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Late 90's/Early 2000's "black tweed" FX Loop Classic 50 212
The amp was purchased in November and played for about 3 hours straight with no problems, then sat in place until 2 months later.
The amp started crackling, popping and the sound best described as "cartoon electric chair"
I smacked the top of the amp and it worked well for another hour, then we took it home
I unplugged the reverb, pulled the preamp tubes(including PI), unplugged the reverb tank, turned all the controls to 0, left the input unplugged (no guitar/cable), jumped the fx loop
Still does this after 10-15 seconds of warming up, seems to have gotten worse, it can be very loud
Also if the amp manages not to pop/crackle, one of the tubes will start to red plate, tried new matched tubes, same thing, not always the same socket, doesn't always follow a tube either.
Bias voltage is -15.6VDC on all 4 EL84 sockets
Pulled the board, cleaned and tensioned all tube sockets, cleaned all pots, checked the ribbons for any obvious frays/breaks near the board, checked the impedance of the ribbons, checked all the diodes, reflowed most of the solder joints, replaced the filter caps with the same values, cleaned every post with a molex connection, tensioned every molex plug.
Chopsticked the wirewound resistors, checked their impedance, checked impedance with every resistor on the power board with a fluke 88V, chopsticking seemed to reveal nearly nothing, though sometimes the power tubes are sensitive (old and new)
Plate and screen voltage with no tubes appears to be appx 417VDC
I THINK the problem goes away when I pull the two molex connections from the FX/FS board to the main board, but I'm not sure what that means, because I don't know all that I'm interrupting.
Out of curiosity, the speaker output jacks are one switched and one typical mono 1/4, what would an intermittent speaker connection cause? would this fluctuate the plate voltages since those go through the OT?
The crackling and popping is from the speakers, happens with extension cab as well (known good cab)
since the bias voltage is used for switching is it possible that one of the transistors or something is fluctuating the bias supply? I guess that would affect all 4 tubes though...
Any ideas or tips would be appreciated, I have my meter ready to check anything you need me to
The amp was purchased in November and played for about 3 hours straight with no problems, then sat in place until 2 months later.
The amp started crackling, popping and the sound best described as "cartoon electric chair"
I smacked the top of the amp and it worked well for another hour, then we took it home
I unplugged the reverb, pulled the preamp tubes(including PI), unplugged the reverb tank, turned all the controls to 0, left the input unplugged (no guitar/cable), jumped the fx loop
Still does this after 10-15 seconds of warming up, seems to have gotten worse, it can be very loud
Also if the amp manages not to pop/crackle, one of the tubes will start to red plate, tried new matched tubes, same thing, not always the same socket, doesn't always follow a tube either.
Bias voltage is -15.6VDC on all 4 EL84 sockets
Pulled the board, cleaned and tensioned all tube sockets, cleaned all pots, checked the ribbons for any obvious frays/breaks near the board, checked the impedance of the ribbons, checked all the diodes, reflowed most of the solder joints, replaced the filter caps with the same values, cleaned every post with a molex connection, tensioned every molex plug.
Chopsticked the wirewound resistors, checked their impedance, checked impedance with every resistor on the power board with a fluke 88V, chopsticking seemed to reveal nearly nothing, though sometimes the power tubes are sensitive (old and new)
Plate and screen voltage with no tubes appears to be appx 417VDC
I THINK the problem goes away when I pull the two molex connections from the FX/FS board to the main board, but I'm not sure what that means, because I don't know all that I'm interrupting.
Out of curiosity, the speaker output jacks are one switched and one typical mono 1/4, what would an intermittent speaker connection cause? would this fluctuate the plate voltages since those go through the OT?
The crackling and popping is from the speakers, happens with extension cab as well (known good cab)
since the bias voltage is used for switching is it possible that one of the transistors or something is fluctuating the bias supply? I guess that would affect all 4 tubes though...
Any ideas or tips would be appreciated, I have my meter ready to check anything you need me to
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