#1
I just recently purchased a "used" original 5150 combo from a local music store, and when I tested it there, though only around 4 at rhythm and lead, it sounded fine. It was tested with a guitar with passive pups though.

But at home, when I turn the lead channel on, I get TERRIBLE feedback. Yes, I realize this is the high gain amp, but when cranked to 5 with lead on, the amp is nearly unplayable due to feedback as well as the weirdest feedback noise (describable as glass shattering along w/ feedback) and even worse with the crunch activated with the lead channel.

The local music store informed me that the amp has mesa tubes that were installed 5 months ago, and the amp "should" be working fine, aside from the missing mid knob.

Alright. Now for the possible sources of this. First off, I have an epi les paul custom w/ emg 81/85 pups. In my basement, I have a computer w/ 500 watt psu that was running during this plugged into a surge protector, along with a crate 2x12 amp, and my marshall dsl100 half stack and some 9v adapters. I believe my instrument cables work fine.

It should be noted that I boost the pre rhythm to 3 o clock, and the lead is around 1 o clock, and the rhythm sounds fine w/ the pre maxed as long as it is kept below 4 in the post. The lead channel just feeds terribly.

If anyone has suggestions or solutions to this supposed problem, it would be greatly appreciated.

p.s.: if anyone decides to flame me for buying an amp that may have problems, just know that I got it for $350 with only the normal wear and tear, as well as a missing mid knob that is temporarily replaced with a push pin, and this is a 5150 from their first year of production.
#2
Well I really don't know what to say if you played it and it worked at the store.

Maybe you knocked something loose taking it to your house? Make sure all the tubes are fully seated in their sockets.

I dont really get too much feedback with my 5150-

What are your EQ/gain/presence/depth settings?

Whats your guitar? Is it wired correctly?

How about your house? Maybe you could try a new socket. Also, you should try unplugging the rest of the stuff, or at least plugging it into different surge protectors/outlets, because thats a crapload of stuff to be plugged into one socket.. that could cause a problem =x

On another note-

you REALLY shouldn't need that much gain.. I usually run my 5150 at about half on both channels with my EMG 81s and it works fine =/
#3
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!