help

[Alexmc1988]

ok so far I haven't really recived much help. I have a JCM 800 4210 that sounds like crap. the distortion sounds very muddy and it hums like crazy! the clean channel isn't terrible but it could definitly use some help. I have replaced all of the tubes and properly rebiased the power tubes. I know the dangers of working on tube amps and I know how to aviod getting shocked by discharging the amp. I would love to just take it to an amp tech but there isn't one with in 1oo miles of me. I have looked at some schematics but I am just not very good at reading and following schematics. I have done effects mods using a book from indyguitarist.com and I found those instructions to be very easy to follow and made the process fun.

[Rob Mercure]

Where are you located - there are techs hiding in the wood work all over and one may be closer to you. I haven't checked it in a while for Weber VST - the speaker manufacturer - was keeping a technician's listing a few years ago and it's still there as best I know.

How did you bias your output tubes? If one of the tubes is drawing too much current it can cause your humming problem. If you've replaced the tubes you could assume that the hum isn't coming from a bad tube but then again I've seen dozens of bad tubes right out of the box especially the current crop of stuff from Russia and China. If you could pull the tubes one by one and see if the hum stops when a particular tube was pulled this would isolate the problem to a particular stage - its pretty damned hard to diagnose your problem from a distance with few symptoms so you've going to have to do much diagnostic leg work and report back to us. Have you already looked at R.G. Keene's "Tube Amplifier Repair FAQ?" If not you should as this will provide you with much introductory - and some advanced - material on systematic troubleshooting.

Rob

[stokes]

Bad filter cap?

[Alexmc1988]

nope I already check the filter caps.

[loudthud]

Shooting in the dark... bad screen resistor to one of the power tubes?

[loudthud]

Does the hum go away when the master volume is turned all the way down? If it does, that points to the output stage, otherwise it's in the phase inverter or before.