Sorry to say, there is no formula I know of that works reliably for the case of current sources. The situation is even worse for preamps with effectively unloaded secondaries. The simplified theory gives L/(R*R) as before, with R being the tube's plate resistance (if the transformer is unloaded) and L being the OPT inductance. This gives enormous capacitors, and the inductance will vary hugely - more than a 10 to 1 range - depending on the frequency and voltage at which it is measured.

This simplified theory ignores the effective resistance in parallel with the OPT inductance. This resistance is a combination of wire resistance, eddy currents in the laminations which appear as resistance, and magnetic hysteresis which is nonlinear but absorbs energy so you can think of it as a kind of resistance.

In practice, these uncertain loss resistances are the critical parameter, so the only reliable approach I know of is to experiment with different capacitor values and make measurements. You'll have to measure the frequency response at a variety of signal levels to be confident of your choice.

Grainger, I hope you are not using a BCP-16 grid choke as a plate choke - it can't handle **any** DC current, and its inductance varies widely as mentioned above for parafeed OPTs. Are you using a BCP-15 perhaps (40 henries)?