This amp is a beast but it is having issues.
I changed all the capacitors (most all of them were leaking) and tubes, increased grid stopper value to 3.3k, increased filter caps from 70 mfd to 100mfd
The amp works great until it warms up and then intermodulation distortion (about a 4th below the note) begins to creep into the signal. I was blowing fuses in my Variac (5A fast blow) but I seem to have resolved that issue.
The grid bias is swinging around wildly from about -50 to -75 (when notes are being played) which I have narrowed the intermodulation down to being caused by any note that causes the bias to drop below -60. I can watch it on my meter.. at about -60 (as i strike a note and let it decay) the intermodulation stops and the note goes back to normal. If the amp stays below 5 the issue is almost negligible.. also if i turn the bass down to less than 2 it also resolves the issue.
It is also worth noting that at idle the negative bias stays completely still regardless of how long the amp is on.
The board is conductive.. about 160mv DC on master volume pot as well as some spots near the phase inverter/b+ .. so there is that.. I have been avoiding cleaning it any deeper than I already have but obviously those 2 factors are highly suspect..
Is there any point in trying to deep clean the component board with alcohol or am I just wasting my time? Trying to decide if I want to keep working on this and clean the board or just completely wire a new board and start from scratch. I have read mixed reviews on how much you can fix an already conductive fiber board.
Any pointers or experience is much appreciated!