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I will point out where something could have gone wrong.

I think I killed a Roland KC-500.

Today, I took my Epiphone LP (stock everything) to school for Jazz band.
Our school owns a KC-500, and I figured I could just play through that, so that I wouldn't have to lug my amp to school and back every day. I've been doing this for awhile, and for the last couple weeks I've had to use a speaker cable instead of an instrument cable. I did not think much of this, as it hasn't caused any problems before, so I don't think that this is the problem. Today though, I had to move the amp into another room. I plugged into the wall, and started playing "Just What I Needed" with a drummer. We played the song a few times, and after about 15 minutes, I smelled smoke. After looking around, I saw that it was coming out of the amplifier, and that it was moderately thick. I turned the amp off and unplugged it immediately, and upon closer inspection saw that the outlet I had plugged into had the plastic covering broken. Though the cover was broken, it appeared to be working fine, and the TV I plugged in there to test this worked like normal.

Does anybody know why this happened?
I see three possible reasons:
1. Guitar through keyboard amp.
2. Speaker cable instead of normal cable.
3. Broken outlet.

But there's three counter points..
1. I've done it quite a few times before.
2. Once again, I've done it quite a few times before.
3. The outlet seemed to work fine with the TV.

Anybody know? And does anybody know what's wrong with the amp? I thought I might have blown a tranny, but I wasn't playing loud, nor did my playing distort terribly at any point.

EDIT: And please, feel free to call me a moron for any of the three reasons. I did not notice that the outlet had the outside broken though.
Main gear (For complete list, see profile):
Epiphone Les Paul Standard Plain-top
Basswood Telecaster, 2 single-sized HB's, both split.
Epiphone Valve Junior
B-52 AT-412 Cabinet
Oh, and I have a Squier VM Jazz Bass too.
Well, you are a bit of a moron for testing the outlet with a TV, when it possibly just fried the amplifier.

The cable shouldn't be any problem; using a guitar into that amp should only damage the speaker (if anything); so that only leaves the outlet; unless it's something else.
keyboards can handle a wide frequency range, which is why they are keyboard amps. so no problem there.

a speaker cable is just thicker than an instrument cable, so no problems there.

the plastic cover dont do shit to how a plug works.

how was the condition of the amp before it started smoking? new? old? battered POS since is a school amp?

if anything, a cap popped.
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Well, he said he "thinks" he killed it, and plus I didn't see anything that would indicate that he re-tested the amp or that he explicitly said that it doesn't work anymore.
Well when I first started playing, I played through a keyboard amp, so I dont think that that would be the problem, as I was doing it for about a year. If anything, I would think that the amp was just a POS and something happened internally.
The licks you were shredding were far too hot. That amp only has a hot licks rating of 5.0, man. Be careful about that stuff! :P

Seriously though, it was probably just old and ready to die.
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It was the outlet or the amp itself.

Speaker cable doesn't mean anything. The only thing THAT will do is mess with your tone (if anything)

Keyboard amp- keyboards have a higher range of frequencies than guitar etc and it generally wont do anything. Even guitar into bass and bass into guitar doesn't hurt the amp itself, just the speaker and even then only when the speaker is being pushed.

So basically either you got too much juice from the wall (sometimes they short and **** up, that's what power conditioners and stuff are for) OR, the amp was just on its last leg and decided to go tits up on you.

Either way, you couldn't do much. No bounce, No play.