Mark IV lead channel strange feedback issue

[Higgs F. Boson]

Hi all,

New to the forum - I recently acquired a Mark IV-B widebody combo. Been enjoying getting to know it the last couple of weeks. Today, for the first time, it did something odd - on the lead channel, when I played a high "A" - 10th fret on the B string - the amp would continue to feedback at that frequency indefinitely, even after I muted the string. The only thing that would stop the feedback is to hit some low notes, loud - an open "E" chord would stop the feedback.

R1 and R2 are fine, no problems. Lead channel developed this issue while playing Class A-triode-tweed power, with Lead Gain and Drive both set around 7.

I've had a couple Boogies in the past (SOB, Rectoverb), never had any amp, Boogie or other, do this particular thing. I'll probably start by replacing one preamp tube at the time. I'm guessing it's not V1 since the rhythm channels are OK?

Anyone have this issue, or know what it could be?

 

[sduck]

I had that happen once on some old amp I had. It was a bad power tube, I think - it's been a long time, but I remember that changing out one of the power tubes fixed it.

 

[Higgs F. Boson]

Thanks. That's a bummer since I just put new Mesa 6L6's in the Class A sockets. Wonder if I oughta return them for replacements?

 

[swbo101]

Did you also change the pre-amp tubes? If it's only affecting the Lead channel, it maybe the V3 tube, I'd also check the V2.

 

[Higgs F. Boson]

Did you also change the pre-amp tubes? If it's only affecting the Lead channel, it maybe the V3 tube, I'd also check the V2.

Thanks, that will be my next step. I actually have not been able to replicate the problem since it happened. Strange.

 

[MichaelC4]

Sounds like a preamp tube going microphonic.
It happens to the V1 in my .50 Caliber+.
Crazy little feedback just starts up when playing.
Smacking on the side of the amp will make it stop for a while.

Tapping on the preamp tube can help to locate the source. (follow swbo101s advice on which one)
A preamp tube roll would be a temporary fix, if that's the problem.

 

[Higgs F. Boson]

Thanks! Yeah, my initial hunch was preamp tube. I have a box of 12AX7's to sub in, if/when it happens again.

Loving the amp more and more everyday, by the way. Easily my favorite of the few Boogies I've owned.

 

[dodger916]

Did you try switching from Harmonics to Mid Gain?