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So I just got a project amp, a silverface Fender Vibro Champ. Its in great condition but needs some electrical work. If you try to play through it you get low volume distorted sound that cuts in and out. Moving the volume control causes a lot of crackles and it cuts in and out. The only thing I have done so far was swap in a set of known good tubes but no improvement. I plan to disassemble the amp so I can date all the components and see if anything inside is visibly wrong. I also plan to clean all the controls with contact cleaner.

Where would you suggest to start troubleshooting? Any suggestions as to components that could be causing the described problems?

Yes, clean all the pots for sure. Make sure the jacks are making good contact with your patch cords. Check for loose wires and loose pins on the tube sockets. And change the filter capacitors in the power supply!

After cleaning all the pots and doing a little troubleshootiong, turns out the only thing wrong with the amp is the speaker. Tried a good speaker from another amp and the amp sounds fine, no problems! Seems like the filter caps are fine too! So ordered up a new speaker. Can't wait to do some serious playing through it!