Tube socket arcing to nearby ground

[dkjay050876]

i am not new to tube amp repair or building, but have a problem that doesnt make sense. 100w pc board guitar B52 AT100 amp. frequent arcing between pin 3 and now pin 4 on 6L6 tube to nearby ground. nearby is about 1/16 of an inch. for some reason, the board designer didnt eat away the copper from around the tube socket mount holes, so ground is within about 1/16 of an inch or so on all 4 power tubes. only 1 tube has done it, and still does it if i move tubes around, same socket, same terminals. i have done very little circuit tracing because even 3000 vdc cant jump anywhere close to that far. any ideas? i bought this very cheap to fix, and if i cant was going to gut the chassis and build myself a 100w tube bass amp. once i fixed the issue it came in with, i decided to see how it sounded as is, as a bass amp. i was surprised that the tone was warm and just what i wanted. i played about 10 minutes or so at moderate to loud volume, maybe 1/2 of the way up on gain and 1/4 on master. out of the blue, snap, which blew the fuse. cleaned up the board, no damage, new fuse then same thing. any ideas?

[nosaj]

i am not new to tube amp repair or building, but have a problem that doesnt make sense. 100w pc board guitar B52 AT100 amp. frequent arcing between pin 3 and now pin 4 on 6L6 tube to nearby ground. nearby is about 1/16 of an inch. for some reason, the board designer didnt eat away the copper from around the tube socket mount holes, so ground is within about 1/16 of an inch or so on all 4 power tubes. only 1 tube has done it, and still does it if i move tubes around, same socket, same terminals. i have done very little circuit tracing because even 3000 vdc cant jump anywhere close to that far. any ideas? i bought this very cheap to fix, and if i cant was going to gut the chassis and build myself a 100w tube bass amp. once i fixed the issue it came in with, i decided to see how it sounded as is, as a bass amp. i was surprised that the tone was warm and just what i wanted. i played about 10 minutes or so at moderate to loud volume, maybe 1/2 of the way up on gain and 1/4 on master. out of the blue, snap, which blew the fuse. cleaned up the board, no damage, new fuse then same thing. any ideas?

Using your meter withe the amp unplugged and 6l6 pulled do you have continuity between pins 3 and 4. If so you may have a carbon track on the board now. Sometimes taking a dremel you can cut it out. Also check the 6l6 pins 3 an 4 or if you have a tube tester check the tube for shorts.

nosaj

[Chuck H]

if you have a tube tester check the tube for shorts.

And probably any tube plugged into that socket now. Since the tubes were swapped around and the problem stayed with the socket there's a good chance of a conductive path on the board and/or socket that may have initially been caused by a tube short in that socket. The voltage spikes in the tube and there's no lower impedance path for current to flow and POW you get an arch and a new conductive path that will continue to fail other tubes plugged in there. The socket and board need to be inspected for conductivity and repaired as needed and the tubes need to be tested and found to be good or replaced.

[mozz]

Change the socket. If you're cheap try drilling out the socket rivets and put a spacer in there to move it further away from the chassis.

[dkjay050876]

i cut away the copper foil that was common to ground and nearby to pins 3 and 4, i have about 1/2 hour on it and issue hasnt resurfaced. i think perhaps it was a carbon track or something from the original arc. when it first happened i was pretty sure i saw a small piece of loose solder, between pin 3 and ground and assumed that is what the problem was. hoping that initial arc left some kind of conductive path that deoxit and a wire brush didnt get off. now that the copper foil is gone there is "clean" board between pins and the remaining ground foil. i WILL pull the tube, drain the caps and check for any continuity between the 2 pins, or ground. THANKS. if it persists, i will be back. the amp just sounds too "proper" for there to be an issue. generally speaking, if an amp sounds good, and works for an hour of loud use, "at aint got no issues"

[Enzo]

There is the answer. You said it up top, they neglected to grind away the nearby copper. SO grind it away yourself. If you ever have an arc, and there is not enough copper to grind away, then cut off that copper anyway, and run a jumper wire in its place.