Hi, I'm new to tube amp repair but have had good luck so far troubleshooting problems. I'm working on a friend's 1965 Fender Bassman and am having trouble solving a problem. Basically if you bump or move the amp the signal cuts out. There is a hum to the amp even when the guitar signal is coming through properly that I'm pretty sure is related to the original filter caps. When the guitar signal cuts out, the hum is still present but very, very quiet. I've checked for any loose wires, components, and grounds, and touched up all the solder joints. I replaced all the tubes with another set I know works. I thought I had discovered the source when I gently wiggled one of the power tubes that had a very loose socket it would cut in and out. So I replaced the socket and I still have the same problem. I sprayed a little component cleaner into the standby switch thinking maybe it was dirty or was shorting, but that didn't help either, and the problem seems unrelated to the switch. : ( The signal sometimes cuts out even when it's not touched. The oddest part is that if the signal cuts out and I have a guitar hooked up, I can play lightly and nothing happens, but the moment I hit the string a little harder, the amps cuts back in! Any ideas of what could be causing this or what else I can check?
An externally hosted image should be here but it was not working when we last tested it.
This belongs in Instruments & Amps per forum policy. 
Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum
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