Generating Bias Voltage from HV tap

[EETStudent]

I'm trying to convert my cathode biased 6AQ5 amp into fixed, but I don't have an apparent bias tap. Actually, there is an used tap that puts out 100vac, but I'm not sure if that will work or how to make it work.

Can't I just draw from one leg of the HV winding? Any schematic references?

[defaced]

That unused tap is probably your bias tap. Just copy a standard bias circuit (it's simply a half wave rectified supply that feeds a resistor network which serves as the bias voltage reference point) and tweak the resistor values until you get the voltage range you need. Because the grid of the output tube doesn't pull current during Class AB1 operation, the bias supply is literally just a resistor voltage divider network. Knowing that, tweaking the resistor values is pretty straightforward. Pretty much all of the big 100w amps use fixed bias, so all of their schematics might be worth looking at to see how it's been done. I know these are out in the wild and use fixed bias: Fender Twin, Marshall JCM800, Mesa Rectifier, Peavey 5150/5150II/JSX/XXX, Soldano SLO.

[Wilder Amplification]

The 100VAC tap uses the center tap as its reference, which is grounded. Copy the bias supply from a 100 watt Marshall to start. If you need less negative voltage at the grids (and you more than likely will), just increase the value of the 27K resistor in the Marshall 100 watt bias circuit.