channel fading probs

Ello all

I've got a jcm900 4500 dual reverb and when on channel b (distorted) everything works fine but when I switch to channel a it seems to pick up gain from channel b. It's intermittent (more apparent when I touch the channel a gain knob) but really annoying.

I recently put new valves in it to cure a fading in and out problem. I wondered if it had anything to do with them so I put all the old valves back in. Now the original fading problem has gone (like the valves fixed themselves :confused: ) but the channel crossing thing is still there. Even taking the footswitch out hasnt helped.

Any ideas? I've got no money to take this in so any help really would be appreciated :(

this is a job for....

LATS MANNN! GT TECH GENIUS!

hmm, so lets get this strait. you're on the clean channel, you switch to the dirty channel, you switch back to the clean one, and it sounds a little dirty?

could it be natural overdrive?
could your ears just be tricking you?

just some questions to ponder.

might it just be that they valve get warmer in the overdrive channel than clean and take a bit of time to cool back down for the clean channel, thats just an idea since i dont really know alot about tubes

no no, let me explain (sorry :D )

when i use the distorted channel it works fine. everying sounds great. when i use the clean channel (whether i've used dist or not) it sounds clean, then when i touch the gain knob it sounds scratchy (like a dirty pot, but i tried that) and it picks up gain from channel b as if its crossing over. the distortion cuts in and out like the noise you get when you go to put the lead in your guitar and touch the jack socket for the first time.

it happens when i play too. i'll be playing clean and it'll be ok then it'll fart and pop into distorted but its not switching channels - its almost like i'm playing clean then someone gets a wire and makes the gain from channel b short with the gain from channel a, then wiggles the wire about.

i also cant seem to get any reverb when i use the footswitch. get it fine without, then i insert the jack and it cuts off like i've broke the circuit or something.

Understand that I have no experience with your particular amp...

It seems to me that whatever handles the channel switching is not fully switching. If its done with mechanical relay(s), there may be some intermittent problem with some of the contacts. If the switching is done with something solid-state like JFET's, there may be some leakage in the control circuit that keeps the channels partially open sometimes.

My guess.

For the real scoop, you need to take it in to a tech who knows this particular model of amp.