PY 81 as power rectifiers?

[Terry]

Not sure that the idea of an RC network in the earthy end of the grid leak to improve the smoothing of the bias on the triode's bias supply would give benefit, I'd suggest the reverse.

Considering that no bias current flows in the grid leak, the series resistor can be quite high - I think I suggested 100k or more - and, with a suitably high value of decoupling cap, should completely isolate the triode grid from any feedback from the pentode's cathode.

The size of the capacitor is limited solely by the RC time constant so that there is no undue delay in generating the bias voltage as the valve warms up.

The alternative, of course, assuming that the valve type remains unchanged, is to AC couple the triode grid and fit a 10M grid leak between the grid and earth.