MArshall AVT 150H inadvertent loss of gain and high pitch squeals.

[fretburner28]

My AVT 150H is losing gain in the OD1 and OD2 channels. Sometimes when I turn it on and switch to an OD channel there is just not as much gain as there usually is. Other times the gain is massive as usual. Its killing me! Also there is a high pitch squeal when I face the pickups even close to my 1960a cab. This squeal is not regular feedback it is more of an electronic noise. The squeal only occurs in the OD channels and at any volume above 4. This squeal is only happening when I have the OD channels maxed out on gain and volume. Or close to maxed out. I usually practice with it maxed or close in the OD channel and master volume at 4 or 5. It never used to squeal and now its bad. So losing gain and squealing? Please help!

[plec.22]

Hello there, have you got some exp in electronics? has you may have to do some checking with the chassis open....but before you open her up...is the front panel pot's feel in any way loose? I've have a few of these amp with knackerd o/p stage and broken front pot's...the thing about removing the front circuit board is it has metal type pcb holders that you have to wiggle about with pilers to free up the board...

If I get one of these amps with burnt o/p stage all I do is whip the o/p stage out and fit a modified power amp stage in and she's ready to jam...

[fretburner28]

hey man thanks for the reply, I will check it out and let you know.

[Enzo]

The high pitched noises when your pickups are right in front of it is still feedback. It is electrical feedback rather than acoustic. I often find it hard to operate amps with the gains maxed.

[fretburner28]

Thanks Enzo, what do you think is causing this electronic feedback. It has just started, I have been playing through this amp for over a year and have never heard this squeal before the last two months?

[Enzo]

Have you tried other guitars? A wire off a part of the shielding would be enough. Cause is excess gain through the amp, usually. Are you playing on 10 now and used to play on 9?

[jamesmafyew]

Have you ever replaced the one 12AX7 tube in the amp? I had one of these at one point and after a few years, the tube in there started to die and went microphonic. That's one possible explanation... in my case, popping in a new 12AX7 solved the problem.