Thank you for the info on the choke. I need to measure the inductance of one of the types I have with DC flowing through it, then I'll make a decision.
About the bias, output current gradually drifts downward in this design over an initial period of 20 minutes. Maybe it was even longer, I don't remember now. If it's set to 80 mA cold, the drift ends at 70 mA or so. This doesn't degrade performance significantly, but I'm a perfectionist and it really bugs me.
I've looked at many different means of controlling this, but most are fundamentally thwarted by A2 operation. Given the fact that average cathode current changes when the amp enters A2, only two parameters can be used for electronic monitoring. One is the DC at the grid of the 211. A high impedance connection there driving an integrator can be used to keep it steady. However, stabilizing the voltage isn't sufficient to compensate for aging of the 211. The amps would still require adjustment from time to time. The second means is to monitor average anode current. That doesn't change when the amp is driven into A2 (if it does, the amp isn't linear). This can be accomplished with a small value resistor in series with the B+ negative return. The "top" of the resistor will be positive, carrying a value commensurate with anode current.
Using SPICE, I originally devised a simple means using a single FET to monitor this point and make corrections. However, startup is complicated. The auto-bias initializes first, and because the 211 isn't conducting yet, it pushes grid bias to the positive end of its window. When the 211 eventually conducts, the circuit senses the over-current, but as an integrator with a long time constant, it responds too slowly. This causes the 211 to be way over-current for an extended time, and it self-destructs. The only solution I've found is to use a timer to precondition the auto-bias circuit so it outputs a full negative voltage. The timer would be set to expire after the 211 conducts, at which point the grid will gradually move less negative to reach equilibrium. Unlike the FET appoach, the timer requires a source of low voltage VCC. Once the need to include that additional power supply is accepted, it's easy to replace the FET with an opamp for more refined control. I spent hours on this problem, because the FET is a really simple, elegant solution that doesn't require additional power. I just couldn't devise an equally simple means of preconditioning it during startup.
Yes, tube amps often eat semiconductors, and I share your concern in this regard. The circuit I've come up with will use the opamp to directly drive the grid of a 6SL7. Nothing else will be connected to that point, so unless the tube shorts anode-to-grid, the opamp is completely isolated from high voltage. Also, if the 6SL7 opens, 211 bias will move to -80V, cutting it off. That's the plan, anyway.
Jack