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 Post subject: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 12:40 am 
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Location: Phila Pa
I have noticed that on the 6L6 tubes that some glow blue and some don't and I would like to add a set of blue glowing ones to an Amp I am building for a friend,
I have never logged which ones had the blue glow, I only know that it's OK,
So my good fellows,
Have you noticed the blue glowing 6L6s and know which brand glows the best?
Thanks for your recollection,
Peter


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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 1:58 am 
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The blue glow you speak of means the tube is gassy. they don't perform as well as good tubes. You may want to rethink using then. As they can cause excessive current draw and weak output. .


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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 2:47 am 
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Blue glow inside the plate structure is bad, but blue glow from stray electrons striking the glass is harmless. However I don't know how you would select for that. It's not a characteristic that manufacturers wanted.


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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 2:48 am 
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I built an P-P ultralinear amp using Electro Harmonix 6L6GCs. They have a nice blue glow that varies with the music.


Attachments:
6L6GC Glow.jpg
6L6GC Glow.jpg [ 162.08 KiB | Viewed 1785 times ]
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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 7:04 am 
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Have seen this many times. Not a bad sign at all so long as the plates do not start to glow orange or yellow. Be ware that the screen voltage must be tamed. Exceed the limits of the tubes, and you will shorten their lives. New ones are simply not as good as the old ones. Build modestly.


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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 8:41 am 
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The kind in the picture is just the glass glowing, this is fine.

Russian stuff seems to do this more often than not, but I've seen some random American tubes do it too (GE, in particular).

I forget which impurity in the glass causes it, but I remember reading that it is common in russian glass (maybe their deposits of soda or lime are high in whatever it is, something systemic like this), and I have similar experience.

Another kind of the same thing, I've seen on one soviet tube. the inside of the plate (on surface) also has this glow, so whatever the plate was made of had similar impurity.


Now, if the 'air' is glowing (like how a 0D3 glows), this is a problem, gassy tube. but this isn't how i mean, just on the surface.

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 1:06 pm 
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I've seen two asymptomatic, hence nondiagnostic, kinds of glow in output tubes.

One of these always appears inside the plate (in fact, it appears to be coating the plate instead of diffusing evenly throughout the plate-cathode space).

It's always a steely light blue, and it showed up in every 6CA7/EL34, EL37, 7591A, 7868, and British Genalex KT66 that I ever saw. It shows in new tubes and old, and seems to be routine.

Another kind of glow, as illustrated above, appears around the inside of the envelope. It's always an aurora-like dark violet. As noted above, it is caused by electrons missing the plate and impacting the envelope, causing the glass to flouresce. It will respond to a magnet brought near the glass, and it will "dance" in step with major current swings inside the tube. I've seen this in very many 6V6, 6L6 and 7027 tubes. The RCA 6L6GC's habitually used in the sixties Fender amps were especially prone to this flourescence. They are the ones with side getters and bulbs that look pinched and sometimes asymmetrical at the top.

Both kinds of glow get brighter when the tube is being worked harder, but I've never known either kind of glow to be symptomatic.

The "gassy" tubes I've seen were usually rectifiers like 5U4's or 80's, and they were always glowing "fuchsia" (bright purplish pink). This glow doesn't look anything like the plate or envelope fluorescence I described above. It's always much brighter.

The tubes pictured above have as good a case of "fluoro-glow" as I've ever seen.

:wink: Larry

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 2:40 pm 
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BigBandsMan wrote:
Another kind of glow, as illustrated above, appears around the inside of the envelope. It's always an aurora-like dark violet. As noted above, it is caused by electrons missing the plate and impacting the envelope, causing the glass to flouresce. It will respond to a magnet brought near the glass, and it will "dance" in step with major current swings inside the tube. I've seen this in very many 6V6, 6L6 and 7027 tubes. The RCA 6L6GC's habitually used in the sixties Fender amps were especially prone to this flourescence. They are the ones with side getters and bulbs that look pinched and sometimes asymmetrical at the top.



http://en.wikipedia.org/wiki/Bremsstrahlung

Yep, it's a harmless (albeit entertaining) effect.

I purposely put a set of GE 6V6GTAs into my Harman A300 amp at home, just because they did this (well, they also sounded good, duh :D ). They're even brighter than those ones in the first post (at least, they glow over a much larger surface area per tube)...

Regards,
Gordon.

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 2:45 pm 
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Bad Glow:

Attachment:
IMG00997-20120110-2131 (Small).jpg
IMG00997-20120110-2131 (Small).jpg [ 53.85 KiB | Viewed 1730 times ]

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 4:59 pm 
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Precisely. That is a perfect "fuchsia" glow.

The tube looks like a UY-224.

Larry

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 6:52 pm 
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Yep, that's the color of ionized air. (Really a mix of nitrogen etc but you know what I mean.)

Quote:
One of these always appears inside the plate (in fact, it appears to be coating the plate instead of diffusing evenly throughout the plate-cathode space).

It's always a steely light blue, and it showed up in every 6CA7/EL34, EL37, 7591A, 7868, and British Genalex KT66 that I ever saw. It shows in new tubes and old, and seems to be routine.


I'm going to watch for that! I feel like I've been asleep and just got slapped awake. Anybody have a guess what causes it?

Dave Wise


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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 9:18 pm 
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Location: Southern NH, 03076
There are two distinct types of gas combination which often causes confusion.

1. Leakage from the tube seals which reduces the vacuum. This does not become visible but if bad enough the filament will self destruct.

2. Out gassing from the anode which is normal in any power tube especially. This can be due to an incomplete gettering when new, a trait very common with import tubes, or when the dissipation is run above the design rating which releases the formerly recombined cases from the anode material.

3 A combination of the above.

For #2 the gas may or may not affect operation. Ive been running PP UX-245's for almost a year with a nice glow and they sound fine. Its when the voltage gets to the point of sustaining an internal plasma arc is when they turn to scrap and at the 250V they run at here that wont be soon. Ive also rebiased for a more efficient Class AB2 operation.

Carl


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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Wed 25, 2012 10:39 pm 
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Burnt Fingers wrote:
There are two distinct types of gas combination which often causes confusion.

1. Leakage from the tube seals which reduces the vacuum. This does not become visible but if bad enough the filament will self destruct.

2. Out gassing from the anode which is normal in any power tube especially. This can be due to an incomplete gettering when new, a trait very common with import tubes, or when the dissipation is run above the design rating which releases the formerly recombined cases from the anode material.

3 A combination of the above.

For #2 the gas may or may not affect operation. Ive been running PP UX-245's for almost a year with a nice glow and they sound fine. Its when the voltage gets to the point of sustaining an internal plasma arc is when they turn to scrap and at the 250V they run at here that wont be soon. Ive also rebiased for a more efficient Class AB2 operation.

Carl


Indifferent gettering has long been a MAJOR problem with offshore output tubes. They aren't called "scuds" and "silkworms" for nothing, and before I'll trust any such tubes in any of my gear, I'll retire it.

8) Larry

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Thu 26, 2012 9:23 pm 
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I dont know about the small tubes but Chinese glass TX tubes are also infamous for insufficient pumping down to the correct vacuum. This is where a hi-pot tester is indespensible before trying them out at a serious B+.

Its time for the government over there to have some more executions of plant managers :lol:

Carl


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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: Apr Fri 27, 2012 12:17 pm 
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:lol: :lol: :lol:

L

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: May Wed 02, 2012 3:49 am 
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I used to see the glow ont he glass in many of the HO tubes in color TV sets.

The correct answer to your question is;:The tube with the most air in it. :lol:

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: May Wed 02, 2012 4:17 am 
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Contrary to that thought, I have only read that the pretty, glass-hugging fluorescent "electric" blue depends upon a good vacuum and a lot of emission. I'll ask some of the physicists/fluorescence specialists that I know. The wavelength(s) of emission may narrow it down. It could well be a trace mineral in the glass.
I'd bet that there may often be others at wavelengths that we don't see.

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 Post subject: Re: Which 6L6 glows blue the most?
PostPosted: May Wed 02, 2012 11:44 am 
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My Holland 7189's have been exhibiting a blue glow external to the plate for a decade or so in my Hamond Organ, although I fire it up probably only once a week or so. Also, most 6L6 types in AB2 service dance around in color. Just a few volts less on the screen grids can take this out on lesser grade tubes.


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