a little update:
today i reversed the OT's polarity. i slowly turned up the amp, then all of a sudden a LOUD squeal.
quickly turned off the amp. after that the amp was dead. it turned out i had killed the preamp tube somehow, cause after i had replaced it, it was back to normal.
i wonder how that happened, since the feedback loop was switched off. i'm also surprised that you can actually smoke a preamp tube that way....
after that i tried the following:
- reducing the gain of the first triode by raising the cathode R to 2,7k
- adding a grid stopper to the 2nd triode
- reducing the gain of the second triode by raising the cathode R to 2,7k (this helped a bit, but on last few millimeters of the pot
there's still that fuzzy, terrible sounding distortion). i reversed everything to the original state (1,5k and no grid stopper).
voltages on the 12ax7 are:
pin1 = 187 V
pin 3 = 1,45 V
pin 6 = 187 V
pin 8 = 1,43 V
this seems ok to me.
both 68k grid stoppers are soldered directly to pin 2 of the input tube instead of the input jacks as it is seen in many amps.
could this be the culprit?
i kind of ran out of ideas, so i'd be grateful for any suggestions before i rewire the whole thing.