poo-bar...jcm800 prob

[tristan]

ok so my amp has pooed itself

i think its a power tube problem. which you would think would be fixed by replacing the power tubes, but im poor and i dont want to waste money on tubes if i is something else.

so when i crank it (which iveonly ever done once before) it looses volume sometimes and craps out, but comes back in for a litttle while. i think ive heard of this hapening before, just want to double check that its power tubes :)

cheers and beers

tristan

[John Phillips]

Probably not, if by 'crapping out' you mean the signal disappears entirely. It could be quite a few things, but the most worrying is if it's a bad impedance selector - these can and DO fail, and if it's this you're risking the tubes and the output transformer. You should check this as the first possibility before doing anything else - apart from checking your speaker cable and cabinet (especially the jack) for the same reason. Try moving the selector very gently - not enough to click it from one setting to the next - while listening to the amp, and see if the hiss/hum varies or cuts out. If it does, you need to get inside and squirt some contact cleaner/lubricant into the switch (it's quite difficult to get in) and cycle the switch back and forth for a while to clean the contacts. You may need to replace the switch if it's really bad (or bypass it if you only use one setting). Even if it's not obviously the cause I'd still be inclined to clean it.

If it's not a selector fault, it's most likely to be something other than the power tubes, since even if one fails entirely the sound won't disappear altogether. Check the fuse holders are tight, and have the correct type and value of fuses in. Check the preamp tubes are making good contact with their sockets (wiggle them about a bit with the amp running and listen for crackling or cutting out). If it's none of that, it's probably a failing component or a bad solder joint inside and you may need a tech, if you're not familiar with servicing amps.

[tristan]

cool john thanks a bunch.

the signal doesnt go completely, just looses alot of volume. but it sounds fine at bedroom volume?

also i have a combo and the impedance selector has not been played with, but i jiggled it anyway.

as i said i can play it at low volumes withoout a problem. so.....?

ta!