blue glow in tubes?

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so i recently got a new pair of Winged C 6L6GC's for my Bassman. it's one of the ones with a balance pot instead of a bias pot, and the tubes were matched but i hadn't adjusted the balance pot (the tubes in it when i got it were not matched). anyway, yesterday i took the amp apart and set the balance pot to exactly halfway, which was where the amp sounded the fullest.

but today, i noticed that one of the tubes has a bright blue glow throughout the tube, with one area having a kind of red-orange-ish glow. the amp sounded fine, and when i put an old 6L6GC in place of that tube, there was no blue glow.

is this a sign of a bad tube? something else wrong in the amp? or just nothing at all? i've noticed other tubes have that blueish glow with a little bit of red, but not nearly this much.
 
Re: blue glow in tubes?

The blue glow is fine - if you Google it, you'll find a website that explains it. Some tubes do it, some don't.

I'm more concerned about the red-orage glow. If that's not coming from the filament, then it's bad. Usually, if a tube's biased too hot, the big grey or black plates around the guts will start to glow. The amp needs to be re-biased for the new tubes.
 
Re: blue glow in tubes?

well i don't think the red-orange glow is coming from the plates, probably just the light from the filament being seen through the blue part. and from what i read online, it would seem that tubes with a blue glow can actually be better than ones without sometimes. but i'll try re-adjusting the balance pot again tomorrow and pay attention to see if the blue goes away. i thought it was kind of weird that i didn't notice it until i adjusted that...
 
Re: blue glow in tubes?

Don't worry about the blue glow .Be concerned about the red one.Try and get rid of the red,not the blue. As said before a Bias adjustment is probably what is needed here.Too low a bias is as bad as too high[hot]and will produce red glow too!
 
Re: blue glow in tubes?

I have a thread in the Vault, showing pictures of me biasing a Marshall.

Ideally, both those tubes should glow the same, so either the bias or balance is off.

Put both the old tubes in and see how they look. If only one of your new tubes is glowing oddly, it's possible it's got a vacuum leak or something.
 
Re: blue glow in tubes?

hmm, so one of the plates in the suspect tube does indeed have a red-orange glow on it, and i can't seem to dial it out with the balance pot. i put the old tubes back in (they were 5881's) and there were no problems. i mean, the amp sounds fine, and i'm guessing if the one tube has a problem like a vacuum leak, it's not going to go away from re-balancing.

so now my question is: if i can't get this to go away, is it still safe to use the amp? or has damage been caused/will damage be caused to other parts of the amp, or just the one tube?
 
Re: blue glow in tubes?

You say the old pair work fine?.If there's no red glow there,but there is in one of the newer pair,then it probably is the tube in the newer pair.If that was the first time you've used them, then it sounds like a dud from the beginning.
If it seems okay[no red glows]It's probably OK to use it.Just keep your eye on it for a while,see how the tubes look every now and then.
 
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