Microphonic Feedback (Maybe...)
Been trying to describe an issue to my amp tech via email/phone, descriptions have pointed towards a microphonic valve, but wanted to get a few opinions from here, see there are any other causes that are easy to diagnose before I just give in and let my tech take a look.
Got a Selmer Treble and Bass 50 head, Treble side was gutted, new enclosure built from the chassis, pretty messy job, not much can be done without essentially a full rebuild. Once I get it up loud enough to compete with a kit, and push it with an overdrive pedal a little, getting a pretty horrible squeal from it, which alters in pitch as I move the guitar around the amp. Having heard a similar pitch when using horribly microphonic pickups before, I guessed it could be that, but had the same issue with a few different guitars, so guessed probably the amp.
The usual tapping the preamp valves test hasn't shown anything obvious when I did it, (though I'll try swapping out a couple spare ones, see what that does, just in case). I had the amp in the studio recently, head in the control room, cab in the live room, and was able to get the noise even with just the bleed through the walls, so it definitely seems to be related to how close I get the guitar to the head, rather than something acoustic from the speaker.
Any thoughts other than microphonic tubes?