No sound - Blue Tubes

[G-Man]

After about five minutes of playing, my DR volume decreases to nothing and then comes back a couple seconds later. I looked at the power tubes and they glow blue. This is not typical for them and I'm only palying with vol on 3. Is this what happens when tubes get old? Any clues on a cause?

[dartanion]

Hey G-man,

I have tubes that glow blue all the time and they continue to work fine. It sounds to me like you have an oscillation somewhere. I would wager that your grid stop resistor is toast, but then again I am not a betting man. Are you betting any squeals associated with the drop outs? I have made the mistake of leaving the grid stop resistor out of a circuit and got similar results as yours. It would work fine for a while and when I hit a chord relatively hard it would just drop out completely.

[G-Man]

Sorry, there is no squealing and the grid resistors are fine. I grabbed a bad cable! That caused the volume to die down and when I went to the amp to look for a problem I moved the cord enough for it to start conducting again. I never noticed the tubes glowing blue before. I thought that was a sign they were going bad. I read another post about checking the glow with a magnet. It does move when I move the magnet so it is just stray electrons. Besides the blue, the plate of one power tube was glowing orange and the PI tube was glowing hotter than the preamp tubes.
I pulled the PI tube and found it was a 12AY7 instead of the 12AX7 called for. I installed a 12AX7 and checked the voltages. They are checking 240v when the schematic calls for 190v. It seems to work fine. I'll have to give it a work out to see if it holds up.

Thanks for the reply!

[teletroy]

Over the years I've had plenty of tubes that glowed blue- nothing to worry about, something to do with the gasses. As far as your voltage readings go- I know with fenders 20%+ or - is generally acceptable, so you're relatively close. Could also be caused by higher wall voltage.

Rock on!

[Plexi]

Another thing i have seen over the years is,if an amp looses a screen grid resistor, then the tube is just sitting there not really working. And the other tube will be doing all the work. But when this happens,one tube will have the blue hazy swirl in it,and become microphonic,or rumble very easy.

Richie