05-11-2016, 09:30 PM
The book “Hacker Radio” provides some clues as to the progression of the late Dynatron valve tuners in its summary of post war models.
The two pertinent paragraphs are:
The "Merlin" B.119 was revived. This now used the same chassis as the -Ether Conqueror" which was almost their last chassis, although it went on being modified, up to Mark V. The most drastic of these alterations was the change to miniature valves (fifteen of them). The "Princess" was a "short" version with one I.F. stage and less output (nine valves), The "Ether Marshal" had preset tuning for Home, Light , and Third, a grounded grid R.F. stage on Short waves, bandspread, and an idea resurrected from the Keates-Hacker in the shape of an overload indicator on the output stage.
And:
Their post-war chassis were:
"Ether Conqueror" Tuner T69 (6 valves + "Magic Eye")
Audio amplifier LF59 (5 valves), 9 watts
Later versions had three extra valves in the tuner and one more in the audio amplifier. They were then re-numbered:
Tuner T99 (9 valves + "Magic Eye")
Audio amplifier LF612 (6 valves). 12 watts
"Princess" Tuner T57 (5 valves)
Audio amplifier LF44a (4 valves), 4 watts
So the T99 does look to have been the miniature valve successor to the T69, albeit with extra valves. And apparently the T99 designation meant 9 valves and 9 tuned circuits.
The T139 tuner was not mentioned by number in the above list, but was described in the entry for the Ether Marshal. This had 11 valves, if one includes the two magic eyes. There were 8 tuned circuits in the signal path, one for the main RF amplifier, one for the interstage, and three bandpass pair IF transformers. The 9th tuned circuit would thus have been that for the local oscillator. Two of the valves had AF functions, one as a gramophone input preamplifier and the other as a voltage amplifier ahead of the passive switched tone controls and notch filter. Anyway, the T139 did not have 13 valves, so its designation was out-of-sequence. Given that the T69 and T99 both had 9 tuned circuits, then their RF/IF signal paths were probably broadly similar to that of the T139, albeit lacking the grounded grid input stage.
The AGC arrangement for the T139 appears to have been unusual. AGC voltage was applied in full to the main RF stage (W77) and mixer (X79, with separate L77 oscillator), but not to either of the IF stages (both W77). The 1st IF stage had a preset gain control in the form of a potentiometer (bypassed) in the cathode circuit; there was also an unbypassed cathode resistor in series with it providing a measure of degeneration. The 2nd IF stage had a fixed and unbypassed cathode resistor, so had a higher level of degeneration. So in effect both IF stages varied in gain inversely according to signal strength, thus contributing to AGC action. This would also have minimized modulation rise distortion , which can be caused by an AGC’d final IF stage chasing its own tail around the bend at the bottom end of the valve’s transfer curve. AGC generation and AGC delay were done by separate diodes, thus obviating differential distortion. Application of AGC to the mixer is somewhat frowned upon for HF receivers, at least those used for communications and DX’ing, but I suspect that any deleterious effects thereof are unlikely to have been major in respect of reasonably comfortable broadcast programme content listening.
The T139 AGC circuitry may be compared with that of the Chapman S6BS, which was quite different, although with similar ends in mind. It’s a pity that the AGC curves for each are not available.
Cheers,
Steve
The two pertinent paragraphs are:
The "Merlin" B.119 was revived. This now used the same chassis as the -Ether Conqueror" which was almost their last chassis, although it went on being modified, up to Mark V. The most drastic of these alterations was the change to miniature valves (fifteen of them). The "Princess" was a "short" version with one I.F. stage and less output (nine valves), The "Ether Marshal" had preset tuning for Home, Light , and Third, a grounded grid R.F. stage on Short waves, bandspread, and an idea resurrected from the Keates-Hacker in the shape of an overload indicator on the output stage.
And:
Their post-war chassis were:
"Ether Conqueror" Tuner T69 (6 valves + "Magic Eye")
Audio amplifier LF59 (5 valves), 9 watts
Later versions had three extra valves in the tuner and one more in the audio amplifier. They were then re-numbered:
Tuner T99 (9 valves + "Magic Eye")
Audio amplifier LF612 (6 valves). 12 watts
"Princess" Tuner T57 (5 valves)
Audio amplifier LF44a (4 valves), 4 watts
So the T99 does look to have been the miniature valve successor to the T69, albeit with extra valves. And apparently the T99 designation meant 9 valves and 9 tuned circuits.
The T139 tuner was not mentioned by number in the above list, but was described in the entry for the Ether Marshal. This had 11 valves, if one includes the two magic eyes. There were 8 tuned circuits in the signal path, one for the main RF amplifier, one for the interstage, and three bandpass pair IF transformers. The 9th tuned circuit would thus have been that for the local oscillator. Two of the valves had AF functions, one as a gramophone input preamplifier and the other as a voltage amplifier ahead of the passive switched tone controls and notch filter. Anyway, the T139 did not have 13 valves, so its designation was out-of-sequence. Given that the T69 and T99 both had 9 tuned circuits, then their RF/IF signal paths were probably broadly similar to that of the T139, albeit lacking the grounded grid input stage.
The AGC arrangement for the T139 appears to have been unusual. AGC voltage was applied in full to the main RF stage (W77) and mixer (X79, with separate L77 oscillator), but not to either of the IF stages (both W77). The 1st IF stage had a preset gain control in the form of a potentiometer (bypassed) in the cathode circuit; there was also an unbypassed cathode resistor in series with it providing a measure of degeneration. The 2nd IF stage had a fixed and unbypassed cathode resistor, so had a higher level of degeneration. So in effect both IF stages varied in gain inversely according to signal strength, thus contributing to AGC action. This would also have minimized modulation rise distortion , which can be caused by an AGC’d final IF stage chasing its own tail around the bend at the bottom end of the valve’s transfer curve. AGC generation and AGC delay were done by separate diodes, thus obviating differential distortion. Application of AGC to the mixer is somewhat frowned upon for HF receivers, at least those used for communications and DX’ing, but I suspect that any deleterious effects thereof are unlikely to have been major in respect of reasonably comfortable broadcast programme content listening.
The T139 AGC circuitry may be compared with that of the Chapman S6BS, which was quite different, although with similar ends in mind. It’s a pity that the AGC curves for each are not available.
Cheers,
Steve
