Smell the burn! OH NO!

[soma_hero]

Have been wrapping up my Liverpool build this last two weeks when I have spare time. Just got home from a weekend trip and had some new tubes waiting for me. Plugged in an old RCA 12AT7 in V2 and turned the amp on. I walked over to the wall to grab a guitar and when I turned around white smoke was billowing out from the amp. It's in an open back combo cab with a back covering the chassis. The smoke was incredibly acrid, I started gaging immediately. Flipped the amp off and opened a window and opened the amp up when everything cleared.

There were no damaged parts that I could tell, no scoring of any sort anywhere on the boards/pots/tube sockets. However, the wiring to the heaters looked a little burned and there was some residue under the wires like the covering had melted and was burning off. I used the wire listed in the BOM which is 300V, 20awg cloth covered for the heaters. I've been using the amp for 2 weeks with no problems at all so far so I'm curious why now it would be a problem? Checked for shorts and both sides of the heater filament measure about 0.5ohm to ground so the filaments aren't shorted.

Do you think it's just the heater wire burning up? Why start now and not when it was first fired up? Maybe it just absorbed all the heat it could and now the cloth is starting to burn....

Thanks,
Michael

[skyboltone]

Well, my first thought would be you got a shorted filament connection in that 12AT7 you stuck in there. I've seen that in power tubes several times. I must admit that I've never seen it in a pre tube, but a mechanical jolt could have sprung something loose to short across the pins inside.

[soma_hero]

I had that thought, but pulled the tube and turned the power on again. It again started to smoke...

I did just put in a pair of 6P14P from Russia but they'd been working fine. I've been gone a few days so I'll ask my roommates if there was any funny business....

[Structo]

Transformers have a very unique smell to them when they fry.

If the only change was the 12at7 then that is the likely cause.
Those tubes do draw more power on the filaments but I can't imagine it would be enough to smoke the tranny.

I would suspect one of the EL84's had a short.
Do you have the diodes on the power tube sockets?
If those fried then you will have to fix them too before using the amp.

[soma_hero]

I don't have flyback diodes on the power tubes since they weren't present on the Liverpool schem.

Also, I removed the 12AT7 and replaced the russian 6P14P with some Mullard EL84s briefly and got the same smoke rising from the heater wires. Didn't bother leaving it on to measure any voltages since I didn't want to burn my lungs up and anything else in the amp. I might add that all the tubes heaters lit up, both the russians and the mullards.

I've removed the heater wires now, and the cloth cover was essentially burnt. It had melted some residue on the chassis that I had to clean up with a little acetone. I have a feeling it was just that this cloth covered 300V solid core wire couldn't handle the current and started to burn.

Any other ideas? I'm replacing the heater wires now, but should I do any checks on the PT and OT to make sure they're in working order? Again, the resistance measured between the heater filament and center tap was 0.5ohm so I don't think it shorted.

[Zippy]

I've removed the heater wires now, and the cloth cover was essentially burnt. It had melted some residue on the chassis that I had to clean up with a little acetone. I have a feeling it was just that this cloth covered 300V solid core wire couldn't handle the current and started to burn.

It's not an issue of whether the cloth-covered wire is rated for sufficiently great voltage - what matters is the diameter of the conductor. But "solid core" raises a possibility: Were there kinks in the wire or parts that were bent/rebent? You may have had hot spots in the wire due to local cold working of the metal when it was bent too sharply or bent too many times. Copper work hardens very easily. When you rewire, make sure to use a sufficiently high gage of wire to handle the current.

Wire gage relates to current as insulation relates to voltage.

The matter of cloth insulation burning might also happen if the insulation was compromised and arced/shorted to the chassis.

[skyboltone]

It just don't make sense. Was it so tightly spiral wrapped that it might have shorted wire to wire throught the insulation? Doubtless, the re-wire of the filaments will fix it. You may find short to ground (with a ct filament winding) or a short at a tube socket.

[soma_hero]

Yep, fired up when i replaced the heater wire with proper 18guage stranded. Everything is now good to go. thanks guys

[Zippy]

Excellent news!

I hate the smell of smoke in an amp.