Glowing 6EA8

[W1EUJ]

Working on an HW-101 last night, turned off the shop lights. Saw this.

Pretty bluish glow, almost looks like a fluorescence, along the inner surface of the glass.

I'll swap it out, but, ain't it pretty?

[W3RSW]

Hmmm,  How purty that glow.
the getter's been gotten.
Keep it if it still works without distortion.

It's just a tube
and slightly lossy.
Won't hurt a thing,
lessen it's bossy.
and screws up IMD and HMD.

[KE6DF]

Maybe use it as a Christmas light. It's the season, after all.

[WD5JKO]

 I often see the same thing with the 6U8 as used in the Central Electronics 20A. The 6U8 and the 6EA8 are pin for pin, and almost identical tubes. I am currently using the 6MU8 with good success to replace the 6U8. The 6MU8 is also pin for pin except it uses a taller bulb, and the pentode portion has higher plate dissipation (a good thing). So if your 6EA8 is running pretty hard (hot), consider the 6MU8 swap after comparing device characteristics. The 6MU8 is readily available and is very cheap.

Jim
WD5JKO

[k4kyv]

For the umpteenth time, that blue glow does not indicate anything wrong with the tube.  This is commonly mistaken for a gassy tube, and has been the cause for many perfectly good tubes to be discarded.

If the tube is gassy, the blue glow will appear from the interior of the plate structure, around the grid(s).  In severe cases, the entire interior of the glass envelope will glow, just like with an 866A.

If the blue glow appears on the surface of the glass envelope, it is an indication of harmless impurities in the glass. 

Nearly all tubes have some electron leakage around the plate structure, and those stray electrons bombard the glass.  Sometimes, impurities will cause the glass to fluoresce, the same principle that makes the screen on a cathode ray tube or TV picture tube glow.

That same electron bombardment is what causes the brown streaks of "coffee stain" on the glass of transmitting tubes that have seen many hours of service.  Those tubes may still check out perfectly, but the stained glass tells you for sure they are heavily-used pulls, not new old stock or "slightly used". 

There is no reason to discard or change out a tube if the only questionable characteristic is a blue glow coming off the glass envelope.

[W1EUJ]

Thanks Don.

[W3RSW]

That's right!

I distinctly remember KT88's, 6550's, etc. glowing in audio service, sometimes seemingly depending on which way the amp was plugged in.   

Sometimes they glowed, sometimes they didn't.

Wasn't there an explanation for this?  Perhaps not how the plug was polarized, just a flip the coin glow or no-glow.   I swear it was almost a 50/50 observable phenomena.

Strange.
'dum de dum duhhm.'

[WD5JKO]

I still say that a lot of equipment runs low level tubes like the 6EA8/6U8 hot, especially with today's higher line voltage. When I see a 6EA8/6U8 glowing like that, failure is often near. Comparing this to a Power output tube like the 6550 is not comparing apples to apples. I've brought up the 6MU8 issue before and all sorts of folks got agitated. I thought this was a technical forum.

Jim

[w8khk]

If you really enjoy the glow, build a beautiful boatanchor, install some 866s, or better yet, 872s or 8008s, and STRAP!

[k4kyv]

The blue glow could signal a tube running @ too high plate voltage or pulling too high current, since either of those circumstances could increase the electron bombardment. If you suddenly notice the blue glow, it might not be a bad idea to check tube voltages Justin Case.  But if everything checks normal, I wouldn't worry about the blue glow.

You need to really worry when you see the blue glow and notice also that the tube plate is red hot or has red spots on it.  I have seen that combination when a tube loses bias or goes into the runaway mode.

[W1EUJ]

If you really enjoy the glow, build a beautiful boatanchor, install some 866s, or better yet, 872s or 8008s, and STRAP!

I built a 872 nightlight in college, used it till it burned out. What a nice glow.

[W3RSW]

I've brought up the 6MU8 issue before and all sorts of folks got agitated. I thought this was a technical forum.

It is but sometimes we have a rough time taking ourselves seriously 
  And it's funny what some half-banked memories or thoughts do generate on this board.  Then we get 'calibrated' in a hurry, learn some history, 'hear' other stories and generally have fun.
Uh, Sorry for getting so preachy.

[AF9J]

That's right!

I distinctly remember KT88's, 6550's, etc. glowing in audio service, sometimes seemingly depending on which way the amp was plugged in.   

Sometimes they glowed, sometimes they didn't.

Wasn't there an explanation for this?  Perhaps not how the plug was polarized, just a flip the coin glow or no-glow.   I swear it was almost a 50/50 observable phenomena.

Strange.
'dum de dum duhhm.'

Yuppers,

Somebody had this very same question (with photos provided) on the Harmony Central Guitar Amps forum.  He was concerned because the new 6L6 power tubes he'd put in his guitar amp, were glowing blue, which he'd never seen happen before.  After discusion of the issue (including some from electronics techs with tube experience), the same thing things that Don mentioned were bought up - glowing blue tubes are not necessarily bad, and due to the hot biasing in some guitar amps, are pretty common.

73,
Elen - AF9J   

[WQ9E]

 I often see the same thing with the 6U8 as used in the Central Electronics 20A.
Jim
WD5JKO

Jim,

You make a good point in that the small bulb size of miniature tubes does increase the heat; for example the 6AQ5 is basically a 6V6 in a miniature package and run close to 6V6 ratings it gets plenty hot.  In theory, better metallurgy and construction techniques that evolved over time should allow little tubes to run OK under those conditions.  As a small data point I have replaced a lot of 6AQ5 tubes that were weak, gassy, or had secondary emission.  I think the only 6V6 I have replaced in the numerous radios I have restored was still working but it seemed to have a small subsonic oscillation that caused little blue dots to appear on the envelope at a 1 CPS rate.  It was cute but I didn't want to risk the associated components in my Halli receiver.  It is sitting on a shelf in case it is needed as a ballast tube replacement for my NC-400.

As to the CE-20A, do you seem to have plenty of drive from the 9 Mhz. oscillator?  I assume you are referring to 6U8 used as the master oscillator and it depends upon grid bias developed by the 9 mhz xtal so if your oscillation is weak it may be running with low bias. 

Best wishes,
Rodger WQ9E

[WZ1M]

How can you tell if a 6L6 is glowing blue, xray vision?
Gary

[W1EUJ]

6L6GT?

[kb3ouk]

I have a Crosley that I was replacing the audio output tubes in. They are 6K6GTs. Iwas trying to find which pair of tubes out of the 8 that I had would have the most output. The old tubes that were in it did have the bklue glow inside the plate and had very low and distorted output. I put a Sylvania tube in there and it glowed blue on the glass but would stop if you gently hit the envelope with your finger, then it would stop glowing. It would start again if you turned it off and then back on.
Shelby KB3OUK

[AF9J]

How can you tell if a 6L6 is glowing blue, xray vision?
Gary

Oops!  Sorry, I should have said 6L6GT, which is very commonly used in guitar amps.

Ellen - AF9J

[w4bfs]

try exploring the area of the glow with a NE-2 neon held by the leads ... of course, don't do this in a way that could shock you .... if the neon should glow, sounds like some rf may be gettin' made, whether or not it is supposed to .... 73 ...John

[k4kyv]

How can you tell if a 6L6 is glowing blue, xray vision?
Gary

Oops!  Sorry, I should have said 6L6GT, which is very commonly used in guitar amps.

Audiophools swear that glass 6L6's sound better than metal ones.  Actually, the original version that came out in the 30's was the metal type.  The glass version is a Johnny-come-lately.

I don't think there was ever such a thing as a 6L6GT. The original glass version with the shouldered envelope carried the -G suffix.  The later models with the straight sided glass envelope carry suffixes like -GB, -GC, etc.  -GT tubes are the smaller straight sided octals used for low level receiving applications (e.g. 6SQ7GT), and lower power audio applications (e.g. the 6V6GT and 6F6GT).

I never could tell any difference between a good glass tube and a good metal one.  But then, my ears never could tell the subtle differences between a speaker wired with heavy gauge zip cord and one wired with Monster Cable, either.

[kb3ouk]

Tubes with a GT suffix usually have a plastic(or most likely, bakelite) sleeve around the base. Most glass 6L6s I've seen are labeled as 6L6GCs. But to me, the metal 6L6s look taller than the glass ones.
Shekby KB3OUK