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Vacuum tube testing

Discussion in 'Electronic Repair' started by [email protected], Jun 22, 2004.

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  1. Guest

    A couple days ago I picked up several hundred tubes at an estate sale,
    for $10.

    The estate was that of a fella in town who used to repair televisions.
    He died a couple months ago and his family is cleaning out a huge barn
    in which he had much of his old gear from his repair days.

    The tubes are still in their original boxes. Other than using a tube
    tester, is there any way to check to see if they are any good?

    For instance, a couple I have looked at have dark spots on opposing
    sides. SOme have a dark spot on just one side. Does this indicate the
    tube is dead, or is it how the tubes look new?

    Would it be common practice to put the old tubes in the boxes the new
    tubes came out of? Do tv repairmen have any reason to keep dead tubes?

    I suppose I could open the back of an old working tv and compare those
    tubes with the tubes I bought.
  2. Virtually all tubes will have a getter spot which is usually silver or dark
    metallic in color when good. Milky white, brown, or red if the tube has
    leaked. Tubes don't generally leak unless broken.
    Not a reputable repair person. :)
    Check to see if the tubes manufacturer matches the name on the boxes.
    If you are looking at something like a common 6AU6, there were many
    manufacturers and it would be unlikely for the mfgs to match.

    If they are indeed never used, the likelihood is that they are good
    even if 50 years old.

    --- sam
  3. Bill Turner

    Bill Turner Guest

    The silver area inside the vacuum tubes were called "getter flashes" and
    were left when a chemical coating was burned or flashed from a small element
    inside the tube in the manufacturing process. That process removed the
    remaining air in the tube. If the tube is cracked, the getter flash will
    turn white and flake off the glass.
    As for why the tubes were placed back in boxes, (1) It was the safest way to
    transport them without breakage (2) There was often a market for marganal
    tubes among hobbyists (3) A common practice with intermittent problems,was
    to "shotgun" or replace all the tubes that might cause the problem.Many of
    the ones removed were good. The repair shops would often use those tubes (
    after testing) to repair used sets for sell. The bad side is that many techs
    would sell these questionable tubes as new. It sure did a lot of harm to the
  4. Ken Weitzel

    Ken Weitzel Guest


    AHA! Finally a question for the real old guy (me :)

    Tubes in their original boxes... the rule of thumb
    back in the good old days was that if you removed one
    from its box and for whatever reason put it back, you
    first pushed down the flap, so that it "stood out"
    when you looked at it.

    And the dark spots on the sides mean nothing... it's
    called "getter". They put it on the inside glass,
    then heated the tube so that it would intentionally
    burn, using up any trace of oxygen so that the
    filament wouldn't burn.

    Hope that helps.

  5. Tim Perry

    Tim Perry Guest

    yes... you can try them in circuit. that is replace an identical working
    tube and see if the unit still works.
    often broadcasters would keep weak tubes on hand as emergency spares. the
    common practice was to write "weak" or "used spare" on the box and/or the

    TV, radio and 2-way service techs in general would not want to grab a
    questionable part during a repair.

    id check and see if any were made in China and use those for target
    practice. nothing political it is just that all the Chinese tubes i have
    seen either do not work correctly or do not last long.
  6. Some of my own reasons to keep old tubes in new boxes were:

    1. Anal retention: can't throw away anything that had some potential life
    (never kept one with an open filament)
    2. Proof to the customer of what they where paying for
    3. Even if the customer did not take them back, I always kept them in case a
    few days later the customer came angry saying that the tubes did not fixed
    the problem, and "I want my old tube(s) back" :)
    4. If the filament was still good they could be used to complete a series
    when testing. Some tubes were designed to work in series i.e(27,
    5. Looking at the pile of old tubes was kind of an index of how good the
    business was going. (just likea pile of extracted teeth are to dentist's
    I had two caddies: the NEW Stock, and the USED stock. I never sold nor
    recommended to use an old tube. I did had a lot of fun breaking them and
    examining the marvelous and mysterious design inside. It was much more fun
    and instructive than today breaking a 16-pin IC chip to check the inside
  7. JURB6006

    JURB6006 Guest

    I don't know if this is called "top-posting", but I backed up to reply here.

    As the tube is used, usually more "crap" gases would un-disolve from the metal
    elements. This would usually cause a brown, or lighter ring around the getter.

    A nice shiny getter is a good indicator, but not 100%.

    If you can't test them but want to sell them it's a piece of info that might

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