Vox AC 15 Issues

[Six String]

I recently installed 3 tung sol 12AX7 tubes in my AC 15 amp. after completing it, the TOP boost channel no longer works. I have never had any issues prior to this so i must have messed something up. Any ideas for trouble shooting?
Any help would be greatly appreciated

[Brent Hahn]

If you swap those three tubes around, does something else stop working?

[Six String]

Originally Posted by Brent Hahn

If you swap those three tubes around, does something else stop working?

ill try that and reply.

[Six String]

Originally Posted by Brent Hahn

If you swap those three tubes around, does something else stop working?

so i opened it up and the middle one wasnt glowing. When i removed the little wire thing from it, it started working? im not sure what the issue is. Is it a big deal if that wire thing isnt completely connected to it? is it designed to hold it in place? i dont think its going anywhere as it holds in place on its own. I dont know why it wont get connected with the wire completely around it. a grounding issue?

[jonfields45]

Originally Posted by Six String

so i opened it up and the middle one wasnt glowing. When i removed the little wire thing from it, it started working? im not sure what the issue is. Is it a big deal if that wire thing isnt completely connected to it? is it designed to hold it in place? i dont think its going anywhere as it holds in place on its own. I dont know why it wont get connected with the wire completely around it. a grounding issue?

Probably some oxidation on one of the tube pins not allowing a decent connection. Just moving the tube around probably cleared it.

Electronic leads that are not gold or nickel plated, are tin plated (most things these days now that you can't use tin-lead). Tin can oxidize reasonably easily forming a resistive layer.

[Rudy4]

Your symptoms point to the very common loose tube pin solder joint to the printed circuit board.

It's a very common problem on the raft of "tube amps" that use tube sockets soldered to PC boards. Attaching the tube bale exerts enough pressure on the base to cause the connection to open. The long-term remedy is to pull the board and re-solder the tube connections to the copper pads on the PC board. Occasionally the copper pad will break where it changes to a printed circuit trace. That can also simply be bridged with solder.

The simplest way to find that type of break is to place a fingertip over each tube pin connection on the PC board and physically move the tube a bit with the other hand. A loose or broken joint can almost always be felt. Obviously this with the PC board out of the enclosure and NO POWER applied. There are bleed resistors that should dissipate any dangerous voltage from filer caps, but if you have ANY doubt about what you're doing in a tube amp PLEASE take it to a qualified technician.

Swapping the preamp tubes is also an easy way to see if the problem stays in that particular portion of the circuit. Tubes are notoriously dicey in their manufacture today, so it's good to swap them around if possible. The most often noted problem with new pre-amp tubes is odd "scratchy or fuzzy" noises from the circuit due to the internal structure of the tube when it was manufactured.