Suspected shorted tube socket

[Esquirefreak]

So, a strange thing happened a couple of days ago. While playing my AB763 Showman, the amps volume dropped/faded out in a matter of seconds until silent.

Looked around, nothing burned, no smell. Strange.

Took the power tubes out, measured voltages and everything appeared to be just fine.

I suspected a shorted power tube, so I put in new ones. Fired up, played the amp for a while and let it run for a while, monitoring voltages and current. Shut it down for a couple of hours.

Fired up again later and wasn't getting any sound.

Then I noticed - 7 out of 8 tubes were cold. Sure enough - no filament voltage, except for the power tube closest to the lamp. That power tube has a steady 6.4 V~, while the other tubes seem to come and go.

It almost seems like it's intermittent, as sometimes the voltage is there and other times it's not.

It's an early silverface and has the thinner (coated?) type of cloth wire. I've heard that that type of wire have a tendency of corroding.

Tomorrow I will look for carbon traces on the sockets as well as solder joints and possible DCv on the filaments.

Any thoughts on more trouble shooting methods?

/Max

[sluckey]

I suspect a poor connection in the filament string between the first power tube and the next power tube. Closely examine the connections on both of those power tubes. A bad connection on either socket will cause your symptoms. Also, a broken wire between the two tubes would cause the same. I'm betting on poor solder connection.

[Esquirefreak]

I'm really hoping that's the case. Really don't wanna swap out more parts than necessary in this old amp.

Thanks for helping me!

/Max

[Esquirefreak]

A loose solder joint, sure enough just like you predicted.

I've almost felt like I was losing my mind on this amp. It sounds sooo good, but this imtermittent issue damn near broke my sanity.

Fingers crossed this was the only culprit!

/Max