amp problem

I think I have this problem figured out, but I am just asking to make sure. The other guitar player in my band has a peavey xxx. He bought it about 2 months ago. Recently at band practice, he has had a problem with feedback. Really bad. Anytime he isn't playing something and there is any amount of volume he is getting a ton of feedback. So we started looking at tubes. We tapped on all the tubes, and one of them made a noise when we tapped it. It only happens on the dirty channels, but I figure the tube must be only used on the dirty channels. Has the tube gone microphonic? I told him that more than likely if he replaced that pretube it would fix the problem, but I just want to make. Can only let me know what they think?

Easiest way to find out is to swap the tube with one of the other preamp tubes. Preamp tubes are pretty much interchangeable.

Peavey Amps which have IC's, are prone to IC failure which presents as feedback. I agree that you should swap around two of the preamp tubes, but if that does not solve the problem, get professional help .

The problem could be where Philip says but microphonic tubes will surely feedback. Some tube sockets are much more critical to the quality of tube used in them. If a gain stage is driven by a microphonic tube it will amplify the signal it recieved compounding whatever trouble there was. Looking from the back the far right socket is usually the first gain stage and the most important to have a good tube. The preamp tube to the far left (before the larger power tubes)is usually the phase inverter and not a bad place to stick a microphonic tube if you dont have any other options at the time. Be forewarned. If the socket at the far right has a 12AX7 tube in it and you replace it with a 12AT7 or 12AU7(amongst others) the amp wont have has much volume overall but you'll be able to get more out of it before it feeds back. Old harp players trick to fight feedback

First of all, an invitation:

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It is NORMAL for at least one tube to 'make a noise' when you tap on it. This is the 'gain hole.'

What you don't want is any squeal when you tap them.

Try moving that tube around with another 12AX7.

ALSO: Try using other guitars on the amp.

The XXX is a high gain amp. Pickups susceptible to feedback WILL feedback with higher gain. High gain + loud volume + small room (and/or close proximity to amp) = feedback issues.

HOWEVER, if he hasn't changed anything, and the feedback is a new issue, you need to troubleshoot to find the culprit.

Your better guitar p'ups are vacuum wax potted, to reduce microphonics. Do you know what he's using?

Try this test. Have him tap lightly on his p'up. Does that induce feedback? Then the p'up is the source.

Actually, since the XXX is very high gain, you may not lose any overall volume. ...just some of the gain. I don't like the 12AT7 as much, because operating characteristics are different than 12AX7. I prefer the 5751 to the 12AT7. The 5751 has about the SAME gain as a 12AT7, but is much more compatible with the specifications. You can sometimes score triple mica 5751 off of eBay. But be warned! I'm currently dealing with a tube seller that GROSSLY misrepresented some tubes!

The 12AU7 will give you a rounder distortion tone, and less gain. I usually prefer the 12AU7 in gain holes that have too much gain with a 12AX7.

but you'll be able to get more out of it

Well, I'll tell you a little about the problem he has been having. He got EMG's installed on his guitar (Les Paul Gothic, Gibson) and this is when it started. Then it progressivly moved on to any other guitar he plugged into the amp. Now it is to the point that he can't even play songs because if he isn't playing a note the amp is producing feedback. We shall see, he is trying a new preamp tube today and seeing if it helps.

What is IC failure?

IC stands for integrated chip IIRC. It means that there are chips in that amp that could possibly have fried. If that is the case, I recommend getting it repaired by someone qualified to do it as it is easy to screw up.

EMG's are hot pickups. Really hot. Did I mention that they're hot?

Heck, I can go direct from my EMG's to my power amp, skipping the preamp all together! Coupled with a triple X preamp (extremely high gain), I'd expect some feedback at some point.

Then it progressivly moved on to any other guitar he plugged into

NOW it's sounding like a microphonic tube that got progressively worse!

Now it is to the point that he can't even play songs because if he

I predict that'll be the cause. Often, it isn't necessary to buy a new tube, you can just move existing tubes around. The problem occurs when the microphonic tube is in the 'gain hole.'

Integrated Circuit, but who's keeping score?