Counterpoint SA-12 wackyness

[TubeAmp222]

I'm new to the forum and looking for a little understanding of an SA-12 I fixed. I bought two of them, neither was working. One had a bad low voltage section of the transformer and had a badly butchered left channel output. The other was in good shape with a blown right output.

I used the MOSFETS from the non butchered channel of amp 1 into the right channel of amp 2. Using a Variac, I set the bias at a nice steady 350mV over a 1ohm resistor, and the DC offset to a reasonably steady 2mV to 9mV for both channels. I did this with the gates disconnected.

When I connected the gates things got wacky. The output started bouncing around wildly from 300mV to 360mV and the DC off set was all over the place. The power supply was putting out 350VDC and the tube heaters were working. I tried different tubes, I checked all of the diodes the tube stage, I even went as far as replacing all of the film capacitors in the tube stage.

This amp is a little different that my SA-100. That amp has a V3 pot for adjusting gain, the SA-12 does not. I hooked it up to an old pair of bookshelf speakers and it sounds great. Is this behavior normal for an SA-12?

[TubeAmp222]

Originally Posted by TubeAmp222

Using a Variac, I set the bias at a nice steady 350mV over a 1ohm resistor, and the DC offset to a reasonably steady 2mV to 9mV for both channels. I did this with the gates disconnected.

Sorry, I just re-read my post. I meant to say I set the bias initially with the tubes out, not the gates disconnected.