Flames from OT, can it still be ok?

[sonicmojo]

So I had an incident with an Ampeg Reverberocket 2 combo. I was tipping it over and looking in the back and accidentally tipped it where it kinda landed forward on my feet with a slight thud. Immediately loud hum and flames were shooting out of the OT where the primaries go in. I saw them! I pulled the plug immediately. Waited a minute, turned it back on, high squeal noise. Dead OT right? I took it out of the cab onto the bench. I could smell the fried paper but not a lot of obvious damage. I measured for shorts and open windings and all looked good. I took a stick and probed the primaries around away from the secondaries where I could see them entering the paper windings.

I turned it back on.

Works, no flames or hum....I thought for sure this must be a fried OT. Has anyone had any experience like this?

What should I do now. Should I drip some 2 part epoxy in there where the wires enter to cover any exposed leads that may have been the culprit. Or is this a ticking time bomb now?

[xtian]

Woah.

Any visual clue about where the short happened?

[sonicmojo]

I can’t see anything obvious...

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[Stevem]

I bet it was arcing from the fall shock at the output tube pins ( tubes rocking around) and it just seemed to be from the OT reflecting around on the shinny insides!
If the amp / OT still outputs 15 to 18 watts of RMS power with no arcing then it's ok, but in general old Ampeg output tube sockets are shot as there not even that great when there new.
If your going thru the amp for a customer then replace the output tube sockets with the Marshall cross pin type that grip like mad!
Dribbling epoxy in the exit holes is a good idea and I do such to all of my amps and customer amp that I know get moved around a lot.

[Reeltarded]

YIKES

I need coffee and a sedative.

[R.G.]

The name of this game is "You Bet Your Tubes And Speakers".

A visible electrical arc in air runs at temperatures of up to 10,000 degrees. The white color is actually the gasses glowing white hot from heating. This is hard on the materials at the ends of the arc and anything it contacts along the way. Ask a welder about the process. The heating ionizes the gasses along the path so that they are actually conductive, so an arc, once started, tends to stay in the same path because maintaining an arc is easier than starting it to begin with. Again, ask a welder.

You definitely have burned materials at the start and ends of that arc, and some burning along the way. Burning transformer insulation leaves carbonized grack along the arc path, which is why it's bad on output tube sockets. However, the carbonized grack is still more insulating than ionized air, so if you extinguish the arc by turning off the electrical power and letting things cool off, the arc mostly stays out.

But it is still easier to start an arc in the same spot than it was before the arc. The arc burned off the insulation (if there was any) at the places where it started, and made itself an easier path to travel the next time. So the second and succeeding times, it's easier to start.

The answer is - yes, it's possible that one visible burn can happen and if you extinguished arcs (and flames) in time, the amp will still work. But it's easier for it to fail in the future. It's not as tough as it once was.

[sonicmojo]

Thanks for the replies! And yes, I don't really trust the amp right now so will be careful with tubes and speakers I use for a while as I crank the heat back up on it. The speaker and tubes test out ok after the event.

I swear the arc was happening at the OT and not the sockets. I did the epoxy thing and some even dribbled down through to bottom where the secondaries come out so hopefully I coated whatever may have been the problem area.

The amp is mine and the good news is I got the entire amp mainly just for the cab as I've had another RR2 chassis for years sitting in a head cab that sounds really nice so this is just a spare chassis. I measured the OT secondary resistance values on both amps and they are close within just a few ohms so I don't think I shorted anything within the secondaries. Will recap the original filters and put a three prong cord on. I kinda wonder if I had the plug reversed and the charged chassis was part of the problem....

[Roe]

I have a big PT with a shorted bias secondary. I'm not sure if I can do anything with it. It smelled bad but wasn't on for many seconds

[sonicmojo]

Can't help myself. The newly acquired Reverberocket 2 has now gone into a head cab that I just made for it actually out of another box with working similar dimensions. It is working quite well and sounds pretty much just the same as the other one even though the smoke incident. I bought the tolex and handles from Fliptops and so I guess I have a another one to use now. I can always get another transformer from Fliptops if this one blows up. As far as I can tell both these amps were made around the same time and do not have PCB on the main board but the bottom one is a little later and has PCB for some of the pots. Same exact circuit as far as I can tell. I even bought new can caps but did not replace them as these are both hum free and working well. Kbobs are different but I'm pretty sure they are both Ampeg knobs from the similar 1967 timeframe.

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