Some beam power output tubes exhibit a blue-violet glow as a result of stray electrons striking the glass envelope. This is an incidental effect, but much appreciated when it happens, as the glow often dances with the music (you get a light show as well as the cool tube sounds). I've seen this effect in vintage 7591A output tubes as well as (faintly) in old 1625s. I have built two sweep tube amplifiers, but have not seen the effect in either. Has anyone seen the glow in their sweep tube amp (screen driven or otherwise)? Just curious - I'm wondering if it's the difference in construction, envelope glass, etc.
My sweep tubes were glowing blue a bit...I was running a few in push-parallel as a vocals amp at a show we did recently. Little glow at idle, higher glow at full out. The amp is 6av5 push-pull parallel, with screens at about 250v pentode. Glows blue on glass more with no load or with oscillation, as I discovered during design & testing. EH6CA7s glow for sure too. Actually, I haven't used any sweep tube that didn't glow blue on the glass when operating near limits.
I've always heard that's the sign of a good vacuum (as opposed to the mica-illuminating interior plate glow of gas) since nothing is impeding the electrons till they hit the glass. It's never a general glow, unless it's gas. Mostly, it's where the electrons can get around the plate. The fun part is when you take an old speaker basket magnet near the glowing glass, it's entertaining for a good 5 minutes or so
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I've always heard that's the sign of a good vacuum (as opposed to the mica-illuminating interior plate glow of gas) since nothing is impeding the electrons till they hit the glass. It's never a general glow, unless it's gas. Mostly, it's where the electrons can get around the plate. The fun part is when you take an old speaker basket magnet near the glowing glass, it's entertaining for a good 5 minutes or so
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I must be doing something wrong, my tubes glow red!
I have been exploring the upper limit of power available in my trusty old pair of Chinese 6L6GC's. I can get over 90 watts at 3% distortion from these guys in AB2 without red plate. I noticed last night that they too have some blue on the glass when cranked hard. It seems that the blue glow might be related more to the operating voltage than the power being dissipated. Currently 500V on plate 400V on G2.
Some serious sweep tube testing is coming.
...toward the plate. Isn't the energy of the electrons striking the envelope related to the temperature of the cathode? I was under the impression that most of the phosphorescence on the surface of the glass was caused by the electrons that escaped through the inspection holes in the plate.
Wait a minute, I just answered my own question.
John
I had some RCA 6BQ6GTB that would strongly fluoresce.
I think fluoresence is directly related to voltage, and somewhat to brand.
In my experience, most GE tubes fluoresce as do most Russian tubes. I even had a GE 6201 that would glow blue. A lot of Sylvania tubes do it. The above noted 6BQ6GTB are the only RCA made tubes I have ever seen fluoresce, and I have never seen a real Tung Sol glow.
My Chinese coke bottle 6L6GC glow blue, but only above about 425 volts.
Bendix 5992 has a great glow, can't speak for the others in their line.
Win W5JAG
I think fluoresence is directly related to voltage, and somewhat to brand.
In my experience, most GE tubes fluoresce as do most Russian tubes. I even had a GE 6201 that would glow blue. A lot of Sylvania tubes do it. The above noted 6BQ6GTB are the only RCA made tubes I have ever seen fluoresce, and I have never seen a real Tung Sol glow.
My Chinese coke bottle 6L6GC glow blue, but only above about 425 volts.
Bendix 5992 has a great glow, can't speak for the others in their line.
Win W5JAG
Last night I was exploring the upper limits of a pair of RCA 6BQ6GTB's (the 98 cent ones from last years AES sale) in screen driven P-P. I had the B+ voltage set on 600 volts. With a 6600 ohm load is was seeing reliable and stable power outputs in the 90+ watt range. There was no glow either red or blue!
Then I got greedy and switched to a 3300 ohm load. I saw the power output display on the audio analyzer hit 125 watts before the runaway started. It seems that I have found a new way to blow up a tube. In extreme screen driven situations (125 watts from 6BQ6's IS extreme) the screen grid is going highly positive while the plate is approaching zero. This makes the screen grid glow. When it glows is can emit electrons which travel to the positively charged plate. The PowerDrive circuit uses a resistor to return the output (screen or control grid) to the negative supply. As the screen grid starts to draw a lot of current it is no longer under mosfet control and the screen voltage will runaway. The screen voltage rose quickly causing the tube current to shoot toward 1/2 amp. I was observing this effect and managed to catch it 3 times, but on the 4th time the fireworks erupted inside thge tube and it doesn't work so good any more. At any rate there was various shades of red, orange and white, but no blue.
I was exploring the limits of these guys too. You can get 90 watts from a pair in AB2 with 500 volts on the plate. The only color visible is heater glow and blue!
w5jag said:
I have to retract this, I had a pair of 5881 in the Simple SE last night, and when I turned the lights out, one had some blue glow at about 360 volts.
tubelab.com said:
I wonder what one of the 17 watt sweeps would do, their cathodes would have to be good for at least half again as much as the 6BQ6.
Win W5JAG
I may try some old 6CD6Gs in my "Miz Piggy" Screen Driven SE amp for that old-timey look. If you cherry-pick the tubes, you can find some with larger plates that look like they'll take some abuse. I have one Raytheon and one GE that might make the nut. The old G-style tubes look maddeningly complex inside, with a bunch of posts for grid. plate, and whatnot, and a big pinch seal for everything that holds stuff in place. Some blue glow would also be appreciated.
Not sweeps tubes but I've got a pair of unused EH KT88's one from '01 and one from '06. The older one fluoresces on the glass. None from the newer one.
Likewise my ECG-Philips 6BG6GA's fluoresce like mad. The pattern on the glass changes based on bias. Lower voltage seems to reduce the fluorescence.
I have a set of SED EL34's that flouresce too, but I've not tinkered with them much.
Likewise my ECG-Philips 6BG6GA's fluoresce like mad. The pattern on the glass changes based on bias. Lower voltage seems to reduce the fluorescence.
I have a set of SED EL34's that flouresce too, but I've not tinkered with them much.
Tekko said:
Mabey. Or it could be the glass composition. Some tubes have their glass "carbonized" to absorb those electrons that hit the glass. Right?
Small amounts of gas can be addressed by baking the tube or running the heaters for a while to allow the getter material to do it's job as suggested by the Book of Jones. Not that I ever do this.
Tekko said:
Not necessarily. The glow from gassiness is different. It often fills the space between the cathode and plate. It is also more purple in appearance. If the glow is due to gas, you can tell by touching the outside of the tube. Gas glow will follow your finger like a plasma globe, or sometimes disappear completely.
What we're mentioning here is the sky blue color that comes from cobalt contamination of the glass. It's quite harmless, and adds more glowey bottle coolness points. Touching the glass opposite the glow won't change it.
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