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LittleSickFish 2nd January 2009 03:41 PM

Cracks and hiss in tube guitar amp
I own Laney VC-50 tube guitar combo. I have a problem with hiss and cracks on the clean channel. When I switch to the clean channel, I can hear some hiss. It is surprisingly louder than hiss or hum on the overdrive channel which should be noisier.
Then couple seconds after there comes some cracks, they become louder and louder and then It turns into terrible rumbling noise. Both hiss and cracks doesnīt respond to volume control on the clean channel. It only responds to pot on the end of effect chain, which is basically master volume control.
I think that the noise rises slower when I play the guitar, but when I stop, nothing is going to stop this. I also noticed that when I switch on the boost button on clean channel, there is less hiss and the cracks donīt come louder.
The overdrive channel is perfectly OK, problems only start when I switch to clean channel.

This is the story how the problem started:
We were playing a show in club.. I moved the amp from car on the stage and I think that maybe I didnīt wait enough to balace the temperature of amp and plugged the amp too soon. Maybe some water condensed there and ruined it. However, for couple minutes the amp was playing perfectly, we did a soundcheck, then I turned the amp into standby for maybe half an hour, came back, put it on and bang, the only thing I heard was this terrible rumbling when swithed to clean channel.

So, to sum it up: Only the clean channel is damaged, boost mode on the clean channel is bit less noisy than clean mode. I think that the problem is somewhere between volume control and the preamp output, because noise is not responding to volume control. It is NOT bad preamp tube, I retubed the whole preamp and power amp!

Here are schematics. the VC50 / VC100 / VH100 amps have the same concept:

Guys, I will be glad for any response and idea:dead:

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