PY 81 as power rectifiers?

[Kalee20]

The audio current is irrelevant as that is AC.
The DC and AC voltages are also irrelevant.
All that matters is the DC currents (especially as it's Class A). So feeding rest of set from one winding and output anode from the other does work. Obviously it works best if the ratio of the windings equals the ratio of the currents.

I think Amie meant something like in the sketch.

Then as you say, the currents flowing in opposite directions will cancel out the magnetisation in the core. And if the numbers are right, complete cancellation will occur.

But as soon as you put a signal into the output valve, you will get audio voltages across winding A (which is the whole point of the stage). Transformer action will then induce audio voltages in winding B, which will be fed to the rest of the circuit.

This could result in weird feedback effects. Or, the 'rest of circuit' could load the transformer leaving precious little remaining for the speaker.