Bias method switching
On my 2 channel homebrew, I added a modification to switch between cathode bias and fixed bias. Here's how I did the switching. If you just want to convert from cathode to fixed bias, then omit the switch and start with the "Creating a fixed bias voltage supply" section below.
A DPDT switch is needed. It handles low currents, so just about any kind will do. The schematic shows the switch in the cathode bias position--the 220K grid leak resistors are grounded, and the negative bias voltage is disconnected. Flipping the switch grounds the cathodes and short-circuits the cathode bias resistor and capacitor, un-grounds the 220K grid leak resistors and applies the negative bias voltage to them.
Creating a fixed bias voltage supply
Many amps with fixed bias have a tap on the power transformer secondary from which the bias voltage is developed. My transformer didn't, so I had to steal voltage from the secondary. The capacitor I found was rather high in value. I've seen schematics that use anywhere from 7 microfarads to 70 mf. It charges somewhat slowly, developing the bias voltage in about 10-15 seconds. Since a low bias voltage allows the tubes to draw more current, it's important to let the amp sit in standby for a while before switching it on to make sure the needed bias voltage is available.
I went with a bias-adjust circuit similar to what Marshall uses. Fender amps draw the voltage from the wiper on the pot; in other words, the wiper isn't connected to an outside lug of the pot as above, but connects to the grid leak resistors (the dual bias supply page shows a diagram). If the wiper ever loses contact with the carbon ring in the pot, bias voltage is lost, and the tubes redline. If you don't catch it, you lose the tubes and maybe the output transformer. Using the Marshall approach, if the wiper ever lifts, the full negative voltage is applied, the the tube shuts down.
Some techs say they put a high-value resistor between the wiper and the hot lug of Fender bias pots to safeguard against redlining the tubes. And some techs have said they have never seen an amp that lost the wiper's connection. How important is this? I don't know.
Preferences
I like the sound of cathode bias with the distortion channel of this amp. Maybe because it drops the plate voltage slightly, and has a more compressed sound. The clean channel sounds better with fixed bias. But the difference between the two is relatively small to my ears with this amp.
