[Telelove]

I just bought a used Fender Twin RI. It worked great for about a month. One day I left it on for about 8 hours by accident. The next day I turn it on, and after it warms up I get this awfull loud continuous screach/ howling/ static sound. If I knock on the top of the cab a little it stops for a while, then starts up again. The amp also seems to have lost a lot of volume. I turned the lights off and looked at the back and noticed that one of the power tubes were sparking blue in sync with the screaching. I was hoping that the problem was caused by a burnt power tube, So, I replaced that power tube, but the new tube did the same thing. This is the only tube amp ive ever had and don't know a whole lot about it. Has anyone else had a similar problem? Should I try replacing all the tubes or just take it to a repairman...did I get ripped off or did I mess it up by leaving it on and just need to junk it? LOL