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Old 4th September 2012, 08:47 PM   #1
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Default How much Blue glow is ok?

I have seen blue glow in the past on amps which had a lot of time on the tubes and in two a tube flash. The blue glow at the time while evident....not really pronounced thru out the tube.

Most of my amps do not have blue glow at all.

One amp has blue glow so bad that the tubes look like neon bulbs....the glow is thick and very pronounced. Previous owner was told the tubes only had 150 hours on them and of course...he hardly used the amp.

In reading about blue glow there seems to be mixed perceptions out there...

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Old 4th September 2012, 09:17 PM   #2
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if you have the correct bias, and grid stoppers in place. (so it doesn't have any parasitic oscillation)
its very likely to be just fine.
blue glow is mainly just stray electrons hitting the glass bulb and exiting impurities in the glass, sometimes does the same to the remaining gasses inside.
i have some kt88s that glow blue faintly but is easy to see with the lights off.
and you should see the blue glow increase when it gets near clipping.

other than that blue glow mainly comes from tubes getting gassy or having the wrong bias voltage/current.

some tubes tend to glow blue when they are new, then just to disappear after some hour's of use.

if all the tubes glow as you say its probably just fine. just check the bias to be sure.
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Old 4th September 2012, 09:30 PM   #3
DF96 is offline DF96  England
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I think it depends on where the blue glow is. Blue where vacuum should be means air is getting in and not being mopped up by the getter. Blue on the glass is harmless.
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Old 4th September 2012, 09:52 PM   #4
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When a single tube glows blue it usually means the coupling capacitor is leaking and increasing the bias. Lots of blue glow means the bias is too high. If you can't monitor the current into the tubes look at the power supply ripple. If the bias is adjustable, lower it a bit and see if the glow drops and if it sounds fine.

One fellow around here had blue glow and bought new output tubes to shortly afterwards buying a new power transformer. (Bad cap!)
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Old 4th September 2012, 10:02 PM   #5
Bigun is offline Bigun  Canada
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air doesn't produce a blue glow when it's ionized, but violet in colour so I'd be surprised if it was due to a tube going gassy. if it's really blue and not violet then I believe it is just electronics hitting the glass. I have seen this in my tubes and I view it as a good sign - the vacuum has to be good for a blue glow.
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
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Old 5th September 2012, 02:10 AM   #6
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air doesn't produce a blue glow when it's ionized,
The color depends on the pressure and temperature of the air and the impurities present. I have seen bad tubes glow in colors ranging from deep blue to hot pink. A glow in the space where vacuum should be indicates the presence of something that is being ionized, often air. A glow, usually blue ON THE GLASS is harmeless phosphoresence.

The tubes in these pictures are ALL BAD and some are BLUE. The last 6V6 was so bad it took out the amp blowing the cathode bypass cap all over the room. I might have leaned on it a bit just to see what would happen
Attached Images
File Type: jpg Blue_211.jpg (172.8 KB, 266 views)
File Type: jpg BlueRay.jpg (177.8 KB, 258 views)
File Type: jpg Tube_1B.jpg (282.2 KB, 259 views)
File Type: jpg Tube_2A.jpg (260.7 KB, 258 views)
File Type: jpg Tube_1C.jpg (189.8 KB, 253 views)
File Type: jpg Tube_2D.jpg (169.3 KB, 100 views)
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 5th September 2012, 03:58 AM   #7
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ok, thanks for the replies...bias is all fine...I do have some new tubes that I could put in to see if the glow is tube related or topology related as mentioned. could also move the blue glow tubes to other amps and see what happens there. Tubes test fine on a decent tester for GM, gas, shorts. My initial thought was that these tubes are high in hours but the comments posted thus far don't really support that notion. So I'll do a tube swap and move the tubes to another amp and see how they behave.
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Old 8th September 2012, 06:47 PM   #8
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I've seen this blue glow in the fuel pond at Sizewell B. Is it radiation or just the underwater lamps?
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Old 8th September 2012, 07:01 PM   #9
DF96 is offline DF96  England
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Radiation. Well, to be more precise, Cerenkov radiation. IIRC it is caused by beta particles (fast electrons) passing through the water at faster than the speed of light in water. You get the electromagnetic analogy of a sonic boom.
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Old 9th September 2012, 12:54 AM   #10
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The 5CZ5s in the SE amp I recently built do exhibit a deep "blue glow". It's on the glass. The bias is right. It looks very cool.
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