When I Think Back... by Neville Williams Vintage radio receiver design 7 Enhanced audio, dualwave tuners & frequency changers During the latter half of the 1930's, the audio systems in Australian radio receivers were upgraded by the adoption of negative feedback—offering better sound for both radio and record reproduction. Up front, dual- or triple-band tuners extended their coverage to the international shortwave bands. Ornate edge-lit glass dials became routine, perhaps in anticipation of the day when they would give place to a video screen. While these and other developments represented the voltage gain of the stage. 6B7S/6G8-G could still serve as plug-in can be identified with the late 1930's, the The Radiotron Designer's handbook alternatives for the original types in most sequence in which they appeared is am- (1940) quotes the 75 as having an in- audio applications. biguous. In the pursuit of market share, herent grid-anode capacitance of 1.7pF However, the full variable-mu charac- manufacturers tended to major on dif- so that, allowing for a gain of 60, this teristic should enable them to be used ferent features at different times, with the translated into a dynamic (Miller effect) with variable bias and the valve(s) did, in advantage accruing to those that input capacitance of 105pF. fact, find limited use as gain-controlled managed to get it right in terms of sales audio amplers, supplementing normal appeal. Duo-diode pentodes front-end AGC systems. In this present article, it will be more While it could be argued that this was In practice, however, they found their realistic simply to discuss aspects of less of a liability than the audio bypass widest application as IF amplifiers in receiver design which characterised the capacitors included elsewhere in the cir- place of the traditional variable-mu period from around 1936 to the outbreak cuitry, engineers saw it as a needless 78/5D6/6K7 pentodes — the difference of war — an event which was to put treble loss of original signal that should being that the IF output could be fed to domestic radio on hold 'for the duration'. be avoided, even if only on principle. the 6B7S/6G8-G's own diodes for detec- Fig.5 in the last article typified the Valve manufacturers responded to their tion and AGC voltage. By so doing, the design of domestic 4/5-valve mains dilemma with the 2B7/6B7/6B8 series of entire tuner could be standardised around powered superhets of around 1935/6. duo-diode general purpose pentodes. As two valves, with an antenna feeding into Fitted with AGC and magic-eye tuning, an R-C coupled audio amplifier, these of- one end and an audio lead coming out the and offering acceptable audio quality, fered a potential stage gain of 100 but, other (see Fig.2). they gave their owners little cause for with a static grid-anode capacitance of The audio system could then likewise complaint. In a laboratory situation, how- only 0.007pF, the Miller effect be self-contained, ranging from a single ever, limitations were still evident — capacitance amounted to less than 1pF high-gain valve for an 'el cheapo' mantel which posed an ongoing challenge to — compared to 105! set, to something more pretentious for design engineers. Not surprisingly, for the cost of a standard or up-market models. So it was that while the 2A6/75/6B6 screen feed resistor and bypass It may seem like a small point, but it series of hi-mu diode-triodes appeared to capacitors, many engineers opted for fitted in with the emerging philosophy of meet the immediate need, engineers the diode-pentode rather than the regarding the audio system as an audio knew that they were lacking in treble diode-triode. amplifier in its own right, rather than response because of the so-called Miller Alert to a still further design option, extra stages stuck on the rear end of a effect. Incoming signals were being Australia's Amalgamated Wireless Valve radio set! 'shunted' by the valves' intrinsic grid- Co (AWV) devised a special variant of anode capacitance, rendered the more the 6B7, the 6B7S, followed by its octal- Power pentodes serious by the fact that the anode was not based equivalent the 6G8-G. Whereas Back in 1936, the greatest single simply an inert electrode but one that car- the original 6B7/6B8 had been classified limitation on audio quality in 4/5-valve ried an amplified version of the signal — as 'remote cut-off' pentodes, the 6B7S receivers had to do with the power output in opposite phase to the grid input. and 6G8-G were redesigned with a full pentode. The end effect, according to J.M. variable-mu characteristic, cutting off at With their high output impedance, Miller of the US Bureau of Standards, -43V — about twice the figure for the these exhibited an exaggerated treble was as if the grid-anode capacitance was original types. response and exaggerated harmonic dis- (M+1) times its actual value, where M AWV engineers reckoned that the tortion, when operating into the highly 28 ELECTRONICS Australia, February 1992 reactive load presented by a loudspeaker. it would be out of phase with the input overall frequency response would be They alsol imposed very little electrical signal at that point. made smoother. damping do the cone, resulting in an un- Inevitably, in counteracting or partially Similarly, if the stage(s) within the naturally (resonant or 'boomy' kind of cancelling the original signal — hence feedback loop generated spurious har- bass. the description 'negative' or 'inverse' monics, they would be fed back to the As noted in earlier articles, designers feedback — it would reduce the apparent input along with the legitimate signal. sought td counteract these effects by Being then amplified in reverse phase, wiring a dubular capacitor (e.g., 1 OnF or internally generated harmonics would 0.01uF) across the loudspeaker trans- tend to cancel themselves, effectively former Primary, and/or resorting to reducing the level of distortion. treble-cutlelsewhere. It sufficed as an in- Again, if the loudspeaker cone tended terim measure, but the need to find a to prolong sonic vibrations of its own ac- more fundamental solution to the prob- cord, the mechanically generated wave lem was hastened by the release of the trains would be fed back to the input of high-po r 6L6 beam tetrode, which the amplifier in reverse phase, and serve could ge crate high frequency transients ultimately to counteract the spurious across aalloudspeaker load of quite star- cone movements which gave rise to them tling --- d destructive — proportions. in the first place. In Ralliotronics > No.71 (December Research in Australia and elsewhere 1936), AWV published a circuit which established that a voltage gain reduction proposedlthe most radical approach of all of around 3:1 or 4:1(10 to 12dB of nega- to the overall problem, namely to replace tive voltage feedback) was sufficient to the outptit pentode with a 2A3 filament impart triode-like characteristics to a type power triode. power pentode output stage, in respect to Adequate drive to the 2A3 involved the vital parameters mentioned above. In the use or a 6C6 resistance-coupled pen- particular, the new and more economical tode, preeeded in the tuner by a 6A7 and 6V6-6 could be expected to behave like a 6B7S. In this so-called 'fidelity' design, a 2A3! the highest level of overall distortion at any signal level, any modulation percent- Simple circuits age and/fr any condition of loudspeaker load was said to be 7%. Since the deficiences in the audio end If this deems high by present standards, of a typical 4/5-valve receiver related to similar tests on a contemporary receiver the disparate characteristics of a power using an output pentode yielded a figure output pentode (or tetrode) and the com- of 30%! plex anode load presented by a Despit this evidence, and to the best loudspeaker, an effective negative feed- of my laiowledge, the idea was taken up back path could most simply be provided only by la few hobbyists. Manufacturers between the anode and grid of the output presumably looked with disfavour on the valve. marginally larger and more awkward Fig.1 shows a number Of possible con- power transformer that would be re- figurations, which appeared in literature quired, find the potentially less rugged of the period. Diagram (a), from output Valve. In addition, a new scaled- Radiotronics 71, is probably the most ob- down beam power tetrode was on the Fig.t: Typical circuit configurations for vious way of placing a feedback loop horizon 1 (the 6V6), which would make applying negative feedback around the around the output valve, with a DC for good sales promotion= even if it output stage of ordinary receivers. Such blocking capacitor and a series resistor didn't amount to much in practice! circuits normally remain valid after simply strung from the anode back to the replacement of the output transformer grid. With the grid shunted to virtual Negative feedback or even the complete loudspeaker. earth through its own 1M resistor and by As id happened, the sante issue of the 250k anode supply resistor and the Radiotrbnics was cautiously optimistic anode resistance of the 6C6/617-G, about die idea of using negative feedback gain of the system and necessitate a slightly less than 10% of the anode signal with pentode (or tetrode) output valves larger input signal. It offered a vital swing would be effective in the grid cir- — for the reason that, while preserving bonus, however.