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hello and thanks for having a look at this,
As the title states, I am having an issue with HF oscillation on a relatively new (around 4 years old) Fender Custom Shop 68 Twin re-issue. This is actually the second amp I have received in the last weeks with this problem, the first amp I was able to help out by rearranging the wiring slightly but the second is more stubborn.
The problem is pretty centrally located to the mix tube, where the reverb and dry signals are brought together. Funny enough, on this model, they have put 10k grid stoppers on every preamp grid! I suspect Fender knew there was an issue and used these to solve it. However, it doesn't work in this case. A sine wave at 400Hz sets it off earlier than other frequencies.
So far, I have tried to rearrange the wiring-- this is built on a pcb so there isn't much to vary. I have tried snubbing the grid, but it takes such a large value to kill the oscillation that it also kills the high frequencies. I have also tried to remove the PI and output tubes just to make sure they aren't part of the equation and it still oscillates. (This also removes the feedback from the equation obviously.) Lastly, as this is an European amp and LDRs are outlawed, Fender has installed a transistor replacement. I also disconnected this circuit but to no avail.
The strange thing is that I would assume the amp has done this since being built but the owner never pushed the volume past three-- it only begins oscillating at around 4-5-- and it was used only sparingly. If anyone has an idea of where to start, I am all ears. Thank you for your time!
Schematic is included
As the title states, I am having an issue with HF oscillation on a relatively new (around 4 years old) Fender Custom Shop 68 Twin re-issue. This is actually the second amp I have received in the last weeks with this problem, the first amp I was able to help out by rearranging the wiring slightly but the second is more stubborn.
The problem is pretty centrally located to the mix tube, where the reverb and dry signals are brought together. Funny enough, on this model, they have put 10k grid stoppers on every preamp grid! I suspect Fender knew there was an issue and used these to solve it. However, it doesn't work in this case. A sine wave at 400Hz sets it off earlier than other frequencies.
So far, I have tried to rearrange the wiring-- this is built on a pcb so there isn't much to vary. I have tried snubbing the grid, but it takes such a large value to kill the oscillation that it also kills the high frequencies. I have also tried to remove the PI and output tubes just to make sure they aren't part of the equation and it still oscillates. (This also removes the feedback from the equation obviously.) Lastly, as this is an European amp and LDRs are outlawed, Fender has installed a transistor replacement. I also disconnected this circuit but to no avail.
The strange thing is that I would assume the amp has done this since being built but the owner never pushed the volume past three-- it only begins oscillating at around 4-5-- and it was used only sparingly. If anyone has an idea of where to start, I am all ears. Thank you for your time!
Schematic is included
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