Last Wednsday, at a gig, I had some problems with my tube amp. It is a Pignose G40V. 2 6l6's, 3 12AX7's, 40w. I run it into 8 10" speakers in 2 separate cabs, for a total load of 4 ohms.
When I powered it up, it worked fine for a few minutes, but then started to cut out intermittently. The output would change from ear-piercing loud to whisper soft. So soft it couldn't beven be heard over the rest of the band during soundcheck. Sometimes there would be no output for a few seconds, and then it would come back on. Sometimes it would cough and sputter. I looked in the back of the amp, and one of the 6L6's was turning blue when the problem was occuring. I pushed the tube into it's socket just to make sure it hadn't come loose during transportation, but that didn't seem to help. The problem was still occuring.
The amp head had been sitting on top of one of my bass player's amps, getting vibrated by the loud bass, all of this time. I decided to move it to the floor, and then turned it off. After about 20 minutes, I turned it back on, and it has worked fine ever sense, throughout the entire 5 hour gig, and during a recording session last night.
What could have caused this? Do I need to invest in a set of spare 6L6's? Could it have come from the amp being vibrated by the bass amp under it?
Oh my, there are lots of things that can cause volume to vary like that. The blue glow on the power tubes isn't a sign of problems, though, that's present in healthy working tubes and will vary according to the volume in Class AB amps.
Go here: http://geofex.com/gtramps.htm Click on "Guitar Amp Debugging Page." There's a link on it for the volume cutting out or some such.
"A cheerful heart is good medicine."
Well it wasn't just a blue glow, it was intermittent, and the whole tube was turning blue. The tube would turn blue when the volume would cut off.
But, it hasn't done it anymore, so i'm not sewating it. I just want to know what it could have been in case it ever happens again.
http://members.aol.com/larrysb/blue_glow.html
The intermittent nature of the blue glow is because the tubes were being driven by a varying signal. I think your amp's problem is elsewhere. Gassy power tubes don't fix themselves. They overheat till something blows, hopefully a fuse.
Did you read the Tube Amp Debugging Page? Can't really add to that, it covers all the likely possibilities.
It sure is a good idea to keep spare tubes on hand. If your Pignose G40V is like mine, its tubes will be short lived unless you modify it. Mine put 7.5V on the heaters, which are nominally 6.3V. That's very hazardous. It causes the power tubes to be more likely to go into "thermal runaway," where the grids begin emitting electrons because they're heated by the overheated cathode, that makes the grids more positive, so more plate current flows, that heats up the tube and cooks hidden gas molecules out of the plate, the gas molecules get ionized and bombard the grid making it even more positive, and the plate current rises exponentially till something blows. The tube manuals used to warn to never run 6L6s above 7.0V. I fixed mine by inserting two 1.8 ohm 5W power resistors, one in each side of the heater circuit. Dropped the heater voltage to 6.0V, which is fine and slightly extends tube life over the nominal 6.3V.
"A cheerful heart is good medicine."
Everything on that website seems to point to some sort of thermal runaway in that tube, which could have caused the arcing inside the tube.
I just realized that when I put the amp on the floor,(when it stopped malfunctioning) I had placed it on it's side, in a position where the heat could have vented out of it much more easily. It was very hot when it was malfunctioning.
Do all G40V's have this flaw where the heaters are run at 7.5 volts? Is there a way to measure this? If my amp has that flaw, would this modification that you performed change the sound of the amp at all?
