Pleiades might have been on the right track after all.
A hard copy schematic has just been preliminarily examined of the DECCA mic/line amplifier. It is dated 15-8-1960.
It is worth noting:
1st stage is EF86 single ended.
2nd and output stage is an ECC82 in push pull.
Input and output transformers.
Vb plus and Vb minus supply.
The anode current of the output stage seems to be only 147μA, (micramperes).
Vb seems to be only of the order of about 9V.
Not shown what the negative supply is but it looks like it should be comparable.
There seems to be pull up on the first stage as grid is connected to inoutbtransformer secondary. The other side of secondary is connected to 470K which is then connected to 1.2K going upwards to cathode and 39K going downwards to -Vb rail.
It makes sense, the Pleiades V0?, V-1?, V1? front end amplifier (can't remember which one) operates with a long tailed pair ECC82, input and output transformer, and just Vb=12V with grids at space potential and sounds great. See precious posts. This amp started of as UCC85 long tailed pair.
Light falling on tubes increases gain as possibly photo electrons emitted from grid make it less negative?
Very low anode current and low voltage makes sense when handling the delicate detail of a mic signal. Low noise too.
Just like the Pleiades V6 battery powered electron tube front end microphone booster amplifier.
See also:
Pleiades bias - euroelectron
Operating features of the Audion - E. H. Armstrong
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