Effects loop feedback

[Cliff]

Just got a TC Electronics Nova Repeater, which was the cheapest delay I could find to go in the effects loop of my Studio 22. It's almost great, except that it suffers from high-pitched feedback. If I dial the tone down (and the treble on the amp), the feedback stops climbing, but is there as a background noise. Still unacceptable. It's only noticeable on the clean channel - lead channel seems fine. It's also not there if I put it at the front of the amp (it has line and instrument level inputs), but that's not why I chose it.
I googled something like 'studio 22 effect loop feedback', and immediately found a post on the TC Electronic user forum from a Boogie owner with the same problem. No decent solution was proposed.
So I wonder if this is a problem with the delay, or with the amp? Do they just not get on together, or do I need to be replacing a valve associated with the effects loop (never used it since I bought the amp second hand)? If so, anyone know which valve that would be?
Thanks,
Cliff

[Cliff]

Noone got any ideas on this? TC Electronics don't seem to know, but googling seems to reveal a few people with similar problems. It's a shame, cause it just does't sound good with the delay out in front with distortion.

[lhrocker]

I had an issue with a high whining feedback when I put an FX in the loop. The problem ended up being that the amp was set for +4 but the FX was -10. When I put the level back to -10, the squealing went away. Not sure if you can do that on a .22 though.

[chrisolson]

Is the loop tube buffered? Maybe try a different tube in that socket if that's the case.

[Hellbat]

Is the loop tube buffered? Maybe try a different tube in that socket if that's the case.

I was thinking along these lines as well, it could be the tube going microphonic. Get one of Mesa's 12AX7 premium tubes (SPAX7A) and try it in the V3 or V4 positions to see if they clear it up. It's a $12 troubleshooting tool.

** EDIT : Just checked a schematic I found online and the loop is buffered by the V3 tube. You can try their standard 12AX7 but I'd recommend using the SPAX7A as they are less liekly to produce feedback than the standard 12AX7

Also, if it's just the clean channel it could be the V2 tube causing the issue.

[Cliff]

Excellent - thanks guys, I'll give that a try tonight. (I have a spare 12AX7 I think, but it's not from Mesa - just some generic tube I ordered a while ago - is this a bad thing?)

@lhrocker - don't think I can change the effects loop, but the pedal has both line and instrument level inputs, which I believe are +4 and -10 respectively. I'll have to double-check tonight, but I know I got the feedback with either.

[Hellbat]

A general 12AX7 will work fine, just the SPAX7's are tested for lower noise and less microphonic feedback.

[Cliff]

Hey Hellbat - thanks for taking the trouble to lookup the schematics.
Seems it's not a tube issue. Listening more closely, if I dial the pedal down a little so it doesn't quite feedback, I instead hear each successive repeat getting more and more trebly. I wonder if the input from the amp is just too bright for it? There's a tone control on the pedal that's mean to simulate the old-school effect of repeats getting darker in tone. I'd assumed having this at max would simply mean no loss in high frequencies, but if I dial it right down I can more or less hear the signal stay pure. Seems to be a working solution for now, at least.