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the audio tube supply's the 135v to the rest of the set.
it is determined by the voltage drop in the tube from the plate to the cathode, this is largely determined by the bias of that tube (the 6GC5).
there is grid bias set by a two resistors, again as voltage dividers, a 1.2 and a 1.0 meg resistor pair. The mid point of those two supply the grid bias.
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