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What is blue glow in vacuum tubes?

  1. Jan 10, 2014 #1
    I have noticed that some of my tube amp power tubes are glowing slightly blueish. Why?

    It has been explained to me that this only means that the tube is ok.

    However, I find it fascinating!

    Because what is this blueish glow, really?

    To me it sounds like secondary emission of either the anode or the glass envelope.

    It is either this or some residual gas that exists inside the tube.

    But, with regard to what has been explained to me, I do not believe in this last part.

    Do you physics experts perhaps have any explanation of this amazing phenomena?
    Last edited: Jan 10, 2014
  2. jcsd
  3. Jan 10, 2014 #2


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    Blue is normal but if you see this purplish/pink
    it usually means a gas leak causing ionization instead of normal blue electron Fluorescence.
  4. Jan 10, 2014 #3
    Thanks, but why is blue normal?
  5. Jan 10, 2014 #4


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    A few 'blue glow' links.
    http://www.jacmusic.com/techcorner/ARTICLES/English-neu/blueglow/blueglow.htm [Broken]

    We still use high voltage triode tubes for ion suppression voltage shunt regulators on a few production machines. When the normal blue glow seem in tubes with good vacuum turns pinkish it's time for a new tube.
    Last edited by a moderator: May 6, 2017
  6. Jan 12, 2014 #5


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    interesting, I have only seen a blue glow from my mercury vapor rectifier tubes :smile:

  7. Jan 12, 2014 #6


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    Interesting. Being somewhat old, when my younger brother asked me about blue glow in his amp tubes, about a year ago, I made up some mansplanation that; "It was probably designed that way, to look cool", as I'd only ever seen the stereotypical red/orange/yellow glow.


    I need to remember that things, change. :redface:
  8. Jan 12, 2014 #7
    Interprating your first nicely provided link for interested people to read:

    The blue glow I am thinking of seems to depend on the fact that not all the electrons in a vacuum tube actually strikes the plate. A few strays off and strike other things in the tube, such as the glass envelope and thus gives a faint blue just inside the glass envelope. Many types of glass will obviously give off a faint blue glow when bombarded by electrons of sufficient energy. I have seen for myself that the holes themselves gives patches to the glow.

    Furthermore, it was interesting to learn that this somehow mainly concerns newly produced tubes. I am a witness of this because my RCA tubes do not glow like this while my friend's Sovtek tubes in exactly the same type of PA do.

    This is very strange to me.

    But thank you, this answers my question!

    Best regards, Roger
    Actually it doesn't. What is this blueish color really?

    Is it perhaps the Oxygen in SiO2 that is broken free, excited and reverts by sending out hf (=hc/λ)?

    Or what happens?

    Please try to give me a physical explanation this time.
    Last edited by a moderator: May 6, 2017
  9. Jan 12, 2014 #8


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    The effect is best seen when the electron beam is in very good (hard) vacuum so my best guess is the Sovtek tubes have very little gas and have impurity molecules like cobalt from alloys that enhances the glow when stray electrons strike the cobalt ions. Iron-cobalt-nickel alloy cathodes are common in power tubes.
    Last edited: Jan 12, 2014
  10. Jan 12, 2014 #9
    You seem to know very much about vacuum tubes.

    Do you mind telling me what the different parts are made of?

    I heard somewhere that the plate is often made of zinc. Is this true?

    And now you say that the cathode is made of some iron-cobalt-nickel alloy.

    But what about the grid?

    And the supporting rods.


    1) Plate
    2) Cathode
    3) Grid
    4) Supporting rods
    5) Connecting electrodes
    6) Heaters/filaments

    Would be very interesting and fun to know.

    Best regards, Roger
  11. Jan 12, 2014 #10
    By the way, this is what I mean with blue glow in tubes:

    http://www.jacmusic.com/techcorner/ARTICLES/English-neu/blueglow/807-3.JPG [Broken]

    These are 807's.

    Best regards, Roger
    Last edited by a moderator: May 6, 2017
  12. Jan 12, 2014 #11
    Except for the complications of the unique socket, I think the 807 is the best audio tube ever made!
    Last edited: Jan 12, 2014
  13. Jan 12, 2014 #12


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    There are still plenty of 'tube' type devices used in industry today with vacuum process systems and I worked on tons of tube equipment in the military during the 70s. They were even used in some primitive tube digital devices that I worked on as a 'Crypto' tech in the Navy.

    Try some Google searches for electron tube construction if you want to learn more.
  14. Jan 15, 2014 #13
    Last edited by a moderator: Apr 14, 2017
  15. Jan 15, 2014 #14


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    The typical blue glow on the surfaces in good tubes is not a Corona discharge. The stray electrons from each side of the dual beam power tube (a 6L6 or clone?) above create a glow on the inner surfaces of the glass. Too much gas would stop most of the strays from hitting the glass and instead you would see a inner glow near and surrounding the plates.
    Last edited: Jan 15, 2014
  16. Jan 15, 2014 #15
    Your first link made me even more fascinated by the "obsolete" tube technology. There is obviously a newly made tube type called "Dual Tungsten Filament Glass Ion Pressure Gauge Tube" :smile:

    And for what I understand from the site, the collecor/plate is at the top. The two tiny spirals at either radial end are the filaments/cathode, and the helix in the centre is the grid, right?

    I do however not understand the input arrangement. It is not simply just open, or is it?

    So maybe the gas lets in the tube and the gas gets ionized and the current is then an indication of the pressure, right?

    No, I do not think I understand this.

    Anyway, this was fun to read!

    Your second link did however not interest me so much but I read a good part of it anyway. The most interesting part was about how and when the KWR-37 was used. The simple use of a punch card fascinated also. So thank you for that link too.

    Best regards, Roger
  17. Jan 15, 2014 #16
    This was an extremely interesting article to read. I will read it yet some 5 times to perhaps understand!

    Preliminary thoughts:

    "4) The glow of the corona is caused by electrons recombining with positive ions to form neutral atoms. When the electron falls back to its original energy level, it releases a photon of light. The photons serve to ionize other atoms, maintaining the creation of electron avalanches."

    Please correct me if I'm wrong, but would not an ionized atom miss it's electron?

    To me this would mean that the atom is at a somewhat lower stage of energy. And recombination would simply mean that the outer shell is completed.

    But for an atom to actually send out light (a photon) the outer electron would have to be excited to some higher energy shell (mgh, I almost said :wink:), whereas it would fall back to its "comfortable" energy and then send out a photon according to the difference (i.e hf), right?

    Considering that I might be right, I do not understand how a "simple" recombination would yield the same.

    But this is probably just me being stupid :smile:

    Take care and thanks for the interesting link!

    Best regards, Roger
  18. Jan 15, 2014 #17


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    The tubes are open ended into the vacuum chamber and they also sometimes have blue glow at e-6 Torr vacuum or better on new and very clean tubes.

    The KWR-37 link was to show that tubes were also used as complex digital switching components in equipment well past their prime when there was a special requirement. The 37 was almost completely immune from the effects of EMP or high energy EM weapons so it was the often the primary device for sending/decoding Strategic missile and bombing launch commands to ships at sea. The 500 or so tubes with about a ~5kHz clock rate implemented a complete Self-synchronizing (the internal Fibonacci shift register clock generator needed to be within maybe 10 seconds of the remote encoder clock for a stream lock using the front panel time dial and tweak buttons) synchronous stream cipher device similar to RC4.
    The 26 was a similar tube device with different requirements that also used magnetic logic devices.

    No blue glow inside these guys
    Last edited: Jan 15, 2014
  19. Jan 16, 2014 #18
    I can't help but being fascinated while mainly being interested in plasma physics.

    1) These apparatus were, as you say, obviously being used well after their prime because the transistor was actually invented as early as 1947 (Germanium PNP).
    2) Being completelly immune to EMP as compared to Solid State (this might however need a physical explanation) tubes were/are great for warfare. In fact, I have heard an unverified rumour about a certain Russian tube (6C33) which where actually used in their MIGs as late as the 90's.
    3) I "like" the low clock rate :smile:
    4) Interesting to know that the famous Fibbonachi series where used.

    Reading your KW-26 link, I finally understood what you said about tubes being used way past their prime. Not until mid-80's the KW-26 was decommisioned. This is way beyond 1947!
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