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I have a TMB that after about 10 minutes of play or so will jump in volume by about 6 db. It's about the amount volume gain you might do with a clean boost for a lead. I'm not using a boost, I'm just using that to describe the amount of volume gain I'm hearing. That will go on a for a minute or two then it will drop back down to what what it was when I threw the stand by switch to on.

I've done some googling on the internet and it seems like it can be anything but there's got to be a way to narrow it down.

It happens on either channel - this suggests that it's not in the pre-amp tubes, but perhaps the inverter or the power stage. In my experience with electronics, it seems most likely a tube or a cap, but I support a back connection or solder joint is also possible? Resisters and connections typically either work or don't, fail fully or not.

Any suggestions?

--chuck

Hi Chuck

You are on the right track. The problem is after the preamp if it affects both channels. Problems like this are usually caused by bad solder joints. Try prodding all the joints and wires associated with the phase inverter and output tubes with a nonconductive object such as a chopstick and see if you can provoke or clear the problem. Don't use a pencil because the graphite conducts electricity.

There's also a chance it's a flaky tube or tube socket.