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Thread: Question about tube glow.

  1. #1
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    Question about tube glow.

    Last night I had my amp open for repair. While the amp chassis was sitting on the table, I had a chance to turn off the lights and watch the tubes glow. Very cool, BTW. I noticed that when I had the lights out, there was an obvious difference in glow from the tubes.

    Now I know that heater glow is supposed to happen, and that a blue glow is supposedly a mild florescence resulting from the imperfect vacuum in a tube that shows the tube is indeed emitting, and that neither need be worried about. But one pair of tubes had a much brighter blue glow than the other pair, while one pair had much less in comparison. I swapped the tubes around in the chassis and the glow moved with the tubes -- it was not a result of the socket they were sitting in.

    What I'm wondering is, can the strength of a the blue glow from an operational tube be used as a rough indicator of it's strength/remaining life, and if so, which way to go? Is it bright glow = plenty of emission = strong, healthy tube with plenty of life left, or is it bright glow = gassy tube = old tube that's worn out and on it's last legs?

    I want to lean towards the former, as the less glowy pair had much less difference in glow between standby and on, one of which had almost no blue glow at all when taken out of standby - I had to look through the holes in the plate to see the glow and verify the tube was on. But I'm assuming here, and figured it'd be best to ask those with experience and know what they're talking about.

    If this is indeed a sign of uneven wear between the two tube pairs, I do run the amp in half-power mode almost all the time due to the mismatch between my cabinet, and the OT. That should account for one pair of tubes being almost dead while the other pair is pretty fresh, right?
    Last edited by Koreth; 03-30-2010 at 10:47 PM.

  2. #2
    Noodle of Reality Steve Conner's Avatar
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    There are three kinds of blue glows in tubes, all fun to watch after a beer or three.

    If the glass envelope seems to be glowing blue in patches, that's fluorescence caused by electrons that somehow missed the plate and hit the glass. This seems to be at its strongest in brand new tubes, and gradually dims out as the glass gets sputtered with a thin film of cathode material. But even once it's gone, the tube should still have plenty of life left.

    In my experience it disappears in the first 10-100 hours of use, but a good tube should last several thousand hours. I have a couple of Sovtek 6550s that glowed strongly until I left my amp in a rehearsal room we shared with a few other bands. When I got it back, the tubes didn't have that nice blue glow on the glass any more, but they still gave full power. That was a year ago and they're still working.

    Sometimes moving the tube to a different amp will make the glow appear again. The different voltages mean that the electrons hit in different places that are still fresh.

    If you see bluish-white stripes on the inside of the plate, same thing. Horizontal zebra stripes for a pentode, and one vertical line on each side for a beam tetrode. I've seen these even in old, worn-out tubes. They're brightest at the beginning of a run when the plate is cold, and fade out as it heats up.

    A cloud of glowing gas inside the tube is always bad news, it means you need to junk it before it shorts out and damages your amp. I've got some really old Sovteks that do this, and it looks quite different to the other two kinds of glow.

    Maybe you should swap the two tube pairs around every so often, so they wear evenly.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  3. #3
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    An update, since I figured resurrecting a 1+ year old thread instead of starting a new one for a recent and related anecdote would be the lesser crime.

    I got a chance last night to see what I assume is the last type of glow Steve mentioned above. After dinner I turned my amp on in order to let it warm up while I did the dishes. 30 minutes later I come back, grab a guitar, plug it in and go to hit the standby switch, and was promptly greeted by a loud cracking and humming sound that startled me enough that my hand jerked the switch back in to standby before I even fully realized what had just happened. After I had a few seconds to go "WTF was that?" I nervously threw the switch again and was again greeted by *crackle-POP! HUUUUMMMM!!!* and this time saw the purple glow on the wall behind the amp. "ohsh!tturnitoffturnitoff!"

    I took the amp down off the speaker cabinet and set it up where I could see tubes so I find which one was going bad. I also hooked it up to a smaller speaker which was set face down into the carpet so as to keep the transformer and the tubes loaded, but not be as earsplittingly loud. As soon as the amp was taken out of standby one of the tubes immediately produced a very visible purple glow, and there was an audible hum, even with the master volume turned all the way down. Briefly the tube started spitting pretty purple sparks from inside the plates out beneath the lower mica, then turned a bright purple-pink, which was bright enough that I couldn't see any of the elements inside the tube, before returning to just having a purple glow. The sparks and bright glow corresponded to the same crackle-pop-hum sound the amp had made twice before. Dude, arcing tubes are LOUD! Also, very pretty with bright exotic colors, but I assume also dangerous, if nature's other uses of bright colors are any indication.

    So I swapped some tubes around just to make sure it was the tube and not a problem tied to the socket, and the purple glow followed the tube. It didn't sparkle or arc again, but the amp continued to hum, even with the master volume still all the way down at 0. I'm guessing the tube in question has become gassy and unstable, and thus is drawing far more current than it should. One tube in a pair pulling down enough current to make pretty colors I'm guessing is causing an imbalance in the transformer, big enough to make it hum.

    I've pulled the tube in question and added it to my collection of curious but useless knicknacks. I guess the tube knew I was planning on picking up a new set of tubes today, so as to have spares.
    Last edited by Koreth; 04-20-2011 at 09:09 PM.

  4. #4
    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by Koreth View Post
    I guess the tube knew I was planning on picking up a new set of tubes today, so as to have spares.
    What, you didn't know that Murphy has ears?
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