My Amp is making a strange sound

[B A Stone]

I have a Marshall TSL602. When the clean channel is on and the master volume is at 40% or more, I get a ringing, almost feedback type of sound when I tap the top of the amp with my finger.

I get the sound with nothing plugged in, no reverb, and only on the clean channel. The 2 overdrive channels are silent.

Is this a symptom of a preamp tube becoming microphonic?

I've owned a lot of different tube amps and have never had this problem.

Thanks

[szulc]

It sounds like microphonics try swapping the first two preamp tubes and see what happens. If it goes away then you should buy a new preamp tube.