Smokin' amp on generator - what's up?
July 4, 2011 8:20 AM   Subscribe

TubeAmpFilter: my 1970 Fender Twin Reverb started smoking when hooked up to a generator. Still works. Opened it up and can't find the culprit. What's the mostly likely diagnosis?

My band were playing in a parade on Friday so Thursday we did a dry run with the generator we planned to use ( 5.6kw - plenty big enough for the calculated current draw).

With the genny running we brought the equipment up piece by piece and it all seemed to work fine, including my Twin. However, once we started playing ensemble, the keyboard amp (a solid state Traynor) packed up and my Twin started venting magic smoke from the Vibrato inputs.

We killed everything and checked the keyboard amp, which had blown a fuse and fried a couple of resistors on the PCB.

My Twin still seemed to be working fine but we retired it for the day.

(We replaced the genny and put in surge protectors and everything was fine for the gig).

I opened up the amp yesterday and can't see any really obvious damage. The heater wires look a bit cooked where they pass over the power tubes and there might be some insulation damage on wires around the vibrato input. All the caps including filter caps look fine and the grid resistors all measure in spec. Amp sounds just fine but that smoke came from *somewhere*.

Best guess is that the generator was putting out too many volts -- I have heard of the same brand running at 150-170v and higher. If this is the case, what's the most likely source of the smoke?
posted by unSane to Sports, Hobbies, & Recreation (18 answers total)
 
Oh yeah -- transformers were cool right after the 'event' and the filter caps under the metal cover are fine -- no sign of smoke.
posted by unSane at 8:28 AM on July 4, 2011


Mr Kimdog here, a long time rock musician: Could have been a lot of things.....dust covered insulation on wires, a resistor or resistors frying, old solder flux paste heating up on solder joints, dust burning off of tubes, plastic tube sockets starting to burn, sealant burning on the power transformer. It was most probably insulation on a wire or wires leading from a resistor. There are a LOT of resistors in amplifiers.

Your generator almost certainly was putting out way too much power. Maybe it was set to 240 volts output instead of 110.

This is a great testament to the tank-like qualities of old Fender amps, but the question you should be asking is why the fuse in the amp didn't blow before things started to burn?
posted by kimdog at 8:59 AM on July 4, 2011 [1 favorite]


Mr. Sproggie suggests there is a possibility of a "one shorted winding" in the transformer. The transformer would still work with this occurrence. He says to try disconnecting the transformer to "ohm it out", or just not worry about it if it's still functioning.

One other way to locate the source, use your nose (discharge the caps first so you don't get a shock.)
posted by sproggie at 9:02 AM on July 4, 2011


Of course it goes without saying (so I'll say it) that the first place you should be looking for charred components is in the preamp section behind the vibrato inputs.
posted by kimdog at 9:10 AM on July 4, 2011


Thinking about what Mr Kimdog just said, you should make sure you have the right fuse in your amp. I'm not sure if this is true for all fuse sizes, but there are fast and slow burning fuses. Slow burning are for things like motors that pull a lot of juice at start time and less when they get up to speed. If a huge current is being pushed through your amp you could have a resistor that is busy making current into heat (and developing higher and higher resistances) while your fuse never sees enough current long enough to actually blow.
posted by Kid Charlemagne at 9:14 AM on July 4, 2011


My thought is that if the generator wasn't putting out a decent sine wave, something in the power supply filter area might be hosed.

Also, charred wire insulation can make a lot of smoke. So maybe that was just it.
posted by gjc at 9:31 AM on July 4, 2011


I would have the power transformer checked, along with the wiring. Given this is a desirable vintage amp you can have it rewound if it's damaged. Smoke always means something has been compromised, better to be safe than sorry.
posted by tommasz at 9:34 AM on July 4, 2011


Update: I checked the generator and it was putting out a nice steady 150v. Yikes.

Filter caps look fine but they are due for a change anyway.

Like I say the transformers were cool so I think they're OK.

I strongly suspect the heater wire. It is suppose to run at 6.3v but these amps are designed for 110v so the heaters were probably running at around 8.6v. The wire is skinny and right above the tubes, which were most likely also running hot. Was also a scorching hot day and the amp was sitting in the sun. If the wire overheated locally and started burning off insulation it would explain why the mains fuse didn't blow since it would not have been drawing anything like enough current -- just enough to fritz the wire.

The fuse is a 2.5a slowblo. The filter caps take a lot of juice when you first turn the amp on so a quickblow would go every time I think.
posted by unSane at 11:18 AM on July 4, 2011


You should look a little harder. Anything that was burning hard enough to give off a steady stream of smoke needs to be found and fixed, and you should be able to find whatever was burning at that level. 2 extra volts through the tube heaters would not be nearly enough to burn the wiring. The hot day had nothing to do with it; that amp gets MUCH hotter than a hot day when running normally.
posted by kimdog at 3:19 PM on July 4, 2011


So what is the most likely point of failure if you hit a tube amp designed for 110v with 150v?
posted by unSane at 6:50 PM on July 4, 2011


There are a few places I'd look.

Any resistors that run near their power spec in normal conditions would be suspect, you will find that a lot of the time they can get very, very hot and even smoke for a short amount of time and still work afterward. I have had power resistors char a plywood bench and still function. They are a prime candidate for early failure after being treated like that though and you may wish to replace them. Look for discolouration and see what part smells bad.

The other part I'd check is the power transformer, 150V is not too bad an over voltage in normal circumstances but 41 year old insulation is a different kettle of fish. The simplest checks are that it does not run hot in normal use and that with 115V in you are getting the right output voltages, if you are comfortable checking that. The best test is to remove (or just disconnect) it and get a tech to do a high pot insulation test.

I don't know your background but if you do go to check operating voltages be careful. The best rule is to use one hand only, or even better, no hands. Clip the probes on and then power up or clip one and then move only the other one. It can be surprisingly hard to keep track of both hands and you do not want an electrical path between your two hands, as that is across your chest. The electrics in a tube amp will kill you if you get it wrong enough.
posted by deadwax at 8:06 PM on July 4, 2011


Thanks, I am pretty used to working with this kind of stuff although not very familiar with tube amps. I know enough to keep one hand in my pocket when the circuit is live and discharge the filter caps when it isn't.

So grid resistors would be suspect since they are right in the signal path? I think they are 470k 2w and measured as such afterwards. If these were smoking hot they could easily have charred the heater wire above them, no?
posted by unSane at 8:13 PM on July 4, 2011


That is far more likely to have charred the heater wiring than excess current in the heater circuit. You may find they still work for evermore but they are very cheap to replace so it might be a good idea.

As a coincidence I'm about to power up a valve cinema amp from 1947. I may have my own smoke to deal with shortly.
posted by deadwax at 8:48 PM on July 4, 2011


"So what is the most likely point of failure if you hit a tube amp designed for 110v with 150v?
posted by unSane at 9:50 PM on July 4 [+] [!]"

Well seeing as the power first enters the power supply, you start there and move stage by stage into the amp from there. But at some point you're going to have to stop pondering over what might have gone wrong and actually open up that amp and use your senses; sight, touch, smell to find what was burning. That's how the majority of this kind of troubleshooting is done. You find a charred resistor or wire by looking for it with a flashlight and touching components with your fingers. AFTER you've disconnected the damn thing and discharged it.

If the smoke was coming out if the vibrato inputs, then open up that preamp and look at all the components behind the vibrato inputs.
posted by kimdog at 6:56 AM on July 5, 2011


kimdog, in my original question I describe opening up the amp and what I found.
posted by unSane at 7:10 AM on July 5, 2011


And you said "magic" smoke came out of the amp. Not a very helpful description. What kind of smoke? What color? If it was white smoke it could have been wax burning off a paper capacitor. Black smoke indicates rubber or plastic insulation or something like tube socket burning. Grey smoke could be high voltage arcing which is maybe not happening anymore now that you have the correct voltage going into the amp.

There is no reason the vibrato inputs would have burned since the only current they carry is the tiny current coming out if your guitar. Your best bet is to examine once again the entire power supply. Could be as simple as the wires going to the fuse holder, since the fuse didn't blow.
posted by kimdog at 10:36 AM on July 5, 2011


It was white/pale grey smoke.

The capacitor directly before the phase inverter has a scorch mark but not enough to account for all the smoke. The filter caps look pristine as do all of the electrolytics. Some of the chocolate drop coupling caps look a bit grimy but these should still have been working within spec I think,

You may be right about the wires to the fuse holder. There is a wire from the mains cord which runs to the (now redundant) ground switch which looks very toasted. This is in the direct mains line, right around where the amp gets hottest above the power tubes.
posted by unSane at 11:27 AM on July 5, 2011


On close inspection I found a loose screw embedded in the wiring of one of the power caps. It's not clear if it was shorting anything out but if so it could have been the cathode/plate... yikes!
posted by unSane at 3:39 PM on July 5, 2011


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