The Williamson Amplifier
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THE WILLIAMSON AMPLIFIER Experimental audio amplifiers were first made about 75 years ago. Since then, a vast amount of work and research has gone into improvements. A major advance came in 1947, when a new design that raised standards of performance considerably was described in the British magazine Wireless World. Although during the period prior to but without a lot of care, the beam tet- plifiers had similar performances and in 1950, there were luxury receivers using rode was hard to stabilise against para- operation would have produced indistin- elaborate audio systems, few qualified sitic oscillations. Fig.2 is an example of guishable results. The deterioration at as being genuinely 'High Fidelity'. As a well designed beam tetrode equivalent low frequencies in the triode amplifier professional equipment was neither af- of the amplifier in Fig.1, again from can be attributed to an inadequate out- fordable nor readily available to the pri- AWV. put transformer rather than the basic vate user, enthusiasts and small manu- The test curves show that the two am- design. facturers usually 'rolled their own', often using designs that appeared in Wireless Weekly and later Radio & RADIOT RON 13.5 WATT AMPLIFIER Hobbies, A.W.V's Radiotronics , Wire- 6J7-G 6V6-G 2A3 less World from the UK and numerous American magazines. Two philosophies Pentode and beam tetrode output valves with their advantages of high sensitivity and efficiency had become standard in receivers, but they had the serious shortcomings of high distortion and the inability to damp down bass RADIO TUNER resonances in loudspeakers, due to high 250 V. 40mA. output resistance. Pf5E°IfOn Quality conscious enthusiasts contin- 9W. ued to insist on triode output stages, the 2,50011' FIELD ultimate valves being the American 2A3 9W. and its British equivalents. A pair of CIRCUIT NO. A 503 these could provide around 10 watts, DATE. 17. e• 39 DRAWN. MAY BE REPLACED BY A SINGLE FIELD 5,000 fl 18 W. but required upwards of 100 volts drive CHECK . between grids. Interstage driver trans- formers were strongly recommended, but suitable types required complicated A 503 FIDELITY CURVE winding configurations and were very [SPEAKER LOAD 4,008) II P-P] Fig.1: This AWV triode expensive. One solution was to use a amplifier circuit of 1939 large driver valve, as in the AWV cir- + 10 shows the performance cuit in Fig.1. achievable without 0 D negative feedback. Note Tetrodes suspect a. that the circuit was 0 drawn by Neville Multiple grid output valves simplified -10 20 design considerably. Their grid drive re- Williams, and checked quirements were less demanding than 2wo.H. by F. Langford-Smith. 4 F, to those of triodes and they were more ef- 5 TN. H. 111 4 T.. H. cc ficient. By using negative feedback, am- to4 0 plifier performance could be made to be 10 102 103 PER equivalent to a good triode amplifier, CYCLES SE CC ND Two factions Sound reproduction technologies have a history of attracting opposing opin- ions, and recent arguments in EA show a that this spirit is still alive, The triode 0 versus tetrode debate became quite 0 heated, so much so that early in 1946, the redoubtable Australian authority, F. Langford-Smith took a hand and wrote 0 putting the 0 an article in Radiotronics _J whole thing into perspective. He re- ported on the work done by the Amer- ican authority J.K. Hilliard, whose con- clusions were that beam power valves could deliver the same audio power as triodes with the same or less distortion. New standards By 1947, a typical high quality ampli- fier would have had a frequency re- sponse varying by no more than 1.0dB D.C. RES.= 500(1 from 40Hz to 10kHz and, above 100Hz, DRAWN harmonic distortion at 10 watts output CHECK air./2v? RADIOT RON CIRCUIT NO. A 146 of between 1% and 2.0%. With the DATE 26 -8 -38 78rpm shellac records and radio trans- missions then available as programme sources, these specifications were ade- quate, but improved methods were fore- I I TI 1_1 I I [ t i Mt cast. Fig.2: The tetrode A 146 FIDELITY CURVE Wartime developments had been in- equivalent of Fig.1, also SPEAKER LOAD = 4,800 a. P-P corporated in Decca's 'FFRW record- from AWV, with negative 1 I ings, improved pickups and speakers feedback taken from the 10 were becoming available, FM transmis- anode of the upper 6L6G ! OuT PuT sions were planned and German devel- to the screen grid of the 0 p-- ' opments in tape recording were being first valve. D revealed. It was time to improve ampli- -10 10 fier performance. One solution was obvious. If negative IC ON 5 feedback improved beam tetrode ampli- NI ,...•,,.. , RM 102 .......10Ill" A fiers, it should make a good triode am- -,....... H „ plifier even better. However in practice 1C 1O2 103 to it was not quite this simple. C YCLES PER SECONC Transformer problem In the amplifier of Fig.2, feedback is formers, causing the amplifier to be- Williamson's answer taken from the anode of the upper out- come unstable before a worthwhile put valve to the screen of the input amount of negative feedback could be In articles published in the April and valve, but it was not sufficient simply to applied. May 1947 issues of Wireless World, substitute low mu power triodes for the Low frequency transformer response D.T.N. Williamson of the Marconi 6L6G's. Apart from there now being in- could be degraded by insufficient pri- Osram Valve Company dealt at length sufficient gain, some distortion could be mary inductance, as the graph in Fig.1 with these problems and their solutions. produced by the output transformer. shows, whilst the high frequency end He concluded by giving constructional Furthermore, it may be seen from Fig.2 was restricted by inadequate coupling details of an amplifier which produced that because feedback came from the between windings. These limitations in- 15 watts with 0.1% distortion, and a upper output anode only, any incom- troduced sufficient phase shift at each frequency response that was absolutely plete coupling between the two primary end of the audio spectrum for the feed- flat between 10Hz and 100kHz, a re- sections would result in there being a back to become positive, with disastrous markable standard of performance. smaller feedback component from the results. Williamson tackled the transformer lower output valve. To avoid instability with a worthwhile problem head on. He produced a design For there to be any worthwhile pro- amount of feedback, a transformer that permitted its inclusion within the gress, the entire amplifier including the would need to have a frequency re- feedback loop operating with a 10 times output transformer would have to be in- sponse wider than that of its associated (20dB) gain reduction, with a stability cluded in the feedback chain. This had amplifier. Without careful design, add- margin of a further 10dB. been tried often enough, but the prob- ing extra voltage amplifier stages to The lack of inductance was countered lem was that serious limitations were compensate for the loss of gain could by the use of no less than 4400 turns of imposed by conventional output trans- also increase the phase shift problems. wire for the primary, on a core as big as that of a large power transformer — providing a minimum of 100 henries in- ductance. To overcome the high fre- I CH 4500 CH 2 quency coupling problem, the windings a •--- 410v1Ps •--• 4.4 MA 5.2SmA IOrA 125mA were divided into no less than 10 pri- R o R 2 —C5 mary and 8 secondary sections, all inter- leaved in two balanced halves. 320V R 7 C2= R23 C3 6 R3 CI RI5 215V 3v 38 v Simple amplifier 100V ' V RII R14 V - R8 R16 b No tricks were employed in the am- RIO ö plifier circuit itself. Just as in the circuit å OUTPUT u 1fl 2.7Y R12 g R21 R22 of Fig.1 there was a voltage amplifier ö R9 Rls (V1) and phase splitter (V2), but to • ____ -_-~. R13 R19 A' 0.190 provide the additional gain needed, 105V e4 - - RI C7 R20 there followed a push pull driver stage R Rza (V3/4). To minimise low frequency phase shifts, the voltage amplifier stage FIR4 i R25 • was direct coupled to the phase splitter and there were no cathode bypass ca- Circuit diagram of complete amplifier. Voltages underlined are peak signal voltages at 15 watts output. pacitors. CIRCUIT VALUES The output stage used triode con- R1 1 Mfg i watt ± 20 per cent R15, R20 1,000 S2 I watt ± 20 per cent Cs 8 µF 550 V, wkg. nected KT66 beam tetrodes. These RQ 33,000 f2 1 watt ± 20 R10, Rls 100 f2 1 watt ± 20 „ c 8 uF 600 V, wkg. needed about half the drive of 2A3 R. 47,000 S2 1 watt ± 20 ,i R,112, 100 f2 2 watt wire- CH, 30 H at 20 mA (min.) R4 470 f2 i watt ± 10 wound variable. CH2 10 H at 150 mA (min.) triodes, but required a greater HT volt- R5, Re, 117 22,000 Cl 1 watt -1- 10 R22 150 12 3 watt -4- 20 „ T Power transformer. R,, Rs 0.47 Mfg i watt ± 20 „ R23, R24 100 f2 i watt 20 „ Secondary 425-0-425 V age. KT66 valves were similar to the 150 mA (min.) 5 V. 3A,6. R10 390 S2 + watt ± 10 R25 1,200A/speech coil impedance, R11, R13 39,000 f2 2 watt ± 10 i watt.