Strange burnt sockets in rain soaked amp

[Randall]

Got a boutique tweed Bassman combo in that was caught in the rain recently. Guy says he let it dry out for a few days and then powered it on, but no sound whatsoever. I opened it up to find both OT wires burnt clean off the sockets, with one pin 3 having only smoke marks where the pin used to be. The tubes all light up and the B+ is there, so I will set about replacing both sockets and taking it from there.

My question is, what particular scenario would cause this? The missing socket pin lying on the bottom of the cab had the solder melted right off it. There is no other visual evidence that the amp was wet. Could it be that the amp got hot, melted the solder on pin 3 and arced until it burnt? And if so, BOTH SIDES? And what else should I look for in this case? Obviously, take a look at the 6L6's.

[Chuck H]

Being as it was both sides I'd be looking for a fault in the bias supply. The amp was wet. That's all you really know. What the customer told you may or may not be the whole story whether you trust them or not. Pride has a way of making anyone a self defensive narcissist. Or, just as likely you're not getting any details about what happened that would be meaningful to you because the customer didn't know those things would be. In other words, when it comes to dealing with musicians and how they broke their gear you're basically like a vet compared to a people doctor. The patient can't talk and, as it happens, it's entirely possible the owner can't either. At least not always honestly or effectively. You have a broken thing and that's all you get to know.

Being as it's a Bassman clone and I can see cloth braid wire I might also suspect moisture in the black fiber board and/or wire insulation (if not actually the transformers!). I might take a moisture meter and poke it right into the cabinet wood to see just how wet that amp got. Tread carefully, take continued drying measures and check the bias supply is my only other advice.

[Randall]

Bias supply, good idea! There is no fiber board in this amp and the wiring and wood looks fine, so I am thinking it was a coincidence as well.

edit: I have poked and probed and tapped and wiggled away, and the bias supply is rock solid. Also, the tubes show no sign of overheating at all. They are very clean and the paint is still JJ red. The Eurotubes sticker on the base looks as new also. I'm going to put a word out to see if these are indeed the tubes that were in it when it failed, or did he swap them before calling me.

[Tom Phillips]

The second photo looks like there was a plate to heater short at that socket. Why it happened if the amp was dried out would be a mystery but I suspect that it happened because the amp was powered on when it was caught in the rain. If so, then the damage may have already been done while it was drying out.

[Randall]

I don't think there was a plate to heater short. Close up it looks like probably the burning spewed in that direction, but I see no evidence of a short there.

[nosaj]

I don't think there was a plate to heater short. Close up it looks like probably the burning spewed in that direction, but I see no evidence of a short there.

The one thing that is not taken into consideration is : At what stage the amp really was right before it getting a bath. A previous plate to heater short with carbon tracks not being replaced can can come back to bite later with another coincidence(rain shower, lightning strike, North Korea, whatever).

nosaj

[Tom Phillips]

I don't think there was a plate to heater short. Close up it looks like probably the burning spewed in that direction, but I see no evidence of a short there.

Note how much of the heater pin #2 is eaten away as shown in photo 2. A significant amount of unplanned energy was flowing there.

[drewl]

I agree with Chuck, customer maybe tried it when wet or amp wasn't dried out sufficiently.

Use a heat gun or hair dryer to make sure.
Replace sockets, check voltages without output tubes in, check for signal on output tube grids, the try one output tube at a time.

[g1]

Another thing that can lead to arcing/burning is running without a load.
And being boutiki, you can't be sure about the build quality. Maybe those pins weren't soldered well or were cracked during assembly. If they were compromised already during build, it wouldn't take a lot to arc or burn.

[Randall]

I talked to customer, he says these are the tubes that he was using. Here is the real tweaker, I asked him what was the situation just before the amp failed, and he said it didn't fail on him. Rain came down, he turned it off, covered it and it just never made a sound after that. I guess it could have burned if he had left it idling while it was getting wet, but what experienced pro would not hit the standby in this case? Doesn't make sense.

[nosaj]

I talked to customer, he says these are the tubes that he was using. Here is the real tweaker, I asked him what was the situation just before the amp failed, and he said it didn't fail on him. Rain came down, he turned it off, covered it and it just never made a sound after that. I guess it could have burned if he had left it idling while it was getting wet, but what experienced pro would not hit the standby in this case? Doesn't make sense.

Stuff happens sometimes things get forgotten. Even pro football players, Nascar drivers, hell anybody will forget something at one time or another that will cost them some dollars. Just part of being human.
nosaj

[Chuck H]

+1

And the guy put his amp in good hands. When the S hits the F you have a support group too There's no way to defile an amp that you can't handle.

[Randall]

This is the best support group I have ever been a part of, truly. I'll wait for the sockets to come in and report. Thanks gang!