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wg_ski 16th September 2020 01:46 AM

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?

audiowize 16th September 2020 01:47 AM

If you want blue, you'll need to use some 866s...

IlikeTech 16th September 2020 01:51 AM

Oh yea, 866s glow amazingly. Kinda toxic though, considering you can see the liquid murcury when they're off.

OldHector 16th September 2020 07:38 AM

1 Attachment(s)
I can't help but look at my poor-man's KT66s!

6A3sUMMER 16th September 2020 08:17 AM

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.

mondogenerator 16th September 2020 09:10 AM

5781a glow nice too, with 480V on the plate/g2.

Is it Cobalt fluorescence? Or just impurity in the inner glass surface?

wg_ski 16th September 2020 11:27 AM

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.

Kei 16th September 2020 12:01 PM

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)

mondogenerator 16th September 2020 12:43 PM


Originally Posted by wg_ski (
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)

wg_ski 16th September 2020 01:02 PM

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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