Help with amp trouble


Hey guys, long time reader, first time poster. I'm hoping ya'll can help me out:

I recently purchased a used EVH 5150III 50 watt head. With the amp hooked up to the speaker cabinet, with no guitar plugged into the amp, as you increase the volume on channels 2 and 3 the noise / amp's "hiss" is louder than what I can imagine should be normal. This is not so much the case with the clean channel.

Furthermore, while practicing with my band at higher volume levels, I get intense microphonic feedback/squealing no matter what guitar I ran through the amp. This happens even with the gain set at less than half. I swapped out the pre-amp tube in the V1 position, and nothing changed.

I feel I'm pretty familiar with high gain amps - prior to this I was playing out of a Mesa Dual Rec and never experienced noise issues like this. For this reason I'm assuming the issue does not lie in my guitars/cables/pedal board. The amp behaves this way in multiple buildings too, so I don't think a issue of the building's wiring.

I'm looking to get a second opinion on what exactly is going on (ground/wiring issue?) before I re-tube the thing, because at this point I can't be certain that re-tubing it will fix the problem.

Any suggestions or help would be greatly appreciated. Thanks again and I look forward to hearing from ya'll.


A few ideas...Is there any squeal with the clean channel at high volumes? If not, I suspect the extra gain stage for the drive channel is the problem. The simplest and cheapest thing it can be is a microphonic preamp tube; try tapping each one with a pencil and listen for the squeal/ring

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