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gasified valves

Discussion in 'Electronic Repair' started by N Cook, Apr 19, 2007.

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  1. N Cook

    N Cook Guest

    What is the white deposit chemically, is it always white, and why does it
    always form on the dome end whichever-way-up the valve is mounted.
  2. James Sweet

    James Sweet Guest

    By valve, are you referring to vacuum tubes? The deposit sounds Rilke
    the getter, if it's white instead of silver that means air has got in
    the tube.

    It could also be evaporated material from the cathode deposited on the
    inside of the glass, that will form wherever there's a clear path from
    the cathode to the glass.
  3. Guest

    Most tubes commonly in use have a Barium getter. This material is
    vaporized within the tube during the manufacturing process to absorb
    any loose gas molecules within the tube. When the tube cracks or loses
    its seal, the Barium oxidizes quickly, thereby turning white (Barium

    There are other getter materials used, but Barium is far-and-away the
    most common.

    Peter Wieck
    Wyncote, PA
  4. Meat Plow

    Meat Plow Guest

    Oxidized mercury?

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  5. Jamie

    Jamie Guest

    Hmm, don't know?, could be the heater falling apart?
    could be the results of the getter used to remove the
    remaining Oxygen.
  6. By far the msot common is the getter. Why is it usually on top? Because
    that's where the original metallic barium or other highly reactive material
    was placed during manufacture. Sometimes it's on the side or near the
    base. But since it's a metal and conductive, has to be placed away from
    an area where it could short between pins.

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  7. The Old Guy

    The Old Guy Guest

    As I remember, the getter used to be a pellet of magnesium. After the
    tube or valve was evacuated the magnesium pellet was "fired" by the
    use of RF. As the magnesium burned it used up any remaining oxygen
    within the tube/valve and "silvered" the inside of the glass (black
    and shiny). If the tube or valve cracks or air leaks into it the
    gettering on the inside of the glass finishes oxidising and turns
    white. This is when we refer to a tube as "gassed".
    Roy the old guy.
  8. N Cook

    N Cook Guest

    I'd thought that was silvering to block stray electrons or something firing
    out of the tops of the tubes. I'd thought this whitening process was some
    sort of safety/failure indicator process as a visual indication of a
    mechanically failed valve. Still learning even if an ancient technology
  9. Guest

    metal oxides

  10. Guest

    OK... let's start at the beginning.

    "Getters" are usually alkaline-earth metals flashed onto the inside
    surface of the tube during the manufacturing process by a number of
    mechanisms that are not terribly relevant, but include inductive
    heating, a high current and voltage applied to a pellet of the
    candidate metal that vaporizes it or any of several other mechanisms.

    These getters are to "get" stray molecules of gas that may remain in
    the tube from the manufacturing process. So, they are going to be very
    'active' metals.

    Far-and-away the most common getter past and present is Barium. Others
    include Aluminum, magnesium, and any of several other materials or
    alloys. Very high-temperature tubes use somewhat less-active getter-
    metals as the temperature overcomes the otherwise limitations of the
    metal and permits greater life and less volatility of the getter.

    Typically the getter is flashed to the sides and top of the tube for
    reasons already noted.

    When the tube envelope fails, the getter very rapidly combines with
    available oxygen and other elements in the air. Barium turns white.
    Aluminum turns white, Magesium turns white. This designates an
    envelope failure. The tube will be worthless even if the filament
    survives by some miracle.

    There are some gas-type tubes and rectifiers that use vaporized
    mercury that have no 'getters'.

    There is a lot more out there on this subject, but this should suffice
    for now.

    Peter Wieck
    Wyncote, PA
  11. James Sweet

    James Sweet Guest

    There's nothing that can fire out the tops of the tubes, a very few HV
    rectifier tubes can emit trace amounts of x-rays, but the vast majority
    of tubes operate at far too low a voltage for this. Electrons can't
    travel in free air even if they weren't blocked.
  12. Guest

    When Electrons travel in free air, they are called "ionizing
    radiation" and cause all sorts of havoc. AKA Photoelectric-effect
    Gamma Radiation.

    Peter Wieck
    Wyncote, PA
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