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Old 16th September 2020, 01:46 AM   #1
wg_ski is offline wg_ski  United States
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Default Pretty blue light

Are there any specific brands that produce the best fluorescence? That pretty blue glow from cobalt in the glass. None of my New Sensorís seem to have any - even at 600 volts on the plate. The most recent tubes I have that do glow are a 1990ís era quad of Svetlana EL34ís, with a B+ as low as 350V. Gets better at higher voltage, of course. Iíve seen a lot of older ones do it over the years. Is it just luck of the draw, or is there some rhyme or reason to it?
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Old 16th September 2020, 01:47 AM   #2
audiowize is offline audiowize  United States
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If you want blue, you'll need to use some 866s...
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Old 16th September 2020, 01:51 AM   #3
IlikeTech is offline IlikeTech  United States
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Oh yea, 866s glow amazingly. Kinda toxic though, considering you can see the liquid murcury when they're off.
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Old 16th September 2020, 07:38 AM   #4
OldHector is online now OldHector  Sweden
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I can't help but look at my poor-man's KT66s!
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Old 16th September 2020, 08:17 AM   #5
6A3sUMMER is offline 6A3sUMMER  United States
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EL34 tubes?

Then put them in Pentode mode, and regulate the screens.
One OD3 150V
Two OD3 300V

Very pretty purple glow.

Or keep the amp exactly the same, except add a resistor and one OD3 or Two.
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Old 16th September 2020, 09:10 AM   #6
mondogenerator is online now mondogenerator  England
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5781a glow nice too, with 480V on the plate/g2.

Is it Cobalt fluorescence? Or just impurity in the inner glass surface?
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Old 16th September 2020, 11:27 AM   #7
wg_ski is offline wg_ski  United States
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Which 5781’s? I’ll bet some do and some don’t. I’ve had old 7591’s as a teenager that were pretty spectacular, but the EH quad I’m playing with doesn’t do squat. Nada from either batch of Tungsol 6550’s that are going in the big amp. I have to go back to my old stash to get anything with even a hint of blue glow. I’ve even seem 12AX7’s do it on occasion - and that’s with maybe 200v a-k if you’re lucky so it can happen at lower voltage.

That particular blue is “cobalt blue” and that would be the particular impurity in the glass causing it. EH advertises “leaded glass” for their KT88/6550, and I assume the tungsol is made in the same factory with similar if not the same processes. Lead doesn’t glow like cobalt does. I’m wondering if it sneaks in occasionally, or if some makers intentionally use it.
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Old 16th September 2020, 12:01 PM   #8
Kei is offline Kei  Wales
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My tung-sol EL34B's produce quite a vivid blue glow in the centre of the valve around the anode cut outs. It's a lot more obvious than the EH6CA7 and JJ KT77's I have. (forum squishes the image)
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Old 16th September 2020, 12:43 PM   #9
mondogenerator is online now mondogenerator  England
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Quote:
Originally Posted by wg_ski View Post
Which 5781ís? Iíll bet some do and some donít. Iíve had old 7591ís as a teenager that were pretty spectacular
Sorry, a bit of discalcula I think, on my part.

Phillip's jan 7581a (not 5781a)
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Old 16th September 2020, 01:02 PM   #10
wg_ski is offline wg_ski  United States
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Well there’s a vote for some New Sensors that are doing it.

Of course it is going to happen around the cutouts in the plate - relativistic electrons have to be hitting the glass and they have to come from somewhere. I’ve seen some with lots of holes in the plate have darn near the whole inside of the glass glowing. Old 7591’s and 7868’s from the old Fishers used to dance like a disco light show when driven hard where the supply sags. Those were GE and Sylvania. I went through several of those units growing up - wish I still had them. The EL34’s I was using were Class A triode strapped and of course stayed rock steady. Best I’ve ever seen was a pair of 6L6’s in a friend’s Peavey, but I can’t recall the brand and it was 20 years ago. First time I ever noticed it was a friend’s Sunn bass rig, probably 6550’s. Then I looked inside our black and white TV set and darned if the horizontal output tube wasn’t glowing just like it. None off the others were. I had lots of small radios and console stereos and nothing ever did that, so I assumed voltage had something to do with it (which it does).
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