Fender Twin Reverb on generator released the magic smoke - help!

[unSane]

Last week my band played on a float. We used a gas generator and brought the instruments online one by one with no apparent problems. However when the bass player arrived and plugged in, we blew one solid state amp (a Traynor - fried several resistors inside before the fuse blew) and my 1970 Twin Reverb started pumping out smoke from the Vibrato inputs.

Thought it was fried but worked just fine on the mains and I played a gig afterwards with no problems.

Opened up the amp today and can't really find the culprit. The heater wires look a little cooked near the power tubes and there looks to be a little heat damage close to the vibrato inputs (which was what I was plugged into). Grid resistors all seem to measure in spec.

I strongly suspect the generator was running hot - I'll measure the output later - if so what is the most likely source for the smoke, and what should I check/replace? FWIW it looked like wire insulation cooking off to me.

(We replaced the genny with a slightly bigger one and put in some surge protectors and the rest of the day went fine).

PS mains fuse was the right kind and didn't blow.

PPS transformers were cool to the touch right afterwards

[Jazz P Bass]

So what model was the bass amp?
My take is that one or more of the amps did not have a three prong cord.
The bass amp may have completed an unknown loop.
There may also have have been some seriuos issues with the generator.
If the heater wires on the Fender look cooked, then that is what got hot.

[unSane]

Bass head was a TC Electronics Bass Amp 2.0 thru a 15" cab.

I put the meter on the generator and it was running a steady 150v!

The heater wire looks pretty thin -- at this voltage level it would have been running about 8.6v, and sitting right above the tubes which were also probably running hot. The whole amp was sitting right in the sun too.

The ground is another possibility. Everything was on three prongs. One of the red wires to the ground switch on the amp looks pretty cooked too but I would have expected that to trip a fuse or circuit breaker. We had GFIs on everything.

[unSane]

So given that I was hitting the twin with 150v when it was designed for 110v, what would be the most likely points of failure?

[Prairie Dawg]

The path to true wisdom is to do as the carpenters say and measure before you cut, rather than the reverse. If there was smoke you cooked something in the power supply. Maybe not bad enough to kill it but cook it you did. If it'e beyond you have a competent amp mechanic check it for damage.

[jrfrond]

Generators are notorious for blowing up shit. I've repaired plenty of gear that this has happened to, and I've also had it happened to me.

Unfortunately, the only way around this is to have a pricey line voltage regulation system. Simple varistor-based surge protection doesn't do a thing, as it is not a surge.

A +40VAC increase in line voltage is approximately 30%, and would DEFINITELY exceed some DC levels in the amp, particularly filter caps.

You said there was smoke. Cable insulation doesn't smoulder unless is is thoroughly cooked crispy-like and obvious. I'd be looking for other things. The most common Fender smoke is filter caps, and it IS a '70 TR, so I'd double-check the doghouse for a cap that vented. If they are the original 41 year-old caps, then they need replacing anyway.

If you know you are playing on a generator before the gig, take a voltmeter with you and check voltage, polarity and grounding FIRST.

[Prairie Dawg]

I think a lot depends on what sort of a generator rig you're using too, and that's something to consider. Although I have never had to be in that position to have to power guitar amps in the great outdoors I have used generators on the flight line to power vibration survey equipment, prop balancing equipment and torque testing equipment-all of which have to amplify small currents from various sorts of transducers and translate that into a stable display. My standard rig was a Honda 5kw generator-nice clean power, smooth and without bumps.

[unSane]

Few more data points --

-- Filter caps are pristine (but should probably be replaced)

-- All other caps look OK apart from the ceramic one just in front of the phase inverter which has a small scorch mark.

-- Wire from mains receptacle to ground switch (now bypassed as it's a three prong cord) is really melted and a bit charred.

-- I found a loose screw nestling in the wires above one of the power tube sockets! Was probably shorting something out...

[olddawg]

Few more data points --
-- I found a loose screw nestling in the wires above one of the power tube sockets! Was probably shorting something out...

This might be a classic case of assumption. The high mains voltage probably killed the SS amp but maybe not the Fender. You may have assumed the generator was the problem when that loose screw rattled around and shorted something out. Of course running an olt amp an 150vac is not advisable either.

[unSane]

Yep it looks to me as if the screw was in roughly the right position to short out the plate and cathode of one of the power tubes. Yikes. Of course it may have settled there as I was disassembling the amp -- but I didn't hear anything rattling about and it was stuck in there pretty good so my guess is it was there a whiles.

[R.G.]

I did a series of articles on doing gigs on generators for the column I did for Musician's Hotline/Premier Guitar. They covered overvoltage, frequency changes, grounding and other things that can fry amps - and people! - when running on generators. It would have made good reading for you before you did the gig, I guess.

[g1]

Few more data points --

-- Wire from mains receptacle to ground switch (now bypassed as it's a three prong cord) is really melted and a bit charred.

-- I found a loose screw nestling in the wires above one of the power tube sockets! Was probably shorting something out...

Not sure what you mean about mains receptacle and ground switch, if it's disconnected then the melting and charring must have happened before the 3 prong was installed. If it's not disconnected then I find this disturbing.
As far as the screw goes, make sure to check that your screen resistors are all ok.

[unSane]

Whoever did the 3-prong mod used the ground switch basically as a junction. It looks a bit scary but actually the switch is completely bypassed. I had me worried at first until I figured out what they'd done. I'll change it once I figure out all the other things I need to do to the amp.

I think they did what is shown here, more or less: