PV Delta Blues intermittent "howling"

[Randall]

Got a Delta Blues that customer says sometimes "howls" before he even plugs in, but then will calm down later. I cannot recreate this in my shop. This is a customer who has very limited technical understanding, he seems to think it has to do with power at various places. It came to me with the reverb knob on 4, it's a rockabilly band. I ruled out microphonic tubes, these are very quiet. The reverb does seem a bit hissy, and also sensitive to cabinet vibration, so I wrapped the tank in packing foam and stuffed it back in the bag. I am inclined to send it back to him like this with the instructions if it ever happens again, turn down the reverb knob to see if it stops, and we'll take it from there.

I don't know what else to do since it is not happening here. Any thoughts?

[Jazz P Bass]

Sure does sound like a reverb issue.

[Chuck H]

Specifics on how the amp is being used may help. But you can't fix what isn't broken. I'll assume you whapped on it and semi roughly wiggled all the knobs? That the problem calms down isn't very reverb like. That usually gets worse, not better. But sometimes getting the springs vibrating at playing frequencies diffuses the feedback. It's very hard to say. Another possibility might be a dirty tube socket. I've heard intermittent pin contact cause a squawk, squeal or screech when the standby switch is first flipped and then it stops once the voltages normalize.

[Randall]

Yes, I whacked it, tried everything I can think of to make it reproduce the problem. I sprayed out all the tube pins and reinserted the tubes, as per usual. This amp has no standby. So hard to fix problems you can't see when your customer doesn't have the chops to correctly explain things.

[The Dude]

If you can't duplicate the symptom in the shop, there's not much else you can do. My bet is that wrapping the tank as you did will likely take care of the problem.

[g1]

If you really want to see if you can recreate the problem (and confirm your suspicion), I would remove your wrap and move the tank a bit closer to the speaker.
If you are able to confirm the problem this way, you can be more confident about your fix.