help with squeeling on twin

[teleray]

in the last few days/gigs my fender custom 15 twin has developed a squeel if i turn the reverb past 1-2. ive changed the v3 12at7 driver tube and thought i fixed it but alas last night at a gig it started again! had to turn it totally off.anyone have any suggestions as to what else might be causing this? ive never experienced this problem before on any amp! thanks for any help.

[muchxs]

It sounds like the problem is in the reverb, if it's not the driver then it may be the recovery. The recovery is the next 12AX7 down the line...

Swap the 12AT7 you're using as your driver into the PI position. If it's screwed up as a PI it's a good bet it's screwed up as a driver...

Same with the 12AX7 that's the recovery stage. Swap that with V1, if there's a microphonic triode in there you'll know about it right away.

[strat a various]

I've had this happen to me with amps where the reverb tank is bolted down too tightly, or touching the speaker or the side of the cab. To eliminate the reverb tank, dismount it from the cab, pull it as far out of the cab as you can, and pad it with some folded towels. If the reverb still squeals, maybe your circuit has some HF oscillation problem. In every case in my experience, resetting the tank very loosely solved the squeal.

[teleray]

thanks for the replys. i;ll try the next tube in line first and see what happens. it seems i have to be playing at gig levels for this to happen as it doesnt seem to happen at home at 'bedroom' levels. l;ll let you know next weekend.