#1
Okay so back around march i bought a used 59 fender bassman reissue amp from guitar center for 800 bucks. Thought i was getting a killer deal and couldnt pass it up. Brought it home and it sounded amazing everything was great for about a month. Noticed one day all of a sudden general lack in tone, and fuzzy sounds. So i looked in the back of the amp at the tubes for the hell of out. And the two power tubes both were glowing blue. The tubes would also pulse blue when i played a note (more specifically bass notes). So i went and bought 2 new groove tubes. They were matched 6l6 tubes. Just like the stock fender tubes. Now i threw them in and the amp sounded awesome again. God knows why but for the heck of it i looked at the tubes after i was done playing, and sure enough, the new tubes have a slight blue glow in them also. The blue glow is not all over the tube tho, But more-so at the two plates. There is a blue ring at the top plate, and a blue ring at the bottom plate. Is this a bad tube? or is this just the glass being bombarded with electrons, or ions, or something of this nature. Because the bad humming and fuzzy sounds are gone. And the amp sounds good, just bothers me seeing the blue. I was under the impression that any color in a tube was bad.
#2
blue tubes are nothing to be concerned about. glowing red plates is something to be concerned about so make sure they're biased in the correct range
#3
tubes will glow blue if they are underbiased, orange if they are properly biased, or yellow and white if they are too hot
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#6
Quote by pmeg568c
tubes will glow blue if they are underbiased, orange if they are properly biased, or yellow and white if they are too hot
NO this is completely wrong. The ONLY way to visually tell if your tubes are underbiased is by looking at the oscilloscope trace. Blue does NOT mean they are underbiased, it is due to fluorescence due to free electrons hitting atoms that are in the tube, when they fluoresce, they emit the blue glow. Which is why you see the blue grow in the gas and around the glass surrounding all the components inside the tube.

If you get orange glow, get ready for your tubes to blow out. That is red plating and it means your tubes plates are dissipating more power than it can handle. You should not be seeing ANY glow on the plates at all. The ONLY glow you should be getting on any mechanical part of the tube is in the heater filament, which has absolutely nothing to do with the bias as the current that flows through the heaters is not part of the B+ that supplies the plate voltage.

Underbiasing will not have any ill effects on your amp, the only problem you might have is crossover distiortion, but that requires it to be biased pretty damn cold. I have a JTM45, which is a Bassman copy, you need to pull the chassis out to bias it. Bring it to a tech unless you want to learn to do it yourself. But all you really need is a multimeter.
Last edited by al112987 at Aug 29, 2009,
#7
Quote by al112987
NO this is completely wrong. The ONLY way to visually tell if your tubes are underbiased is by looking at the oscilloscope trace. Blue does NOT mean they are underbiased, it is due to fluorescence due to free electrons hitting atoms that are in the tube, when they fluoresce, they emit the blue glow. Which is why you see the blue grow in the gas and around the glass surrounding all the components inside the tube.

If you get orange glow, get ready for your tubes to blow out. That is red plating and it means your tubes plates are dissipating more power than it can handle. You should not be seeing ANY glow on the plates at all. The ONLY glow you should be getting on any mechanical part of the tube is in the heater filament, which has absolutely nothing to do with the bias as the current that flows through the heaters is not part of the B+ that supplies the plate voltage.

Underbiasing will not have any ill effects on your amp, the only problem you might have is crossover distiortion, but that requires it to be biased pretty damn cold. I have a JTM45, which is a Bassman copy, you need to pull the chassis out to bias it. Bring it to a tech unless you want to learn to do it yourself. But all you really need is a multimeter.


I biased mine at 28/29. They're still orange. Izzat bad? I'm just curious because those trannies are irreplacable.

I always figured it was just the heaters.

Edit: pics.
Everyone is entitled to an opinion.

Feel free to express yours so I can make an informed judgement about how stupid you are.
Last edited by bubb_tubbs at Aug 29, 2009,
#8
just heaters, those are fine.

I always just bias to 70% max plate dissipation
Last edited by al112987 at Aug 29, 2009,
#9
^ Nice.
Everyone is entitled to an opinion.

Feel free to express yours so I can make an informed judgement about how stupid you are.
#11
they are most likely alright...

did a quick google search

That is what red plating is going to look like (the second tube)
#13
well keep an eye out tubes can drift in bias for the first several hours of use, and it doesnt necessarily happen right away, just keep an eye on it. but generally, yes you should be fine.