Help with a Smoking Super Reverb

[macula56]

I recently did some work on an old '66 Super Reverb. New power tubes, rebias, clean up and what not. I played it for at least an hour and it sounded great, no issues. The guy picks it up and takes it home. I get a call from him a little later and he tells me that when he turned it on it was making crackling sounds then started smoking. And the smoke was not coming from the power tube/tranny side, but from the input jack side. I got the amp back and opened it back up to see what got burnt and I could not find any signs of overheating components. Nothing. Everything looked great. Usually when I have had something start smoking I see components that have discoloration or bubbling. So I turn it on and I have some bad crackling. Three of the pre tubes are looking pretty worn out so I replaced them and the crackling disappears and the amp sounds great again. I would have replaced them originally but there was no crackling at that time so no need. I played it for a while and left it on for at least an hour with zero problems. My question is this. Could a bad preamp tube been the cause of the smoking? Could a pre tube itself have been smoking? All 3 tubes are in the reverb channel, if that helps. I can't find any other bad components and voltages look great  everywhere. Any thoughts? Thanks, JMac.

[sluckey]

Could a bad preamp tube been the cause of the smoking?

Not likely, but the reverb driver might.

Could a pre tube itself have been smoking?

no

Any thoughts?

Look under the dog house. This one may come back again if you can't find what actually smoked. How much smoke are you talking about? Just because the smoke left the chassis from the 'input jack side' doesn't mean the fire was near that side. Usually smoke comes from power supply or power amp circuit.

[macula56]

I checked there and all the caps look great. Everything looks great. No weird smells either. As far as the amount of smoke, I can't say for certain as I wasn't there when it happened and it has not happened again since I got it back. My first thought was power tube socket and red plating tubes causing PT smoking, as I have had that problem on a couple of other old Blackface Fenders. But it hasn't done it again.

[eleventeen]

Sometimes a bad socket contact on the power tubes can defeat bias being fed to them.

[pardon me for suggesting what you already proposed]

How are the plate resistors on the preamp tubes? Still old/orig 100K 1/2 watts?

Measure the 470 ohm screen resistors on the 6L6 sockets. Are they cooked?

[macula56]

tube sockets are in good shape. I wiggled them quite a bit to see if I could get any red plating and they work great.

How are the plate resistors on the preamp tubes? Still old/orig 100K 1/2 watts?
No. the plate resistors are not the originals and they all measure very near 100K.

Measure the 470 ohm screen resistors on the 6L6 sockets. Are they cooked?
Nope. they measure right around 470 and look good.

I have had the amp running for almost 2 hours without any problems. the only thing I changed were the 2 reverb tubes, the 12at7 and the 12ax7.

[Toxophilite]

In my limited experience electrolytic caps  and resistors can smoke a little and still retain a non burnt look, especially electrolytics
I would suspect the electrolytics
I think it's very strange that there's no lingering smell, especially if there was visible smoke
I find the electronics burnt smell is pretty strong and distinctive


Maybe the smoke came from something 'flammable' outside the amp

[sluckey]

Like the kid hiding behind the amp tokin' on a dooby!   

[eleventeen]

Yeah, short roadies can stoop down behind Soopers. 

[macula56]

I took a really good sniff at the PT and I smell something burnt. Not real heavy but it's there. I'm gonna replace the power caps.

[eleventeen]

Appreciate where you're coming from, but in my experience, once the filter ecaps go bad, at least bad enough to cause smoke, they are bad from then on. They should thus not allow you to have 2 hours+ of good operation without blowing fuses or cooking the tranny. It's also not uncommon for a 50 year old power transformer to have a burny sort of smell to it....and still work. I am not sure how to advise you on this.

[macula56]

well at any rate replacing the power caps won't hurt anything. i have tried tapping, slapping, shaking, smacking and anything else I could think of to get this amp to repeat what it did and I can't get it to do anything but sound great. I am at a loss to explain this one. I wish I had been there when it happened so at least I could have heard and seen what went on.

[Fresh_Start]

If you can, you might want to measure the capacitance of both the old filter caps and the new ones. I was very surprised to read someone's test results recently on some new caps showing much higher values than expected. Could give the amp a different feel.

While you're at it, you might want to check the bias filter cap.  Bad things can happen if that goes bad.

Cheers,

Chip

[super&plexi]

no joke, something similar happened to me and my SR...turned out to be grease from burrito I was eating while working. Maybe guy was poking around, checking out your handiwork, got something on tubes....just a thought. My prob was further exacerbated by the fact that tube cover masked the point of where smoke was emanating.  I have no problems with humility.....