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In Reply to: Blue glow inside GE 6550 posted by DynaMite on August 14, 2002 at 14:26:55:
Check the bias on the power tubes - letting to much current pass will cause that blue glow too - that coupled with a poor vacuum will cause it. Sounds like the tubes are pushed to the max.
Follow Ups:
> > letting to much current pass will cause that blue glow too - that coupled with a poor vacuum will cause it. Sounds like the tubes are pushed to the maxHuh? I've never heard that before. It was my impression that tubes that glowed actually tended to have better vacuums. On my OTL amp, a few tubes glow out of 10 and others don't. They're pushed the same. I also find it very common with old 45s. I like it, it looks cool in the dark.
Yep, that's what most say, but I have read that impurities in the vacuum are stuck by electrons (ions?) and excited and start to glow. Makes sense to me. If it were a true, pure vacuum, it would not glow.Achieving a pure vacuum is difficult in the manufacturing process.
True, even now, getting a perfect vacuum is impossible. I'm don't know about impurities, but it could definately relate to the materials used in older tubes along with the gas that was used to purge the air during the vacuum process.
Both are right.When there is blue glow outside the plates, aroud the glass - this is electron bombardment and usually indicates a very good vacuum. My 350B's in my main monoblocks exhibit this, and they always did - and they have been running fine for years. A good sign & great to look at, perfectly safe.
But when there is blue glow inside the plates, around the anodes - this indicates poor vacuum and a gassy tube. This is a problem. For example, I have a very old Arcturus 180, that on turn-on fills with intense blue light from the anodes, virtually lighting up the entire top of the bottle bright as a lightbulb, settling down after a minute or so, to a blue glow just inside the plates. Obviously, this is a problem tube with severe gassiness, it is quite unsafe to use as..well...that gas could very well blow.
As to the state of vacuums, there are no perfect vacuums, not even outer space - there are several scientific studies, beyond my comprehension, that suggest the vacuum in the universe is decaying over time. But a long, long time. Such is the way with tubes also, the vacuums are decaying on all NOS tubes, the only issue is at what rate, and is this rate even relevant yet.
Yes, you are right about the bombardment, but the blue glow on the electrode structure is perfectly normal and the gentle blue glow near the glass is due to the collision of ionized gas molecules with the glass - usually indicating a soft tube. Valves during manufacture have the air pumped out of them and any remaining gas is removed by the getter. Try reducing the negative bias on a tube and you will start to see this blue glow - even on hard (good vacuum) tubes. All tubes have gas to a certain degree - some glow, some don't for the same type tube.
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