Bias pot mod for ampeg vt-22 & vt-40

[FriedWires]

Hey, anybody ever mod an ampeg vt-22 or vt-40 with an adjustable bias pot? I've got a 71' vt-40 toploader and a distortion circuit vt-22. I've recently aquired both of these amps and am going to do caps & tubes to bring them up to speed. I've worked on fenders and marshalls, but I just don't have any experience with to many ampegs. I know ampegs are just a bit different and I don't want to blow it up. Any help would be great. thanks, sean

[Diablo]

It's pretty easy to do. You replace the 75K resistor in the bias circuit with a different value resistor and a linear pot in series. For example...a 56K resistor + a 25K linear pot. Start the amp up with the pot turned to max resistance, and then adjust your bias from there.

[FriedWires]

Thanks Diablo, Much appreciation. Now time to tinker!

[CharlieP]

Originally Posted by Diablo

It's pretty easy to do. You replace the 75K resistor in the bias circuit with a different value resistor and a linear pot in series. For example...a 56K resistor + a 25K linear pot. Start the amp up with the pot turned to max resistance, and then adjust your bias from there.

You can also set the pot and coupled resistor to 75 k as a starting point since the resistor you replace is 75 k if for example your pot is 50k instead of 25k. It depends on the pot you use.