#1
 
The amp (Carvin Legacy 3) will sound great and then all of a sudden the amp will cut, and you can't hear much (it sounds like when you crank the master volume but have no channel volume). If I slam the top of the amp with my hand, it'll go back to normal. I took it to my local music store and they replaced the tubes. Tubes 1 and 4 are Groovetubes and 2 and 3 are JBL (or vice versa) since they were out of JBLs and I had 2 groovetubes lying around. They never biased it, which makes me suspicious because I never heard of the legacy having any "autobias" features, but he said that if it needed a bias then it wouldn't make any sound.
Anyway, I bring it home and it sounds better, fresher and lively, but the problem persists.
I assume that this is some sort of loose connection the circuitry. If I had another amp with an FX loop, I'd test to see if the issue is in the pre-amp or after it, but unfortunately this is not the case. I might be able to identify the problem myself with the assistance of the manual and/or youtube, but I don't know, that is why I am asking what YOU think: Do you have any ideas as to what is wrong? Could I maybe identify the issue myself, as an inexperience tech? Could I maybe fix it myself?
Also if by chance anybody knows a good amp tech in the greater Cincinnati area, do me a favour and drop me a name.
Thanks in advance!
#2
One possible thing to check is the effect loop and the pre/power amp loop.
Sometimes the switch in the sockets that the plug activates can get a slight oxidation on them.

Simply plug a patch lead between out and in of them both. If this cures the problem, then the socket switches need a bit of cleaning attention.

"but he said that if it needed a bias then it wouldn't make any sound"
Not so. Biasing selects the operating range of the tube. Slightly out will hardly be noticed. If it was a fair way off, the tubes can run too hot, or too cold.
But still won't cause no volume until there is a component failure.
#3
Sounds to me like it could be a bad/broken solder joint.  If you are comfortable and knowledgeable enough to safely poke around inside a powered on amp I'd suggest poking every solder joint with a wooden chopstick.  Usually cold solders or broken contacts  will pop or crackle when you poke at them with something non conductive.  My Mesa does this because I have a bad connection on one of the pins of the 1st 12ax7.  I've been getting around to fixing it for about 2 years.
Not taking any online orders.
Last edited by CorduroyEW at May 30, 2017,
#4
Vreid tried to patch the effects loop, problem persists. I may just borrow a friend's amp to combine their pre amp with my power amp and vice versa to narrow it down to one of those. The guy I got a hold of at Carvin said that there is a good chance that one of the jacks (input, speaker out etc) may just need to be replaced. We'll see I guess. Thanks for the suggestion!