Bias drift and burnt insulation

[Smokebreak]

One of my gigging amps is a homebrew 2x6L6 BF Super, basically. I've been using it regularly for 6 months or so.

The other day I noticed that it didn't quite sound like it used to. Not a big change, just something a little off. I opened it up and the bias was around 30mA for 470V plates. Now I remember biasing it a bit cool, but not that cool.

Upon visual inspection, I notice what is in the picture attached. I cleaned it up, resoldered, biased back up to 40mA, and played it that night. Sounded like it used to.

The only variable in this amp's life(I always run it around 4 on the volume) is that 2 weeks ago, I had it cranked in the studio. It performed well, so I had no cause for alarm.

So what the heck happened? Could any of these 3(bias drift, carbonized(?) plate pin/burnt OT sec lead, and cranking it) occurrences be related?

[Zippy]

Did you have a weak/loose solder joint that arced?

[Smokebreak]

It's possible, and I moved the wire around for the pic(which makes it look kinda suspect), but that joint seemed solid. That would be a good explanation though.

[xtian]

Don't we add grid resistors to prevent this kind of thing?

[Smokebreak]

That's the plate pin whose wire's insulation is burnt. I've got 2K2 stoppers on the grids.

[Stevem]

Is that solid core wire that you used?

[Smokebreak]

The brown wire is the OT secondary and is stranded. Sorry guys I could have been clearer about what we are looking at.

[Structo]

Sort of looks like a cold solder joint which didn't pass current efficiently.

When you resoldered it, you corrected the problem.

[Smokebreak]

The more I think about it , Tom and Zippy, you guys are right, that had to be what happened.

[JazzGuitarGimp]

I'm thinking it's a bit curious that it acted up after (or maybe during?) running the amp full-out in the studio. I think if it were me, I'd get the amp in a dark room and crank it all the way and look for red-plating, just to be sure.

[Leo_Gnardo]

I think if it were me, I'd get the amp in a dark room and crank it all the way and look for red-plating, just to be sure.

Good idea, also look for sparks & arcs, both outside & inside the tubes.

Sometimes this sort of arcing is due to the amp not seeing a load, even for a fraction of a second, while trying to pass signal. Bad speaker cables, cab connectors, speaker connections can do this. Plus of course someone pulling out the speaker cable while the amp is being played. Seen it happen - lost a rack of 4 fresh Siemens EL34's one day while I watched a well-known band's stage tech unplug speakers from an amp I just repaired while in use. Geek should have known better - a little short on brains.

[JerryFJA]

Flyback voltage damage. Running an attenuator? Wrong speaker load? Driving it hard with fuzz?
Jerry

[Smokebreak]

Jerry, guitar was straight into amp, running 4ohm load. I made the engineer show me the 2x12 8ohm speakers

Leo and Lou, next time I get the chance to crank it, I'll observe. I do remember, at the time of cranking, the tubes glowing blue in tandem with big chords, etc, but no red plates .
Since then, I DID find a speaker cable here at the house that was intermittent. That was the cable I had been using on gigs, but cannot remember if I took it to the studio, knowing there'd be cables there.

So if this was the case, could it have "slightly" damaged the OT, or just the tubes, or both? I gigged it again last night and it seemed fine.

[Structo]

The blue glow is normal for some power tubes.

I had a bad speaker cable once that caused all kinds of problems, thinking I had blown the OT.

[martin manning]

Since then, I DID find a speaker cable here at the house that was intermittent. That was the cable I had been using on gigs, but cannot remember if I took it to the studio, knowing there'd be cables there.

So if this was the case, could it have "slightly" damaged the OT, or just the tubes, or both? I gigged it again last night and it seemed fine.

Probably is fine then. Maybe you are lucky in that the weak point was outside of the OT.