more changes to the schematic

 

...Just keeps looking better and better.

A few resistor and cap values changed.  But the biggest change is, now I'm going to use two 120V Zeners in series to drop the regulated 260V B+ rail down to 20V, so that the range of the bias control is more reasonable.  (I could, more plausibly, also use one 20V Zener to do this, but now that I have the 120V units in stock...)

I guess I could put a capacitor on this newly-created voltage reference, and then feed it to both legs of the dual-gang bias pot; but I'll probably just duplicate it and keep everything separate, for (conceptual) simplicity.  I'm not initially building out the second section of this circuit anyway, until I can test the first and find out whether it actually behaves any better than the "take 2" circuit.



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