jins. Max. height 5 h ins. Completely wired and ready for use. with the ad- dition of a speaker and output transformer. Two controls— Volume and Station switch. Valves used : 10C1, 10F9 or UF41, 10LD11. 10P14. U404 or UY41. LASKY'S PRICE, 69/6, less Valves. Postage 3/6 extra. PRICE COMPLETE, £5.19.6. LASKY'S RADIO CONSTRUCTOR PARCELS NO. 1. A.C. SUPERHET.— 4 valves plus metal rectifier. Long, medium and short wavebands. In very attractive cabinet, either walnut veneer or ivory or walnut plastic. For 200/250 v, 50 c.p.s. All components, valves, mains transformer, cabinet, dial and metalwork. LASKY'S PRICE, £7.19.6. Post & pkg. 2/6 extra. NO. 2. A.C.T. R.F.— 3 valves plus metal rectifier. L. & M. wave- bands. For construction in attractive wood or plastic cabinet, walnut or ivory finish. For 200'250 1 .v. 50 c.p.s. All components, mains transformer, cabinet, dial and metalwork. LASKY'S PRICE, £5.10.0. Post & pkg. 2/6 extra. NO. 3. 4-WATT A.C. AMPLIFIER.— Uses 1 each 6SL7, 6V6 and 5Z4. All components, chassis, valves, output trans., mains trans. LASKY'S PRICE, £4.5.0. Post & pkg. 2/6 extra. Instruction Books for the above Kits, price 1/- each. coil. Less output trans. 5in..... 14/6 8in. 6iin. 15/- lOin. 19/11 .19/6 ENERGISED SPEAKERS 8in. With O/Trans. 600 Q 15/6 Sin. Less O/Trans. 600 a 12/6 8in. Less O/Trans. l,200n 12/6 6iin. With O/Trans. 600 o 14/- PLASTIC ESCUTCHEON MASKS With dark screen filter. 12in 12/6 16in 25/- BRIMISTORS Type CZ.l. 1/6 each. CZ.3, 10i — I agree with the comments of your corres- pondent, Mr. S. T. Pinder, in his letter in your January issue and in connection with my contribu- U u. n i What is Hi " Fi ? " For an y de gree of pleasur- able listening, balance is perhaps one of the most important matters, but the Hi-Fi enthusiast usually claims a large measure of a reality of reproduction. We have heard in a number of recent demonstra- tions some astonishing reproductions of percussion broken glass, and other " noises," but somehow although one admires the accuracy of such repro- ductions there appears to be still something lacking in the reproduction of certain musical instruments and orchestral effects. This in spite of assumptions that extended range covers harmonics, and deals February, 1955 with transients, and that cross-modulation and phase distortion is not present. One is left with a feeling that in many cases the extension of the top range has satisfied the Hi-Fi enthusiast that he has put realism into his repro- duction. A non-technical but keen violinist recently suggested to me that Hi-Fi reproduction now gave a fairly true rendering of violin tone in a general sort of way, but it was still incapable of giving reproduction which would discriminate between the tonal qualities of a good violin played by a master and an inferior instrument. It would be interesting if some of your readers with a keen musical ear and some knowledge and experience of instrumental work, would give us their considered and critical opinion of Hi-Fi reproduction such as is so called to-day. Are there still some fields to conquer in this direction ?— " Grip Current " (Middlesex). Series Modulation gIR— Your correspondent C. McLean in the December issue of Practical Wireless uses the subject of series modulation as a plea for the granting of higher power. I should like to draw his attention to your article in the November issue, which gives some good examples of modulation at a very reasonable cost to anyone wishing to build them. The most reasonable one being, of course, clamp modulation, and, if Mr. McLean will also consider it, there is NBFM. Any of these will modulate a full 150 watts efficiency, enough for the needs of the most exacting amateur. On the subject of higher power, I would rather suggest a decrease to something like 75 or even 50 watts maximum, and if your correspondent should ever visit this country and listen to the 14 Mc/s 'phone band when the DL4 stations are in full swing with their BC610 and ET4336 transmitters, he will see my point of view.— K. Smethurst (DL2UY) (B.A.O.R. "Poetical ***** 0. 0W Newne., ^|TAttt?RW WC* Revered at the ®CB iiW^^"^, Modern Reflex Receiver §IR — I built the " Modern Reflex Receiver," two months ago, for use on A.C. mains with a trans- former for supplying the heater current. On moving to my present address, I found that the supply was D.C. As the normal current dropper was rather large and would not fit into my cabinet, I tried a 60 watt bulb in series with the filament and 230 nd iTc lt WOrked exceJ,ent, y> tne supply being I have now fitted the lamp to the set and so have a compact reading lamp and wireless. Perhaps other readers will find the above suggestion useful. The set gives good volume on all stations using the fender as an aerial. The set measures 7in. x 7in. x 4m.— H. Parker (Clacton-on-Sea) Thone : Temple Bar 4388. — ■ — — iw transmission oy Canadian Magaa February, 1955 _ PRACTICAL WIRELESS 123. MONTROSE mULTimETEB 7 1-^^ft MODEL 44A )A.C. & "ctrrlnt^ U 0-30 U mA-300"rnA-3 Amperes < D.C. Circuit Tests-Self-contained, easily accessible U v. cell wreuu i« provides quick continuity testing. ReliabilitV-Robustly built moving iron movement, designed Keiiapiiiiy ^ ^ ^ _ everyday use . Convenient posket size— Weight 8 oz. Price £2. 10. 0. Prompt delivery. Please note that other Taylor instruments are available on » no Please note in ' 3 montbs - cre d,t scheme. Ci« h£ PurfhSe terms for .0 and .5 monthly instalments. Some typ/co/ examples are : ass*** 3 MONTHS Deposit £1.8.6. And 3 monthly payments of £3.0.7. Deposit £1.17.6. And 3 monthly payments ot £3.19-8. Deposit £2.17.0. And 3 monthly payments of £6.1.2. 10 MONTHS Deposit 19/5. And L 10 monthly payments of 19/1. Deposit £1.5.8. And 10 monthly payments of £1.5.1. Deposit £1.18.0. And 10 monthly payments of £1.18.3. 15 MONTHS Deposit 19/5. And n monthly payments of 13/6. Deposit £1.5.8. And 15 monthly payments of 17/9. Deposit £1.18.0. And 15 monthly payments of £1.7.1. TAYLOR ELECTRICAL Montrose Avenue, Slough, England INSTRUMENTS LTD. Telephone : SLOUGH 21381/3 -WEYRAD SIGNAL GENERATORS "t oduced": ^ circuit which provide superior performance and higher efficiency. Fundamental coverage 100 Kc/s-70 Mc/s Calibra- tion accuracy ±2% on all bands R.F output r w or M C W. to co-ax. cable. Separate tixea 'A - r output Controls on front V™*-™K fwiich! aaenuator, tuning, modulation on/off i and mains on/off. . ^ ;« ctont metal case finished cream with & '"handle! Front pane, black with natural lettering. 1 S.G.M.I. — Mains operated with double-wound Impregnated transformer. | S G>B . |. — All-dry battery operated. Illustrated Leaflet, 2d. WEYMOUTH RADIO MFG. CO. LTD CRESCENT ST., WEYMOUTH. DORSET T/V TECHH0L0GY RADIO ENGINEERING ELECTRONICS RADIO SERVICING There's a big future in T/V and Rad.o. Act now I Increase your knowledge. Back up experience with a sound theoretical background. I.C.S. offer courses of instruction in — ^hSS^^^AVI radio Sl^mentary elect r onics FREQUENCY MODULATION I.C.S. will also coach you for the following examinations : — HIRE • P.M.G. Certificate for Wireless Operators; Radio Servicing .Certificate (RT.E.B.) ; C. &G. Telecommunications, etc., etc. noWT DELAY— WRITE TO-DAY for free descriptive until successful. Reduced terms for H.M. Forces. Dept. I70D, I.C.S.,JIJXost. Complete construction booklet with theoretical and practical diagrams, also price list of recommended parts. 1/6 post free. THE SUPEREX " ATTACHE " ALL DRY PORTABLE (BUILDING COST S//15-). A really super*. » VALVE SUPERHKT receive r. giving nrst-ekiss results on Long and Medium Wavebands. Cabinet is compact (Size Hi in. X8 4 in. x 4]in.) of attache case type, covered twin colours, high quality leatherette >\ fight complete receiver Jess batteries 8lbs. All eompo- Au'iM'f'l 0 ' h '« hest f? rade: OSMOIl JliCH O COILS and FRAME- *--.W J ' . PI( ' SS( T-Amplion Midget I.F.T.s. Provision also made for Tin. X . vt " al Speaker. Valve line-up : 1 U5. 1T4, IS.".. :;V4. Send for BOOK- free glving ful! building details and piactical wiring diagrams. 1/6 post THE SUPEREX UPRIGHT PORTABLE (BUILDING COST £7/15/-) Chassis assembly and components almost identical to attache model with exception ot cab.net. This is of the upright model with la rye speaker ape r- ture in front, dial and controls under lift-up lid in top of - abinet. Cover-d «nni?? iT% , 1 i' athcreU( '- Cabinet size: J 0J in. ,. 8 j in. x 4 ; in. fcend for iJUUKJiLl, 1/6 post free. S.R. 3-4 WATT AMPLIFIER KIT (iiriLDlNO COST £4/15/-). Designed on attractive lines and incorporating Bass, Treble, Middle, and Volume control*. Suitable for most types pick-ups. Chases bainin-i-finished mottled grey or bronze and given professional finish with engraved control knobs. Complete P. IT. and L.S. panels, O/P Transformet, Mains Trans- former, etc. Aalve Une'np : 0SO7, CX.-gt, HVligt. For A.C. Mains 210/240 v ..its . Send lor LKAFLFT giving home construction details. 6d. post free, ihe above amplifier is available ready built and tested, price £5/5/- post Large range of Radio and Television components and special purpose valve* at competitive prices. TERMS : Cash with order or C O.D. Extra charge for C.O.D. Please add postage. (U.K. and N. Ireland only.) OPEN : 9 a.m. to li p.m. WELCOMED ' SaUUVl:i - 1 P ' m ' Thursday - PERSONAL CALLERS SUPERIOR RADIO SUPPLIES 37, Hillside, Stonebridge, London, N.W.I0. Fhone : FLGar 3644. SOUTHERN RADIO'S WIRELESS BARGAINS TRANSRECEIVERS. Type " 38 " Mark II (Walkie-Talkie). With 5 valves and ready for use. Metal carrying case. Less external attachments. 30/- per set. TELESONIC 4-Valve Battery Portable. Complete with Hivac Valves In Metal Carrying Case. Simply converted to Personal Portable. £2 including Conversion Sheet. TRANSMITTER-RECEIVERS. Type " 18 " Mark III COM- PRISING SUPERHET RECEIVER and TRANSMITTER ' TWO UNITS CONTAINED IN METAL CARRYING CASE. CompleS 8-Valves. £4/10/0. R F£f'^ E n RS TYPE " 109 HALVES WITH VIBRATOR PACK FOR 6-volts BUILT-IN SPEAKER. 1 .8 to 8.5mc/s. Contained in Metal Case. Perfect. 100 ONLY. £5 BOMBSIGHT COMPUTERS. Ex-R.A.F. New. Contains Gyro Motors, Rev. Counters, Gear Wheels, etc. etc. Ideal for Model Makers, etc., £3/5/0, plus 10/- carriage. CRYSTAL MONITORS. Type 2. New in Transit Case. Less Crystals, 8/- each. m UF , B v R a A "OLE CUTTERS. ADJUSTABLE fin. to 3Jin. For Metal, Wood, Plastic, etc., 6/6. RESISTANCES. 100 Assorted. Useful Values, Wire End 12/6 per 100. ' CONDENSERS. 100 Assorted. Mica, Metal Tub, etc., 15/- 103 PLASTIC CASES. I4in. by lOJin. Transparent, Ideal for Maps,' Photos, Display, etc., 5/6. f T * R IDENTIFIERS. Type I A-N. Covers both Hemispheres. In Case, 5/6. CONTACTOR TIME SWITCHES. Complete in Sound Proof Case. 2 Impulses per sec. Thermostatic Control 11/6 REMOTE CONTACTORS for use with above, 7/6. MORSE TAPPERS. Standard Type ex-Govt., 3/6. Heavy Duty IfB™ 0 *! 8/6 COMPLETE MORSE PRACTICE SET with bUZZcR, o/9. SSSSSPlS RELAYS SWITCH. Bakelite, 5 c/273, 2/6 each. METERS AND AIRCRAFT INSTRUMENTS. Only need adjustment or with broken cases. TWELVE INSTRUMENTS IT^MS*'" 8 3 brand New Aircraft Instruments), 35/- for TWELVE Postage or carriage extra. Full List of RADIO BOOKS, 2Jd. SOUTHERN RADIO SUPPLY LTD., II, LITTLE NEWPORT STREET, LONDON, W.C.2 GERrard 6653. February, 1955 PRACTICAL WIRELESS 125 G2AK This Month's Bargains G2AK RACK SIZE CHASSIS.— I7in. long x 2im. deep x I2in., U/6: x IOin M 15/- ; x 8in., 14/-. P. & P. |/-." All 16 s.w.g. ALL POCKET VOLTMETERS. Dual range, 0-15 v. and 0-250 v., 345 O.P.V., M.C. Worth 50/-. Our price 17/6 post free. CRYSTAL HAND MICROPHONES. High quality, very sensitive. Chrome finish, complete with screened lead and standard jack plug. Our price only 25/- ea. Few only. VALVES. B7G base, IT4, IS5, IR5, IS4, 3S4, 3Y4, 7/6 ea., or 4 for 27/6. 807 's, 10/- ea. or 2 for 17/6. Most of the 1.4 v. B7G range available at 8/6 ea. HEADPHONES. Low resistance type CLR No. 3,-9/6. DLR No. 2, 13/6. High resistance CHR Mar 2, 17/6, and the most sensitive of all DHR, No. 5B, 18/6 per pair. P. & P. I /- pair. METERS.— 0-5 ma. 2in. square, i0/-. 0-50 ma., 7/6. 0-10 A., D.C., 7/6. 0-1 ma., 20/-. 0-350 ma. thermo., 7/6. 0-4 A., 5/-. 2Ain. flush 0-100 ma., 0-10 ma., 12/6 ea. Germanium Diodes, 2/- ea., or 6 for 9/-. Deaf-Aid Crystal mike units, 12/6 ea. V.H.F. FANS. Air Space Co-axial Cable, 150 ohm, good to 600 Mc/s ; normal price, 3/1 1 per foot. Our Price, 20 yard coil, £1. Very limited quantity available. FISK SOLARISCOPES — Complete with charts. Give World time, light and darkness paths. Invaluable to the DX man. List 21/-, our price 7/6, post free. PANL Home Crackle. Black, Brown or Green, 3/- tin. P. & P. 8d. ' TEST METER. 7 ranges as follows : 1.5 v. 3 v. 150 v. 6 ma., 60 ma., 5,000 ohms, 25,000 ohms, 2Un. Dia. scale M.C. meter. Rotary selector switch. Black bakelite case, 6 x A\ x 4}, fitted with removable lid, also provision for internal batts., ranges can be easily extended. Bargain Price, 30/-, plus 1/6 post NEW MULLARD QUALITY AMPLIFIER. Chassis and all specified parts, available from stock. Postage free on all orders over £1 except where specifically stated. PLEASE PRINT YOUR NAME AND ADDRESS. C. H. YOUNG, G2AK All callers. Mail Orders to Dept. M P " 1 10, Dale End, 102, Holloway Head, Birmingham 4 (CEN 1635) Birmingham I (MID 3254) /t5te4f&46*ty?Mviays specify ERSIN MULTIC0RE to be precise Wherever precision soldering is essential, manufac- turers, engineers and handymen rely on multicore. There's a multicore solder just made for the job you have in hand. Here are some of them. SIZE 1 CARTON 4 specifications for radio enthusiasts. TAPE SOLDER MELTS WITH A MATCH ! Real tin I lead sol- der containing cores of Ersin Flux. Needs no soldering iron or extra flux. 1'- PER CARD HANDYMAN'S CARTON Sufficient for 200 average BIB WMSTWPPER AND CUTTER , , ,„ , „,„; For stripping insulaKon rough edges and splitting J^g EACH extruded flex. MANUFACTURERS ARE INVITED TO WRITE FOR DETAILS OF BULK PACKS AT BULK PRICES MULTICORE SOLDERS LTD. MULTICORE WORKS, HEMEL HEMPSTEAD, HERTS (B0XM00R 3636) |j MONEY BACK II GUARANTEE \J* & GRA66771 Eebl » [621 ROMFORD RD. LONDON. E.I2. | T.V. TUBES £5 - 12" Site 6 TMc n TT S n 11,ng f °^ e fe* three years - Picture shown to callers. INSU-Carr. and Packing 15/6 extra. hS S L^y ] r.??H?S / : , T M S st J nak l s and types - al] work Perfectly. • r^H™ V L C e ^ h ~ de t° Hea J*2 sh ? rts or sh % ht burn . Weal for testing- or spares. Insured Carriage 15/6. AMPLIFIERS, 77/6.— Push-pull. J 4 valve and rec. Full tone range, variable Output 3-7-15 ohms matching. A.C or Universal. Ready to plug in. Post 2/6. ■ '^5KF^? S V 57/ ft~t watt , s .' 3 valves. Switched tone range plug ?n Post 2/6 5u;.. 6iin., 8in.. 10in.; prompt cash WM. CARVIS LTD., 103, North St., Leeds, 7. AUCTION SALE7 E^t^rT~late Jan.', Electrical Components. Postal bids accepted. Catalogue 6d. from SAMP- S ONS, Aucti oneers, Dawlish, Devoa A.M.I.Mech.E., A.M.Brit.I.R.E. City and Guilds, etc. on " no pass— no fee " terms; over 959r successes. For details of exams, and courses in all branches of engineering, building, etc., write for 144-page handbook free. B.I.E.T. (Dept. 242Bi 29 Wrignt's Lane. London W.8 CITY AND GUILDS (Electrical, etc.) on " no pass — no fee " terms. Over 95 f /r successes. For full details of modern courses in all branches of Electrical Technology send for our 144-page handbook— free and post l£ e ?^, B.I.E.T. (Dept. 242A). 29, Wright's Lane, London, W.8. ALUMINIUM CHASSIS made to your requirements; 16 & 18 gauge; any quantity, large or small. We will be pleased to quote. MACHINE CONTACTS. Building 336, Hurn Air port, Christchurch, Hants, AMERICAN MAGAZINES.— One-year 'Audio Engineering," 35/-, specimen copy, 3/6; " Popular Science," 28/6; High Fidelity," 50/-. specimen copy Utt t rJ r ? e ™ b0( * let Quoting others. WILLEN LTD., Dept. 40, 101, Fleet Street, London, E.C.4. THE "BRENELL" TAPE DECK in- corporates many important design features originally developed for the famous Sound Master. A new toggle- action grip between capstan and pinch wheel ensures a slip-free tape drive and eliminates the chief cause of " wow " and flutter. Three motors are used and the highly ingenious interlocking switching gives instant control of the tape drive in either direction without the slightest risk of damage. Three speeds are avail- able 3|in.. 7iin., and 15in. per second with a playing time respec- tively of 2 hrs., 1 hr. and 30 min. The Brenell Record/Play- back head is a twin-track high-im- pedance type requiring no matching transformer. Due to its exception- ally small gap it will extend the frequency response to at least 11 Kc/s at a tape speed of 7 J 2 in. with a suitably corrected amplifier. The Brenell Erase head is a low-im- pedance type which requires to be led from a low-impedence winding on the oscillator coil. Beautifully made to the highest precision stan- dards the Brenell Tape Deck used with an appropriate amplifier is ideal lor the new H.M.V. pre-recorded Tapes Price £15/10/- complete. S^ n AJ or illustrated leaflet to HOTAX PRODUCTS LTD.. 59, Grays Inn Road, London. W.C.I. Trade en- quiries only to BRENELL ENGIN- EERING CO.. LTD.. 2. Northington Street. London. W.C.I. MIDGET AMPLIFIERS, £3/19/6, p. & p. 2/6, having built-in power- pack for 200-250V A.C. Dimensions 10 x 3 x 2i, fits all normal record players leaving room for speaker 4 watts quality output, suitable for all speakers and with standard or L.P. pickups. Valves 6J7 and 6V6 avail- able at 20/- per pair extra if re- quired. Other models from £4/12/6, oo^ TA ? abinets ^ and accessories. SPECIAL : Our rtew RDJ midget 4 watt amplifier to fit into the Philip s " Disc Jockey " Is now avail- able Send 6d. for new 8 page illus- trated catalogue. ELECTRO-ACOUS- TIC LABORATORIES, Tain, Ross- shire. Scotland. THE INSTITUTE "of~Pr^c~tical Radio Engineers Home Studv Courses are suitable coaching text for I.P.R E and other qualifying examinations, fees are moderate. Syllabus of l e J£R modern courses post free from SECRETARY, I.P.R.E., 20, Fairfield Road, London, N.8. FREE ! Brochure giving details of Home Study Training in Radio, Television, and all branches of E.eccronics. Courses for the Hobby Enthusiast or for those aiming at the A.M.Brit.I R.E.. City and Guilds, k.i.e.b. and other Professional examinations Train with the college operated by Britain's largest Elec- tronics organisation; moderate fees. Sx, 1 ^ t0 E \ M - 1 - INSTITUTES, Dept. PW.L-8. Lo ndon. W.4. SEE THE WORLD as a Radio Officer. Short training; low fees- scholarships; boarding/day students' Stanip for prospectus. WIRELESS COLLEGE. C olwyn Bay. BOOKLETS. "How to Use Ex-Gov Lenses and Prisms." Nos. 1 and 2* price 2/6 ea. Ex-Govt. Optical lists f r - e( L f £ r s - a - e - H - ENGLISH, Ray- leigh Rd.. Hutton. Brentwood, Essex. «?A?' E \ 1 " rECHN,CAI - PUBLICATIONS. 6,500 Alignment Peaks for Super- heterodynes, 5/9, post free. Data for constructing TV Aerial Strength Meter, 7/6. Sample copy The Practical Radio Engineer, quarterly publication of the Institute 2/-- membership and examination ' data' 1/-; Secretary I P.R.E., 20, Fairfield Rd., London, N.8. M E R C H ANT" NAVY Wireless School Overseas House. Brooks' Bar. M'cr 16' THE WIRELESS SCHOOL.-Training ^ttS 1 " 61 ^ 8 ' Telegraphy. RADIO HOUSE. Manor Gdns.. N.7. W,REI -ESS.— Day and Evening Class instruction for P.M.G. Certificate of Proficiency and Amateur Wireless Licence. Morse instruction only if required, also postal courses. Apply B.S.T. LTD.. 179, Clapham R±, London. S.W.9. SERVICE SHEETS.— Send TT- now 4,000 Sheets, listed from 2/3 RadfS Tt' V ' . sh eets; post only. RATEL, 171, Noms St., Preston. 7 ■■■I February, 195 5 PR A CTICAL WIRELESS CLASSIFIED ADVERTISEMENTS 127 WANTED, Midget Power Pack for M.C.R.l. CORNISH, 4, Bay Villas, Long Rock, Penzance. THE CHEAPEST Multi-range Meter on the Market. Measures : 0-6v D.C., 0-240v D.C., 0-30mA D.C., 0-6v A.C., G-240v A.C., 0-30mA A.C. Plus a circuit testing scale powered by internal battery, and a valve socket for testing English 4 & 5 pin valves. Brand new in makefV boxes with instruction leaflet at 29/6 each, post 1/6. Neon Testers, screwdriver type, operates from 100-500v A.C. or D.C. and is a very useful pocket screw- driver, complete with protecting cap and pocket clip, 6/- each. VCR517S O.K. for TV, etc., no cut-off, 19/6, post 2/6. 2in. 0-lmA Meters, 14/6 each. SERVIO RADIO, 156/8, Merton Rd., Wimbledon, S.W.19. (LIBerty 6525.) TELEKIT SUPPLY MAIL ORDER DEPT. Chantry Lane Works, Chantry Lane, Bromley, Kent. Phone : Rav. 5845. Please mention P.W. and enclose 6d. postage VALVES, NEW & SURPLUS. GUARANTEED OZ4 # ID4 5/6 ILN5 (soiled) 5/- IS5 IT4 IU5 3V4 5763 6A8 6AL5 6AM6 6AT6 6B8 6BA6 6/- 6/6 6/- 6/6 8/- 8/- 7/6 6/- 7/6 6/- 6/6 6BE6 6BR7 6BS7 6BW6 6CH6 6J5 6V6 6X4 6X5 7D5 7D8 7H7 7S7 9D2 9D6 6/6 8/6 7/- 6/6 6/6 5/- 7/- 6/6 7/6 6/- 6/- 6/- 8/- 5/- 6/6 11D3 6/' 12A6 1299 (soiled) 5/- 12BE6 7/- 6SH7(soiled) 51- 15D2 4/- 19AQ5 10/- AL60 EF50 VR21 W77 1B24 4/6 5/- 3/- 6/6 50/- BECKENHAM, KENT THE SHOP FOR THE CONSTRUCTOR NOW OPEN AT 104, II Kill STREET. BECK EN II AM 3720 STAN WILLETTS 4 & SPOIV r -ANE, WEST BROMWFCII, S\4?EIi Tel : WE S. 2392. VALVES. — Brand New. Every one guaran- teed. 6SN7, 6SL7, 7/6. 6K7, 4/6. VT52. (EL32), 5/-. 6F6G, 5/6. 6V6GT, 6X5GT. 6/9. 67/- doz. 6K8G, 8/6, 90/- doz. RK34, 2C34, 1/9, 18/- doz. ATP4, 3/6. VR65, (SP61), 2/-. KA50 1/9. 6BW7, (EP80). 7/9. EC52, 4/9. 1T4 1R5. 1S5. 3S4, or 1S4, 24/9 Set of 4, 6/9 each. P. & p. 6d. TltlPEETT 1M/A METER RECTIFIER. Brand New with Circuit, 5/6. P. & P. 4d. GERMANIUM XTAL DIODES. — G.E.C. Brand New. 1/-, 10/9 doz. P. & P. 3d. SET OF PARTS.— For 2-Waveband Crystal Set with Circuit. 7/11. P. & P. 6d. VOLUME CONTROLS, with S.P. Switch. spindle. I and 1 meg., 2/-. Post 4d. HEATER TRANSFORMERS.— 230 volts input. 6.3 at 1J amps, output. 5/6. P. & P. 9d. WANTED— VALVES Best Prices Offered PULLIN SERIES TOO TEST METER AC/ DC IO.OOO/1/V 21 RANGES lOOpA to IOOOV COMPLETE IN DIE-CAST CASE WITH TEST LEADS CLIPS AND PRODS FULLY GUARANTEED SENT POST FREE FOR £2. lOs. DEPOSIT AND TEN FURTHER MONTHLY PAYMENTS OF £|. CASH PRICE 1 1 GNS ^jTrttA "Hone 56927 _ J RADIOCRAFT^ 69-71 CHURCH GATE LEICESTER RADIO UNLIMITED, offer: Valve Sets, 6V6, 6K7, 6K8, 6Q7, 5Z4, 31/-; 1T4, 1S5, 1R5, 3S4, 28/-; 6V6M, 6K8G, 6SN7, 6SL7. EBC33, 5U4, 6X5G, 6BE6, EF91, EB91, EF39, 807, EF50, 6J7M, 6AK5, EF92, 7/6 each; ECL80, 9/-; EF80, 10/-; KT33c, 9/-; EF37a, 9/-; EBC41, 8/6; UY41, 8/6; ML,6, 6/6. Heater Trans., 200/240V, 6.3v at 1.5A, 5/9. Mains Trans., 350/350v, 6v, 5v, 4v, 4v, 80 MA, 19/6. V/controls, \- meg, lin. Sp., 1/- each. Crystal Diode, 1/6. Full range Instrument Wires, Nuts, Bolts, etc. List. Two-valve + Rec Amplifier kit. incl. valves, 59/6. Full range Teletron Coils in stock. Full lists available. RADIO UN- LIMITED, Elm Road, London, E.17. Callers to Eastern Road and Hoe Street, London, E.17. (Telephones- KEY 4813 & LAR 6377.) TELEVISION The advance of Radio Technique will offer unlimited opportunities of high pay and secure posts for those Radio Engineers who have had the foresight to become technically qualified. How you can do this quickly and easily in your spare time is fully explained In our unique handbook. Full details are given of A.M.Brit.I.R.E., City & Guilds Exams., and particulars of up-to-date courses in Wireless Engineering, Radio Servicing, Short Waves, Television, Mathematics, etc., etc. We guarantee "NO PASS— NO FEE." Prepare for to-morrow's opportunities and future competition by sending for this very informative 144-page guide NOW -FREE and without obligation. BRITISH INSTITUTE OF ENGINEERING TECHNOLOGY (Wept. 242), COLLEGE HOUSE, 29-31, Wright's Lane, Kensington, W.8 MULTIMETER KITS. 2 lin. M/C meter. Reads D.C. volts 3-30-150-300-600 mA, 0-60 ohms 0-5,000. With Bakelite Case, 27/6. 6 v. VIBRATOR PACKS. Self contained in Steel Box. Output 150 v. 30 mA, 22/6. Ideal for Battery Radios^ FISHING ROD AERIALS. 12ft . Set 3, 7 /6. Mounting Base, 3/6. 0 /5 AMPMETERS, 2jin. " sq uare M/c, 11/-. GERMANIUM DIODES, 3/9^ FREQUENCY CRYSTALS, 9,100 and 4,500 Kc., 10/6 ; 4,860, 4,800, 2,400, 594, 4,250, 505 Kc, 6/6. Jin. Space Pins. SELEN I U VI RECTIFIERS. F.W. 6 or 12 v. 4 A., 22/6 ; 6 A., 30/- ; 3 A., 14/6 ; 100 mA., 3/6 ; 1 A., 8/6 ; 24 v. 2 A., 30/- I 250 v. 100 mA. H.W.,9/- ; 250v. 275 mA..17/6: 250 v.60 mA.,6'6. TRANSFORMERS, 200-240 "volts, tapped 3-4-5-6-8-9-10-12-15-18-20-24 and 30 volts at 2 A., 21/6. Tapped 17-11-5 volts 5 A., 22/6 ; Tapped 17-11-5 volts 1 1 A., 16/6 j 6.3 v., 2i A., 8 /6. One year guarantee. M/C MICROPHONES AND TRANS- FORMERS, 15/6. Ex-W.D. PHONES. Low Resistance, 9/6. NEW OAK, 6 v. 4-pin Vibrators; 10/6. MORSE KEYS, 3/-. BUZZERS, 3/-. 12/24V. BLOWER MOTORS, 14/6.A.C./D.C. CARBON TWIST DRILLS. Set of 7 l/16th to * or 9 l/16th to 3/16th. 3/6. U.H.F. AERIALS. 180-200 Mc/s covering band, complete with case, 6/6. OVERLOAD SWITCHES, Breaks at 1 Amp., 3/6. TR1196. Transmitter Section. New and complete. 4.6/6.8 Mc/s. Easily converted. Less valves, 15 /- ; with valves, £2^ Throat Microphones with cord, 4/-. Pocket V oltme ters, gin. M/c 0-20-200, 11/6. New VR65/SP61 Valves, 5/6, 4 for 20K ALL POST PAID IN U.K. THE RADIO & ELECTRICAL MART 253b, PortolM'llo Road, London, W.ll. 'Phone : Park 6026. WALNUT Radiogram Cabinets of distinction; stamp details. R.SHAW, 69, Fairlop Rd., E.ll. UNIVERSAL METER, Taylor Model 90A, BS.89, first-grade accuracy, 1,000 o.p.v., AC/DC, £8/10/- only; also Valve Tester, 45A/S, with adaptors measuring mutual conductance of almost all modern valves, £14. both in perfect condition, practically new. Write: STRAUSS, 5, Gloucester Ave., Slough, Bucks. AMERICAN RADIO Plans and De- vices. Now available. Hundreds of new and startling devices you can make. Only designs of this type available in UK. Receivers, waikie talkies, amplifiers, recorders magic eye alarms, etc., etc. Full lists, data, illustrations Free for stamp. Send to-day. — A.P.S. (PW), Sedgeford King's Lynn. CONDENSER CLEARANCE ! ! All new goods. Types at same price may be mixed to make up dozens. MICA. — 10, 15, '20, 30, 50, 70, 75, 120, 150, 200, '2'20, 1,300 pP., 3/6 doz. Larger type 400, 4.50, 700 3,000, 4,500 pb\. 2/6 doz. Our assortment. .50 for 8/9* REALLY MIDGET TUBULARS. — M ire ,nded' 0.04, 0.01, 3/6 doz. STANDARD SLZK, o.yj »,fd' 1,000 v., 3/6 doz. METAL BLOCK. — 0.2.5 mfd. 350 v.. 0 for 1/3 6 MFD. 2,000 v. TEST. — Metal block, 3/9, post J/-' COAX . — Stranded core, 80 o, app. jin., 6d. yd Coax fittings: universal plug Hid., socket, 1/3, outlet box, 4/6, line connector (double socket) 1/3 SPECIAL : 32 mfd. 450 v. (U.S.A.) 4 1 in. x 2in , 1/6 ELECTROLTTICS. — 40 mfd. 450 v., 4/-; 8 + J(j 450 v. (small), 4/3 : 16 + 32, 350 v., 4/9 ;' 32 + 32 350 v., 5/3 ; 32 + 32, 450 v. 6/-. PV(J covered types : 16, 450 v., 2/9 : 50, 50 v., 2/- : 25, 25 v. 1/9 WIRE WOUND RES. — Cement coated, wire ended 25, 50, 100, 150, 200, 250,350, 500, J, 000 J r>on' _',oon, -J..500, 5.000, 1.0,0000, 5 w., 1/3 ; 10 w i/e : 15 w., 1/9 ; 15, 20, 25, 30 K, 5 w., 1/9 ; 10 w.t 2/3 : 50 K, 5 w., 2/3 ; 10 w., 2/9. ' Over £1, post paid. List 3d. REED & FORfl 2A < BURNLEY ROAD, hceu ot rvnu, ainsdale, southport RECEIVER R1155 BRAND NEW. AERIAL TESTED In maker's original transit case. Now is the chance to get one from the best delivery we have had from the Ministry. £11.19.6. Carr. 10/6 Send S.A.E. for further details or 1/3 for publication giving circuit diagrams, etcr Others available from £9.10.0 according to condition. HEADPHONES.— High Resistance Type CHR. New. 12/6 pair, post 1/6. MIC ROAM METERS.— 250 F.S.D. 3ifn Flush Model S37. Specially scaled for test meters. Knife edge pointers, magnetic shield. Brand New. 55/-. VOLTMETERS.— 0/300 Flush D.C. Moving Coil, 10/6 ; 0/20, 2in. Flush Moving Coil, 7/6 : 0/40 21n. Flush M.C., 10/6. post 1/-. CONDENSERS. PAPER BLOCK TYPE .5mfd.. 1,000 v. 2/6 I 8mfd., 500 v. ... 7/6 4mfd., 350 v. ... 4/6 8mfd., 750 v. ... 10/6 4mfd., 2,000 v. ... 7/6 I lOmfd., 350 v. 10/6 WILCO ELECTRONICS 204, Lower Addiscombe Rd., Croydon" H A C EQUIPMENT Noted for over 18 years for . . . S.W. Receivers and Kits of Quality. Improved designs with Denco colls : One- Valve Kit, Model "C" Price, 25/- Two , "E" „ 50/- All kits complete with all components, accessories, and full instructions. Before ordering call and inspect a demonstration receiver. or send stamped. addressed envelope for descriptive catalogue. "H.A.C." SHORT-WAVE PRODUCTS (Dept. TH), 11, Old Bond Street, London, W.l. 128 PRACTICAL WIRELESS Februar y, 1955 An OPPORTUNITY to secure a super quality TRUVOX BX.II I2in. moving coil permanent magnet loudspeaker at a bargain price. Brand new in maker's carton. Available in 3 ohm speech coil only. Limited quantity at the special price while stocks last of ONLY SOf- carriage paid. Our full list of loudspeakers will be sent on receipt of your name and ad- dress and 2!,d. stamp. A TESTER IN YOUR POCKET Combined Neon Tester and screwdriver. Complete with clip for pocket and shield for screwdriver blade. Tests A.C. and D.G. voltages 1 10 v. to 500 v. An essential pocket instrument for every engineer and constructor. Send for one to-day. PRICE ONLY 6 - POST PAID We carry the full range of components for tha " Fury Four," Mullard 5-10 amplifier, G.E.C. " 912 " amplifier, etc. Send S.A.E. for price list. Dealers and bona fide radio engineers send for our trade list of condensers and resistors, etc. HOME RADIO Of MIICHAH 187 London Road, Mitcham, Surrey MIT 3282 Open every day including Saturday until 6.30 (Weds. 1 p.m.) If you lack the qualifications which would get you a better job; more pay and quicker progress; if you wish to know how The Bennett College can guarantee to teach you up to qualifi- cation stage by one of the easiest, quickest and soundest methods of mind training; if you wish to learn how Personal Postal Tuition can prove that you are cleverer than perhaps you think you are — if you like the idea of studying in your own time, at your own pace, with your own tutor guiding you, helping you, teaching you by post — send at once for this recent- ly published important book — 'Train your mind to SUCCESS'. It is quite free. Just fill in the coupon below and name the subject you are interested in (some of the many Courses available are listed here). Then send in the coupon to us TODAY. You will never, never regret it. But do it today. Act NOW ! COLLEGE (DEPT. B101F), SHEFFIELD Please send me, without obligation, a free co*y of " Train your mind to S UCCESS" and the College Prospectus on: subject , _AGE (if under 21) Im — - ; PUau wrltt in Block UVtf DIRECT FROM THE MANUFACTURER C H A S 8 I DULCI RADIO/RADIOGRAM Built to Highest Technical Standards FULLY GUARANTEED All chassis Hi in. x 7in. x 8iin. hisrh. Latest type valves 6BE6, 6BA6, 6AT6, 6BW6, 6X4. 1 Flywheel tuning-. Negative feedback over entire audio section. En- graved knobs. 3 tone positions for radio and gram. For A.C. Mains 100/120 and 200/250 volts Model B3.— Loner, Medium, Short 5 Valves. Output 3i watt Plus Push Pull Stage 6 Valves. Output 6 watt _ Stage. 7 Valves. Output 6 watt £18/18/0 Model B6.— Six Wavebands. Med., Long, 4 Short. k 1+ z»lg\ (3 Bandspread.) 5 valves. Output 3J w.** ■ *>/ I O/U Model B6.— Plus Push Pull Stage r-IQ/1Q/n 6 Valves. Output 6 watt *IO/IO/U Model B6.— Double Feature with P/Pull & R.F. f oO/O/n Stage. 7 Valves. Output 6 watt »^.0/ Jt / KM ALL PRICES TAX PAID Escutcheon for 9in. x 5in. dial, 4/9 extra. Matching speakers P.M. type 3 ohms. 8in. or lOin. available. Chassis sent under monev back guarantee conditions against remittance. Free particular;} from the manufacturers. Model B3.- Model B3.— botrtriTTFratare with P/Pull & R.F. £12/12/0 £15/1 5/0 THE 93 VILLIERS RD., DULCI CO. LTD., LONDON, N.W.2. Telephone : Willesden 7773 BUILD THIS AMAZING RADIO POWERFUL! PERSONAL! PORTABLE! # Selective tuning. 0 Acorn low drain valve. # Loud clear tone. © Long range. # No earth. # Short aeral, 2ft. 9 Welded steel case. # Easy to assemble. # All parts for this set are sold separately. Ideal for : £ Fishing, Camping. 0 Cycling, Touring. 0 On the beach, etc. MAIL ORDER ONLY This little set was de- signed to give you a real personal portable radio that you can enjoy anywhere without disturbing others. Use it on camping trips, in bed, in your offica, or just anywhere. Send 2/- for layout, Wir- ing diagram and Component Price List. Details of our 30/- Shart Wave Receiver are now read/. Send 2/- for Layout, Circuits and Component Price List. R. C. S. PRODUCTS (RADIO) LTD., 11, OLIVER ROAD, LONDON, E.17 EV PRACTICAL WIRELESS February, 1955 Build this HIGH QUALITY LOW COST AMPLIFIER it Circuit designed by Mullard research engineers -Ar Specified components available from most radio dealers. Here's an entirely new amplifier circuit which brings high quality sound reproduction within the reach of thousands more enthusiasts. It has been designed by Mullard research engineers with special regard for easy construction and low cost. Full details of the circuit are included in the 2s. 6d. b6ok which is obtainable from radio dealers or direct from Mullard Ltd. Valve Sales Department — 2s. lOd. post free. Get your copy now. EASY TO BUILD AT LOW COST GOOD TRANSIENT RESPONSE LOW OUTPUT RESISTANCE LOW HUM AND NOISE NEGLIGIBLE DISTORTION AT ALL OUTPUT LEVELS DESIGNED ROUND FIVE MULLARD MASTER VALVES EF86 ECC83 2 x EL84 GZ30 or EZ80 UNIFORM FREQUENCY RESPONSE IN AUDIBLE RANGE Mullard ^■■■■m I Mullard 1 mmmmmmtmmmmm^ MULLARD LTD., CENTURY HOUSE, SHAFTESBURY AVENUE, LONDON, W.C.2 J MVM3- 8C