LED bias cathodyne

[mashaffer]

Any advantage of using LED to bias a cathodyne PI rather than cathode resistor bias (bootstrap)?

[kevinkr]

I can't imagine it would hurt at all in a capacitively coupled self biased cathodyne phase inverter and it should very significantly increase the input impedance.. (As the grid bias resistor from an AC standpoint is sitting very close to the cathode potential as opposed to a significant % below that due to the cathode bias resistor and cathode resistor.

The only concern might be the variation in operating point due to using effectively fixed bias instead of the local feedback mechanism implied by the cathode bias resistor. Note if you really need high input impedance you can also place an electrolytic bypass capacitor across the cathode bias resistor. Using LED bias might force you to select the tubes to make sure that the swing requirements are met under all circumstances.

[SY]

I've tried it, works fine, and I would absolutely use in in place of a bypass capacitor across the bias resistor (as shown in the RCA Tube Manual). Kevin's concern about swing is well-taken, but in practice, the example I built up (using ECC88) worked perfectly right out of the chute.

[mashaffer]

Thanks. Appreciate the insight.