Ok, so I have a used 5150 combo that I bought from a local music store. It feeds back ungodly. I have an ISP decimator on the way, but I'd like to actually have the amp working properly.

I've got an epi lp w/ emgs, but this doesn't seem to be the main problem. I've determined the noise comes from the amp, b/c when I turn it on w/o anything plugged into it, I get a fuzzy/ white noise sound, and when I switch to the lead channel and crank it to about 5, it starts feeding back horribly despite nothing even being plugged in.

I turned off my comp, lights, and other electrical appliances, and plugged it into a working outlet, and tried other outlets, but the noise persists.

If anyone could tell me what the main source of this noise is so as to prevent me from being screwed over by an amp tech, I would be ecstatic.
First, let me warn you that 5150s are noisy amps. They hiss like crazy, and there's not much you can do about that except usign lower gain tubes, which defeats the purpose of a high gain amp. It's the nature of the beast with this amp. However, if it's giving feedback while nothing is plugged into the amp, it sounds like your preamp tubes are microphonic and need to be replaced.

You'll need five 12AX7 preamp tubes, and possibly two 6L6 power tubes. You can replace them individulally, but IMO, if your preamp tubes are going (they usually last a lot longer than power), it means they're old and you should replace them all. And chances are, the power tubes, which have a much shorter lifespan, will also need replacing.

If you just bought it (even used), then the store should have some kind of warranty on it. Check with them to see if they'll do it for you. If not, the nice thing about the 5150 combo is that it has no biasing controls, therefore it makes swapping tubes easy. Just pop the rear panel off the amp, remove the old tubes, and replace with new tubes. This will get rid of all your feedback problems and probably a bit of the hiss.
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Hey thanks for the help, I was told it had new mesa tubes put in 6 months ago, but who knows.... do you recommend a certain brand, like groove tubes or EH?
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Use JJ's preferably, but EH's are also good.
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i have a cure for that noise,dont stop playing. you only hear the noise when you stop. or be quik with the foot switch back to the rythm channel.
Did you ever solve your feedback issues, Im a fellow 5150 owner and have the same problem. I am looking into buying a rack mount noise eliminator like the rocktron hush, but wanted to see if you found a good solution. I also am looking into a Behringer Feedback Destroyer, Im just afraid that the Feedback Destroyer wont kill the hissing and humming, just the feedback. Let me know, thanks!
Not trying to revive a dead thread, but it turns out the tubes were microphonic, so I replaced em, and bam. problem solved.
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