JCM 800 Kit Amp Arc?

[norby3]

I was hoping I wouldn't deal with this crap with yet another tube amp, but...

I've completed a JCM800 kit. Sounds beautiful, really nice clean and low hum. But I'm getting what I think is arcing someplace. It sounds like when you move a pot and it's scratchy, only it goes on for maybe 5 seconds. Happens almost immediately after coming off standby.

I have tried these things: adjusting the pots, tapping around the inside with a piece of wood, removing the preamp tubes one at a time, turned it on with the lights off, looked for wiring errors, tried both inputs, lightly tapped the tubes, tried different speakers, tried with and without something plugged into the input, looked for bad solder connections, lightly pushed on components to see if they were loose.

I don't see anything going on weird inside the tubes, and I biased it about 36ma.

It's a 2204 master volume w/turret board.
What fun.

[Enzo]

Arcs? SOunds more to me like a noisy power tube or cap. Pull ALL the small tubes. Still does it? Noisy tubes don;t look any different.

Does it ONLY make this noise at first power up? Tubes are physical things, and they rely on the metal getting HOT. SOme tubes are slightly loose inside, so thermal expansion makes noise. If it behaves all the rest of the evening after that 5 seconds, I'd call it OK.

For that matter it could be a flaky standby switch. Leave the standby switch ON, (better yet wire across it with a clip lead) power down and let it cool off. Now turn the amp on with the standby ALREADY on (or shorted across). Still do it?

[Jag]

I agree with Enzo.

One other thing I'd add is to check the plate resistors. Are they carbon comp? I often find this symptom and replacing the CC resistors with metal film resistors usually resolves it.

[norby3]

Well I figured out that if I let the amp warm up for a few minutes before taking it off standby the problem does not happen. I've been kind of negligent about letting my amps warm up. I can see where there could be a possible problem down the road if there is a real electrical issue, but I guess it will make itself known at some point.