terrible noise from JCM800..possibly a bad preamp tube socket. looking for some input

moonboots

Member
Messages
67
I'm having this awful, randomly occurring noise coming from a JCM800 2203. it sounds like feedback almost. really loud, ear piercing feedback. or like something heavy is falling and smashing to pieces. but it doesn't happen all the time. just for a couple seconds every minute or so maybe. something like that. anyway. i poked around beneath the chassis to see if i could find anything out of curiosity. underneath the v1 preamp socket, the lug for pin 2 seems looser than all the other ones. like if i pulled on it hard enough, i could pull it out. also...probing it seems to affect the bad sound i'm hearing.

now my question is, could the loose lug be causing the noise? if not, what could it be?

i was going to replace the socket, but before i did all the work and bought the new socket, i wanted to see what other people might think.
 

goldtop4g63

Member
Messages
259
Have you tried swapping tubes to see if you have one going microphonic? It kinda sounds like a bad tube. If you think you have narrowed it down to that pin, the tube is good, and there are no burn marks on the contacts in the socket there is no need to replace the socket. I would re-tension the contacts in the socket. Then get some contact cleaner and spray the socket and put the tube in and out to clean off the contacts. While I was at it I would do it to all the sockets.
 

moonboots

Member
Messages
67
Have you tried swapping tubes to see if you have one going microphonic? It kinda sounds like a bad tube. If you think you have narrowed it down to that pin, the tube is good, and there are no burn marks on the contacts in the socket there is no need to replace the socket. I would re-tension the contacts in the socket. Then get some contact cleaner and spray the socket and put the tube in and out to clean off the contacts. While I was at it I would do it to all the sockets.
not all the tubes. just the preamp tubes. no change. i have a couple of new el34s. i've been waiting for the right time to put them in my vox ac50 but ill swap them around and see what i find and report back. i'll clean the contacts and attempt to retension the sockets, but I've never tried to retension tube sockets before. i understand the idea behind it..but how you actually do it is what I'm unsure of.
 

gldtp99

Member
Messages
3,905
Yes, clean tube pins and tube sockets---- re-tension sockets as needed----make sure tubes are stable and not microphonic---Also cleaning Input and Speaker jacks every now and then is a good idea.
The soldering on the tube sockets wasn't always the greatest in Marshalls of this era---- If some connections need to be touched up, do it.
It is possible the preamp tube socket is too far gone to be successfully cleaned/re-tentioned/solder touched up----- but not likely.
Usual tube amp maintenance should cover it................................gldtp99
 

moonboots

Member
Messages
67
i re-tensioned the preamp amp sockets but there was no change. then i gently tapped the power tubes and figured out the issue. the v5 power tube is bad. i swapped it out with a different one for a quick comparison and the noise is gone. I'm going to get a new matched quartet ultimately. thanks for the advice. definitely need a new socket after all!
 

gldtp99

Member
Messages
3,905
I'm glad you found the problem---- a new matched quad of EL34's, a bias check/adjust and your 2203 will be back in shape !!.....................gldtp99
 




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