I've made some progress. The squealing and distortion is gone. But there are some anomolies that I can't figure out. I've been doing a lot of measuring, but it's difficult because the voltages that everyone says to look at are on the original 1176 schematic which is vaguely like the Mnats one, but only vaguely so. It has the same basic sections, but is pretty different in the details.
When I try to do the Q-bias adjustment, the voltage at the outputs are supposed to stop going up at a fairly reasonable voltage as the Q-bias trim pot is cranked up, like in the upper 1 to 2VAC range I think. Mine doesn't stop anywhere near there. It will go up to like 16V or more. That's clearly not right.
I found one other person who said they had this problem. But then they subsequently said it went away for some unknown reason, so that didn't help. It would seem kind of reasonable to think that it would be some resistor being too low resistance at some point, but I'm not sure. And all the resistors were measured going in and have been checked by eye where there's any question during measurement and all seem right.
The Mnats calibration videos say to crank the Q-bias trim all the way up, then bring up the output to +1. But with the Q-bias trim all the way up, it's putting out enough juice to weld with, so clearly it's not just that the meter based calibration scheme isn't valid, since the same issue exists the other way.
I was at least trying to measure through up to where it started running up really high when the Q-bias is raised up. I kind of ran out of steam a bit ago but I think it was good up to the gate of the second to last transistor, then the drain of that one (which feeds into the last transistor's gate) seemed to exhibit the wild output level. But, does that seem reasonable, that that transitor could raise the level that much?
But whatever was going into the gate of that last transitor seems to be what shows up at the XLR outputs. It seems to drive the brown winding of the output transformer, and that seems to be what directly drives the other side of the transformer, with the others being involved in some sort of feedback'ish type of loop back into the power output driver section. So it doesn't seem like that last transformer is adjusting the level downwards. But all the resistors around those last two transitors seem to be correct, so I don't think that's an issue.
The schematic is at the bottom of this PDF file if anyone cares to look at it:
http://www.geocities.com/m_natsume/f...6LNREVD_V2.pdf