Intermittent Amp Problem (Ga-20T)

[MilkNCereal]

The problem is a little confusing. It deals with a Gibson ga-20t. When I play at around noon, the amp sounds fine, but once it gets to about 9 o'clock on the volume I start to hear this crackling noise. Also when the tone knob is anywhere past noon it starts to have nasty buzzing sound like there is a tear in the speaker. On the high e,b,g it does not really crackle, but the other string it sounds really bad. It sounds like a loose soldering connection because it only happens on high volume settings (vibrations causing the connection to make contact with other things).

Now the weird part is that it does not always happen. I will play for about 5 minutes without any problem. Then while playing with same settings the crackling starts to happen. But when the volume is low again, this problem disappears.

There are replaced caps. But I have been told that the coupling caps may have failed. I have swapped the original working tubes with other tubes, the sound still occurs. I know that this is a lot but any help would be greatly appreciated! Anyways I would be taking it to a tech somewhere in the Seattle area. Any ideas on that? Thanks

-Ryan

[donnyjaguar]

A quick guess is that one of the capacitors is breaking down and passing DC. Easy to find with a multimeter.

[doc]

Have ALL the electrolytic caps been replaced? It sounds like a cap - if the filter caps are new you may still have a cap in the preamp circuit that is failing - change the electrolytics first.

[RedMan]

Sounds like it just might be a dirty tone pot to me.

[Amplitude]

If I read your post right, and the noises happen with vibration (volume), it could be a loose component. I have noticed sympathetic crackling noises with regard to CERTAIN notes only, like the chassis vibrates, or is tuned to certain frequencies which vibrate the cabinet more (usually in the low register). Try to notice if certain frequencies cause more noise. If you find it's not that, it probably is a cap letting DC through...

[redtoploader]

A quick guess is that one of the capacitors is breaking down and passing DC. Easy to find with a multimeter.

+1 this would be my guess. They can be a pain to get to in a ga20t though.

[5E3]

A quick guess is that one of the capacitors is breaking down and passing DC. Easy to find with a multimeter.

For future reference, how do I test a capacitor with a multimeter? What do you measure for? Thanks.

[donnyjaguar]

Given that a capacitor passes only AC, if you measure one that has the same DC voltage on both sides then its a pretty fair guess that its whipped. :)