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Unread 01-20-2011, 06:52 PM   #1 (permalink)
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blue glow in vacuum tube

peeked in the back of my amp yesterday and noticed one of my rca 6v6's had a smallish spot of blue glow to it.

is this something i should be concerned about? i know ive read about this b4, but cant remember what the verdict was...it still sounds fantastic, but i did notice a faint pop now and then, nothing obnoxious, pretty subtle really. but a faint pop nontheless.

could these things be related?
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Unread 01-20-2011, 07:10 PM   #2 (permalink)
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Re: blue glow in vacuum tube

no. large amounts will indicate ionization and slow tube death but, small amounts are normal.
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Unread 01-20-2011, 07:17 PM   #3 (permalink)
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Re: blue glow in vacuum tube

Not at all. Blue glow is a common effect. If anything it can indicate a hotter bias. As I've understood it, it's a phenomenon that occurs when electrons and ions strike each other at high velocity inside the vacuum tube.

The glass envelope of the tube is actually very mildly conductive at high temperatures (inside the vacuum), hence the blue glow tends to gravitate towards certain areas of the bottle, as you'll notice. Sometime's it's just a spot, other times it's more of a fractal design.

Obviously, if you decrease the negative bias voltage (bringing it closer to 0, hence LESS negative) and more current is passing, more electrons are moving within the bottle and at a faster rate, hence more blue glow.

Last edited by Alabama Thunderpussy; 01-21-2011 at 06:54 PM.
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Unread 01-20-2011, 07:23 PM   #4 (permalink)
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Re: blue glow in vacuum tube

When tubes are made, they pull a vacuum on the tube. The better the vacuum, the better quality the tube. Once the vacuum is pulled a device inside the tube called a getter is activated. The getter splatters a barium laced stain on the tube (generally on the top). This leaves a shiny, mirror-like stain on the tube. The purpose of the getter is to further eliminate any extraneous air left in the tube. The effects of the getter last throughout the life of the tube. Even though all of these precautions are used, all the air cannot be completely removed. The small amount of gas in the tube will cause the light blue glow you see. It is harmless.

In current production tubes, there is a chance for more air (gas) to be inside the tube. Therefore the blue glow is even greater.
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Unread 01-20-2011, 07:30 PM   #5 (permalink)
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Re: blue glow in vacuum tube

great, thanks guys.
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Unread 01-20-2011, 09:54 PM   #6 (permalink)
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Re: blue glow in vacuum tube

Be very worried if the gas in the tube is glowing violet. That's a tube that's getting air in somewhere. Beam power tubes seem to be more prone to the blue glow. I think I have some pics of each.

Here's a dying 5Y3:



This is a pair of 5881s with the blue glow:

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Unread 01-21-2011, 03:16 AM   #7 (permalink)
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Re: blue glow in vacuum tube

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Obviously, if you increase the negative bias voltage and more current is passing....
INcrease the negative voltage on pin 5 and you DEcrease the amount of electrons flowing to the anode... that's how it works
yes i like screwing ants
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Unread 01-21-2011, 02:00 PM   #8 (permalink)
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Re: blue glow in vacuum tube

I think the blue glow is cool. A budy of mine had a Mesa amp back in the late 80's that looked like a purple and blue plasma storm in the back of the amp. He used that thing for years and years that way with no troubles.
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Unread 01-21-2011, 06:53 PM   #9 (permalink)
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Re: blue glow in vacuum tube

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INcrease the negative voltage on pin 5 and you DEcrease the amount of electrons flowing to the anode... that's how it works
yes i like screwing ants

YES!! Thank you for catching that. I meant voltage increase as in closer to positive. I should have said DEcrease.

Fixed.

Now, tell the fine folks here WHY increasing the negative voltage on pin 5 results in a decrease of electrons flowing to the anode. ;-)
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Unread 01-21-2011, 07:20 PM   #10 (permalink)
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Re: blue glow in vacuum tube

It's more negative so it repels more electrons back towards the cathode (as like charges repel), preventing them from hitting the plate, or passing the plate and whacking their tiny little negative heads off the glass
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Unread 01-21-2011, 07:58 PM   #11 (permalink)
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Re: blue glow in vacuum tube

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whacking their tiny little negative heads off the glass

funny...
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Unread 01-21-2011, 08:00 PM   #12 (permalink)
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Re: blue glow in vacuum tube

Hey, you'd be blue too if you smacked your head off a piece of glass a few billion times thicker than you while traveling at the speed of light
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Unread 01-21-2011, 08:36 PM   #13 (permalink)
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Re: blue glow in vacuum tube

well great the baboons are loose!
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Unread 01-21-2011, 09:37 PM   #14 (permalink)
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Re: blue glow in vacuum tube

How about a misplaced circus chimp?
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Unread 01-22-2011, 03:40 PM   #15 (permalink)
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Re: blue glow in vacuum tube

As others have said, blue glow is just an indication that there isn't a 100% vacuum inside the tube. That's normal. If the tubes glow blue while the amp is in standby, then you have a problem.
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Unread 01-22-2011, 06:29 PM   #16 (permalink)
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Re: blue glow in vacuum tube

IMO the blue glow in a valve is one of the most badass things about them. Looks kinda eerie on some of them.
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Unread 01-22-2011, 07:58 PM   #17 (permalink)
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Re: blue glow in vacuum tube

Mercury Arc Rectifiers;
Now THAT'S badass blue glow.
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Unread 01-23-2011, 02:25 PM   #18 (permalink)
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Re: blue glow in vacuum tube

it is very cool looking, beautiful color.
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Unread 01-23-2011, 04:41 PM   #19 (permalink)
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Re: blue glow in vacuum tube

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Originally Posted by demonufo View Post
Mercury Arc Rectifiers;
YouTube - Hewittic Mercury-arc Rectifier of Manx Electric Railway
Now THAT'S badass blue glow.
Ive seen a few of those Demon,mostly in old cinemas.I wouldnt want one to break if i was near it though!
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Unread 01-23-2011, 05:30 PM   #20 (permalink)
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Re: blue glow in vacuum tube

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Ive seen a few of those Demon,mostly in old cinemas.I wouldnt want one to break if i was near it though!

Duck and cover...
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