Fixed Bias question

[Howlin' Mad Mac]

Designing a "lite" version of a Brownface/Blonde era Princeton sans extra inputs/vibrato. I know the "simple" way is to just Cathode Bias the power tubes but I'd prefer a Fixed Bias with an adjustment pot. The Bias Circuit as drawn should accomplish this but how do I figure the values for the fixed resistors marked "???" on my schematic? Also my PT lacks any sort of "Tap" for Bias Voltage so what's the preferred method to create one? Help will be greatly appreciated!

Bias values needed.pdf

[loudthud]

First of all the cathodes of the 6V6s should go to ground and the bias connected to the grids vias the 220K grid resistors.

I did a quick simulation with PSUD-II assuming a 300-0-300 volt transformer and 60Hz power. The resistor in series with the pot was 15K and the resistor connected to the transformer is 82K. That will give you about -32V with an adjustment range of about -19V to -32V. With 22uF for filtering the ripple will be about 0.6V .

[tubeswell]

how do I figure the values for the fixed resistors marked "???" on my schematic?

Depends on the plate voltage and tube current. loudthud has given you a sim for a 300-0-300 HT for 2 xz 6V6s - its so happens that the 5G9 tremolux is a fixed bias amp with the same value HT and fixed bias 6V6s. It runs it bias off a 50VAC bias tap and uses a 56k and 82k voltage divider.

Also my PT lacks any sort of "Tap" for Bias Voltage so what's the preferred method to create one? Help will be greatly appreciated!

Use the BF Princeton method of tapping the bias supply off the HT with a 100k/(rev bias diode)/27k divider.