Here's some info from my shop notebook. I'm working on a late model Twin Reverb that has an extra tab on the bias control pot with three things attached to it. There's the bias feed through a 3.3k resistor, an electrolytic cap with the positive to ground (it was an 80/75 so I ended up with a 100/160 number) and a lead going off to the vibrato pedal RCA connector center post.
After thinking about converting this to the more desirable level adjust and studying it a bit I went ahead and converted it as you see in the third sketch, the one in ink. Vacate the extra post, connect the supply, capacitor and vibrato pedal lead to the lower pot tab, the center tab now supplies bias voltage to the revised phase inverter setup, and the upper tab is grounded through a 27k resistor.
When you think about it, the extra tab on the bias pot makes it easier to construct the bias balance setup, but if you're only going for a level adjust it is unnecessary.