Adjustable cathode bias

I haven't finished with this, but there is some interaction with the pot also controlling the bias. I haven't attempted to solve it yet because it was a happy accident--as you turn down the volume, the bias current drops, which is a combination I like for clean sounds. The volume control effect on bias is well within the safe operating range. Other amps may feed the power tubes from a phase inverter, or not have a pot here, and not have this concern--McIntyre's design did not because he uses a phase inverter. If I don't like the interaction, 2 possible solutions may be to put a DC blocking cap between the pot and the 220K resistor, or to wire the pot so it is the grid load resistor, and the signal from the tone network is applied to the wiper. In the Tube 10, there would be no DC voltage, so the pot wouldn't sound scratchy.

Cathode bias is set by a cathode resistor, and since resistors come in certain values, getting the right value may require wiring several together, or in settling for the closest nominal value. John McIntyre used a variation on cathode bias in his "Prince of Wails" mod published in Guitar Player March 1996. By making the cathode resistor larger than it should be, the tube draws less current than you want it to. A circuit is added to draw a positive voltage off the cathode and apply it to the grid, which will increase the current drawn. This is similar to a silverface Fender scheme that had a cathode resistor that was too small (the tube would draw too much current), and used a fixed negative voltage applied to the grid to bring it into the correct operating range. This is supposed to sound poor,