PY 81 as power rectifiers?

[Terry]

As an extreme, consider the triode grid hard-coupled to the cathode. If the pentode current wiggles a bit (noise) then the cathode voltage will wiggle, and the triode grid will be taken with it. So there will be no wiggle in the voltage between triode grid and cathode.

Consider the other extreme, the triode grid hard-coupled to 0V. If the pentode current wiggles a bit (noise) then the cathode voltage will wiggle, and as the triode grid is grounded, the full wiggle will appear between triode grid and cathode. This will be in such a phase to reinforce the original wiggle, hence positive feedback and oscillation is likely

The real problem is that the triode needs a different bias voltage to the pentode, so option 1 above is not viable. However, if the grid voltage is decoupled as I said before but the decoupling cap is connected to the cathode, rather than to ground, the problem will no longer exist.

A better solution, though, would be to connect the 0V line directly to the cathode and use a 10M or greater grid leak on the triode returned to ground for the triode bias - which is the way that Grundig did it in the previous circuit.

I'd prefer a valve with separate cathodes for the triode and pentode sections but that's a personal preference. However, as Grundig have demonstrated, there is no reason why Amie's ECL80 can't be used in a configuration that guarantees success.