01-20-2018, 09:51 AM
OK. I think this is resolved. At least, I hope so.
Thursday evening I cracked the amp open and replaced the load resistors on the input jacks (pain in the butt - all wires and resistors wrapped around the lugs in opposite directions - took ages to de-solder and pull apart). I also installed stoppers on the grids of the first 6SL7. I used 22k, which was one of the options i had, based on spare parts. The end result was absolutely no change. Noise reappeared almost immediately. I can say that the grid stoppers did reduce the amount of radio I was hearing, so that is a good thing, I guess.
Running out of options, I again rummaged through my spare parts and went back into the amp and replaced the volume pots and the input jack for channel 1. One oddity of the original input jack was that it did not ground the input when not in use (very noisy when nothing was plugged in). In fact, that part of the jack had been removed. Very odd. The new jack grounds the input. I did that through a 5.6M resistor, the same as the jack on channel 2. The ground issue is immaterial to my problem, I think, but it is possible that the jack itself was not making a firm connection to the guitar cable plug. There is no evidence of this, though. Jiggling the cable did not make any noise.
The pots also tested fine. But they were ancient, and there is the possibility that there was some residual deoxit or something left over from cleaning.
So, the amp appears to work now as expected. The problem could have been with the jack or the volume pot. It could also have been with one of the solder joints on the board, as I simplified my life by wiring the pots prior to installation, so all the connections to the board were redone. I guess it could also have been a wire.
It's been a very Dr. House kind of process. Try this. Try this other thing. Maybe try this next thing. Eventually the patient is saved. Not super satisfying, but effective in the end.
Thursday evening I cracked the amp open and replaced the load resistors on the input jacks (pain in the butt - all wires and resistors wrapped around the lugs in opposite directions - took ages to de-solder and pull apart). I also installed stoppers on the grids of the first 6SL7. I used 22k, which was one of the options i had, based on spare parts. The end result was absolutely no change. Noise reappeared almost immediately. I can say that the grid stoppers did reduce the amount of radio I was hearing, so that is a good thing, I guess.
Running out of options, I again rummaged through my spare parts and went back into the amp and replaced the volume pots and the input jack for channel 1. One oddity of the original input jack was that it did not ground the input when not in use (very noisy when nothing was plugged in). In fact, that part of the jack had been removed. Very odd. The new jack grounds the input. I did that through a 5.6M resistor, the same as the jack on channel 2. The ground issue is immaterial to my problem, I think, but it is possible that the jack itself was not making a firm connection to the guitar cable plug. There is no evidence of this, though. Jiggling the cable did not make any noise.
The pots also tested fine. But they were ancient, and there is the possibility that there was some residual deoxit or something left over from cleaning.
So, the amp appears to work now as expected. The problem could have been with the jack or the volume pot. It could also have been with one of the solder joints on the board, as I simplified my life by wiring the pots prior to installation, so all the connections to the board were redone. I guess it could also have been a wire.
It's been a very Dr. House kind of process. Try this. Try this other thing. Maybe try this next thing. Eventually the patient is saved. Not super satisfying, but effective in the end.