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 Post subject: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 11:52 am 
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So it's safe to assume a lot of us enjoy the blue hue of a power tube. My personal favourite is that of an 807! But can other tubes give off the blue hue? Say a 6K7G or a 6D6 or a 6Q7G?


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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 1:14 pm 
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arianskre wrote:
So it's safe to assume a lot of us enjoy the blue hue of a power tube. My personal favourite is that of an 807! But can other tubes give off the blue hue? Say a 6K7G or a 6D6 or a 6Q7G?
Except for a tube with mercury or the shadows of electron beams on the glass, a blue glow is pending doom. BTW there is UV in the glow...

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 1:20 pm 
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Many thyratrons will glow blue on purpose.

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 2:19 pm 
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Blue = usually OK
Purple = almost always bad

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 2:46 pm 
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Um, I suppose you already knew this but 807 tubes are not supposed to glow blue. Even if you run the power up until the plates are dull red, they don't produce a blue glow unless there is something unusual about them or the way they are being used. Except for fluorescence of compounds used in certain glass formulations it takes gas molecules inside a tube to make a blue glow. Ordinary vacuum tubes contain vacuums from which every possible gas molecule has been pumped out or trapped in a getter, so they do not glow blue under normal operating conditions.

Tubes into which gas has deliberately been introduced often do glow. Mercury vapor rectifiers like the 866 make a pretty blue glow but sitting there and staring at them is a bad idea. That blue glow is full of short wavelength UV and is harmful to the eyes. The same can be said for mercury thyratrons, ignitrons, and other kinds of tubes.

Voltage regulator tubes like the 0B2, 0A2, etc. are filled with mixtures of argon and other gases. Most of them glow various shades of violet by design. So do 0Z4 rectifiers, if you can find one with a glass bulb. The light from argon is long wavelength UV which is not much cause for concern. Argon bulbs used to be sold for night lights and indicator lamps.

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 3:58 pm 
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I suppose a correction is in order... guitar guys will know this but when you plug a tube into an amplifier there is that blueish/purpleish (I'm colorblind to a degree so my explanation of colors is a little off) hue, almost like a mist of light forming on the glass or around the anode. Purple light is bad I know that but I'll post a picture of my 807 for a bit more context.


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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 4:04 pm 
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https://blog.thetubestore.com/understanding-blue-glow-in-your-amp-tubes/

If it's blue glow on the glass... maybe not bad.

There is a difference between gas ionization and fluorescence.

Rich

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 4:24 pm 
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Here’s my 807. So the change the question a bit… what other tubes (eg 6Q7G, 6D6, 6K7G) can get a blue hue on the glass?


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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 4:35 pm 
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I suppose any power handling tube could do it. I have a 41 that fluoresces.


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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 5:03 pm 
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And this is bad:

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Spock finds a gassy tube :roll:

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 5:28 pm 
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You’d be most likely to find fluorescent compounds in the glass of tubes made before the early 1950s. After that time they switched to different glass formulations which don’t do it. Also, since it takes some energetic electrons striking the glass to make it glow, you’ll find that power tubes do it more readily than small signal tubes.

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 6:29 pm 
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The kind you show is just a 'defect' in the glass. I've had newer production do it too.

Not something you can really bank on any specific model to do, somewhat rare IME... does seem more common in power tubes to me, maybe just because the electron density is much higher, I don't know.

The only guaranteed glow are tubes with added gasses - blue for quicksilver like 866A, 83 (though 83 must have much less, and the plates shield the current path so their glow is kinda weak).

xenon is sorta purple, big rects like 3B25 use it, or 0D3 and similar regulators. Much less of a pain to use... developed as a drop in for the above.



the original "blue LED"

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Mon 08, 2022 10:11 pm 
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I had a high-end RCA console that used 4 EL-84's. They were all original, and 3 out of 4 of them glowed blue

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Tue 09, 2022 2:05 am 
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phlogiston wrote:
And this is bad:

Attachment:
smoke.jpg


Spock finds a gassy tube :roll:
There goes another mnemonic memory circuit!


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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Fri 12, 2022 1:02 am 
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elektron wrote:
The kind you show is just a 'defect' in the glass. I've had newer production do it too.

Not something you can really bank on any specific model to do, somewhat rare IME... does seem more common in power tubes to me, maybe just because the electron density is much higher, I don't know.

The only guaranteed glow are tubes with added gasses - blue for quicksilver like 866A, 83 (though 83 must have much less, and the plates shield the current path so their glow is kinda weak).

xenon is sorta purple, big rects like 3B25 use it, or 0D3 and similar regulators. Much less of a pain to use... developed as a drop in for the above.



the original "blue LED"

Image


Ballast tubes have helium in them, but you need a Tesla coil to make them glow, also many light bulbs have a gas in them.

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Fri 12, 2022 5:36 pm 
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Arianskre, it would be interesting if you bring a magnet near the 807 and see if the blue glow moves.
The magnet would deflect the electrons.

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Fri 12, 2022 6:15 pm 
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Notimetolooz wrote:
Arianskre, it would be interesting if you bring a magnet near the 807 and see if the blue glow moves.
The magnet would deflect the electrons.



I tried this with several mostly dead 6U5 eye tubes. The magnet attracts the beam and concentrates it as a bright spot on the target - as you would expect with a CRT. It might be a bad idea with a good power tube, You might get a hot spot.

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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Sun 14, 2022 9:32 am 
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Notimetolooz, the hue deflected around (mostly up and down the glass) when I moved to volume control pot. I believe it was .5meg. Turning the volume up, the hue would deflect up and sideways. Turning the volume down, the hue would deflect down and inwards. I don't believe I have a recording of it unfortunately but how does the volume control (with no signal coming through it) affect the deflection of the hue? I guess the resistance changes the amount of "free" electrons on the grid but then again I don't think it changes to such an extent that it effects the hue deflection...


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 Post subject: Re: What tubes can glow blue??
PostPosted: Aug Sun 14, 2022 4:36 pm 
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That sounds like the volume control is changing the tube bias voltage. That is odd, volume controls don't do that unless the
design is very early (like the 1920s).
If the pattern moves maybe the location on the grid where the electrons are passing through changes with bias.
Some tubes were designed to do something like that, remote cutoff, but I didn't think any power tubes used that design.

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