PY 81 as power rectifiers?

[Terry]

I will try your ideas on the coupling capacitors, the 470uF was at hand but it seems the trouble was because of the cathode resistors, I will try some different decoupling and possibly a grid stopper for the triode too, it may help. Also if I get the grid biased spot on it may help.

Ah, I obviously misinterpreted your previous post - I thought you meant that you couldn't stop the instability unless you used such a high value.

There is a good reason here for going big, however. If you have insufficient decoupling, there will be feedback from the pentode to the TRIODE, because of the common cathode, and this feedback will be positive. You will get motorboating. Without the capacitor, the circuit forms a cathode-coupled multivibrator. Snip it out and try it!

Agreed - which is the reason I suggested the additional decoupling at the earthy end of the grid leak as it will be considerably better at isolating the triode from the pentode.

Agree also about the adjustable triode bias suggestion. I would suggest that Amie implements both suggestions together to cover all bases.