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When a vacuum tube fails

Discussion in 'Electronic Basics' started by Ryan, Oct 27, 2003.

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  1. Ryan

    Ryan Guest

    My friend's ~1960's model corn moisture meter (accurate!) recently
    failed. Inside he found a blackened RCA 12AU7A vacuum tube. I want to
    help him out. This is the only tube it has and he says there is not
    much stuff inside.

    I've found these about on the internet.

    Is it reasonable to expect that merely replacing this tube will render
    the unit functional, or must I consider the possibility that a failed
    tube can damage of other parts?

    For example, in modern day stuff, it is my understanding that the
    failure of a tranistor or mosfet, may kill other parts around. (Such as
    a shorted gate-to-drain killing a driver transistor upstream in an
    amplifier, or one burned mosfet in the power supply leading to the
    demise of the whole bank.)

    Does this same thing happen with tube era machines? How likely is it?

    Among the same part numbers, how is one tube with a silver top different
    than a clear tube? A¹Íthere other considerations to note before

    Thank You.
  2. tempus fugit

    tempus fugit Guest

    I'm not too sure about the other parts around the tube, but you could easily
    check them with a meter to see if they are OK. Before you order off the
    internet, I would check at a local music store. The 12AX7 is a very common
    tube used in guitar amps, and it may be OK to replace the AU7 with an AX7 (I
    think the AX7 has higher gain).

    As usual, the more experienced are welcome to correct me here.
  3. The silver coating on the inside of the glass is a very thin coating
    of a very active metal that grabs oxygen. This is intended to collect
    any stray atoms that are still floating around, after the tube is
    pumped down and sealed. If the tube has an air leak, this silver
    coating will turn to a semi transparent dusty oxide. So tubes with a
    nice silver coat that does not have dusty looking edges is a sign of
    still good vacuum.
  4. Don Bruder

    Don Bruder Guest

    I think that advice is bad. Well-intended, but bad, nonetheless. With
    something like a moisture meter, you're going to want the same gain,
    otherwise, you're likely to get bad readings. For a guitar amp, more
    gain from the same tube socket is no big deal. It might even be
    desirable (if your'e into distortion, I guess). For a calibrated
    instrument (especially an older item which may not have enough
    information available to recalibrate to compensate for the different
    tube) like this meter, extra gain may literally translate to "umpty-nine
    thousand head of livestock dead" due to the meter reading false.
    Especially if it reads false low... In corn, that's especially bad,
    because it's subject to a specific form of blue mold (name forgotten
    right now) that produces a mycotoxin known to kill and/or permanently
    cripple horses and cattle that eat just a few (less than a dozen, in one
    case I know of) contaminated kernels.
  5. Don Bruder

    Don Bruder Guest

    I can buy PART of that...
    Care to offer an explanation for the mirror-ish spots that form on the
    inside of some tubes after they've been run for a while? As in, they
    start out completely clear, but after a varibable amount of time,
    develop mirrored and/or blackened, or even both in the same spot, areas
    on the inside fo the glass?

    Some kind of atom-by-atom "spray-coating" of metal being applied to the
    glass by the electron flow, perhaps?

    One of those mysteries I've never heard an explanation for...
  6. Guest

    It's likely only the tube.

    Tubes should have a nice silver lining inside the top. This is a very
    reactive element that immediately "removes" any traces of air that
    might leak in. If the vacuum is lost, then all of it reacts and it
    goes black instantly.

  8. Don Bruder

    Don Bruder Guest

  9. I think that high power tubes (that run plates cherry red) do move
    some metal atoms around and produce dark shadowy coatings, but I have
    never seen one that produced a mirror film. Anyway, the intentional
    films are boiled off a metal hoop that has a groove to hold the
    reactive metal, which is heated to incandescence by an induction coil
    after the tube is evacuated. If there is no sign of the boiling ring,
    then the coating has some other source than intentional gettering.
  10. You better keep in mind that a lot of tube equipment was built with
    paper, or oil impregnated pare capacitors that have a limited life. The
    become leaky with age, and it only take a little leakage to upset the
    bias on the tube. I have a military audio generator on my bench right
    now that was built in the early '40s. It was built with the best
    capacitors available at the time. (Oil filled bathtub capacitors) Every
    capacitor in the unit is leaky, yet the equipment looks unused. If they
    want to return the unit to service, they should completely rebuild it,
    not patch it.
  11. Don Bruder

    Don Bruder Guest

    Ahh, but the OP was wanting to know if the failed tube was likely to
    "take out" anything else, as sometimes happens with modern devices - A
    failed transistor causing transistors in the next stage of an amp to
    cook off, ferinstance.

    *ASS* *U* *ME*-ing that the tube alone puked (perhaps the unit finally
    got dropped one time too many and cracked the tube, allowing it to leak,
    or vibrated the filament into breaking, or the impact mooshed a grid out
    of shape or into contact with another internal part of the tube causing
    a short, or some similar "purely physical damage" scenario that I can
    envision being *REALLY* common failure modes for hand-held bits of farm
    equipment) I can't see any reason that a simple tube swap wouldn't
    restore the beast to 100% functionality.

    On the other hand, it could be, as you say, a cap failure, or a resistor
    gone south, or any number of other things that won't be repaired by
    replacing the tube...

    Not really enough info in the original post to be sure either way...
  12. Don Bruder

    Don Bruder Guest

    Right. That follows. Getters that are put there on purpose are usually
    pretty obvious. But the film I'm talking about probably ain't caused by

    The most pronounced instance I saw was in a monster of a tube that
    apparently had something to do with a commercial AM radio station's
    transmitter - can't be much more specific than that, since there was no
    useful labeling on the box it was in - however, the box that it was
    cabled to was tagged very clearly as "Modulator", and had other info on
    it that indicated the beast was from an AM station - very possibly the
    one about a mile up the road from the abandoned, partially collapsed
    shed I found the gear in. Nobody at the station at the time knew
    anything about it, though, so I'm guessing if it WAS theirs, it was last
    in service sometime just before "ancient history". :)

    This particular tube was a serious piece of work... Memory is foggy, but
    I want to say it had more than 20 very large, thick (probably an eighth
    of an inch diameter, maybe somewhat larger, and nearly an inch long,
    except for one that almost looked as if it had been nipped off at about
    half an inch, if you ignored the fact that the end of it was rounded
    like all the rest - a keying pin, perhaps?) pins hanging out of the
    bottom. It was about as big around as a baseball at the base, flaring
    out to nearly softball sized at the top, and as I said, about 10 inches
    high, with a large connector on the top. (FWIW, the wire going to this
    connector was *HUGE* - About 3/4 inch thick, wrapped in at least 6
    layers of varnished/shellaced/doped cloth on top of the rubber-like
    insulation. I'd guess it was close to 6 gauge wire inside the various
    layers of insulating material.

    Aside from three spots, most of it was crystal clear glass. The three
    spots (one covered most of the top of the tube in a somewhat irregular,
    but generally round, "splatter", the other two were smaller patches near
    the base that were, allowing for the shape of the tube, pretty close to
    perfectly circular) seemed to be related to some sort of pieces inside
    the tube. The top one wasn't visible (the film was a near perfect
    mirror, except near the center, where it was black, and any other view
    was blocked either by the film, or the hardware inside the glass) but
    the bottom two could be "looked around" to see that there were two
    ring-like pieces of metal, each one connected to a separate pin,
    directly in line with the patches. Precisely what those two rings were
    is obviously debatable, but from the "spray pattern", it seemed pretty
    obvious that they had something to do with the film being there. In the
    center area of each "patch", the finish was a nearly perfect (albeit
    curved to conform with the shape of the tube's glass) mirror. The
    further from the center of the patch you looked, the thinner the
    coating, until it finally petered out at about 1.5 inches from the
    center, leaving perfectly clear glass.

    Care to take a whack at the "WTF-izzit!?!?" on that one? :)
  13. What a beast.
    Any ring structure (especially if it connects to only one pin) is
    almost certainly a getter induction heating loop.
    I doubt I have ever seen any thing like this.
  14. Fred Abse

    Fred Abse Guest

    Those rings are how the getters are deposited. If you look closely, you'll
    see that they aren't solid rings, but a channel section, a bit like a car
    tire cut down the middle of the tread. The channel is filled with a
    mixture of barium oxide and magnesium powder. After the tube has been
    evacuated, a high frequency induction heating coil is passed over it. This
    "fires" the getter. The magnesium "burns", using the oxygen in the barium
    oxide, giving magnesium oxide and metallic barium, which either evaporates
    or sublimes, I can't remember which, and condenses on the relatively cool
    glass envelope. This is the "mirror" you see. Because barium has an
    affinity for oxygen (though not as great as magnesium has, obviously),
    plus it has an affinity for hydrogen, which is also slowly evolved from
    the internal structure of the tube ("outgassing"), it acts as a "chemical
    pump", soaking up residual gas molecules. If the tube fractures, or
    otherwise "goes down to air", the barium oxidizes rapidly, and the
    "mirror" turns white.

    BTW, barium and some of its compounds are nasty if ingested. AFAIK, the
    oxide is relatively safe, but don't take chances with busted tubes, there
    are other nasty things in there, too, like the cathode coating material.
  15. Fred Abse

    Fred Abse Guest

    No - it goes white. (metallic barium -> barium oxide)
  16. Don

    Don Guest

    12au7 tubes cost less than $10, so don't replace with a 12ax7. A music store
    or old-timer tv repairer shop may also have 12au7 tubes in stock. It also may
    make sense to have a tv repair shop replace any electrolytic capacitors in the
    unit, as these are the most likely components to fail. In fact, the tube may
    have failed due to cap failure. Tubes also fail after many hours of use.
  17. Luhan Monat

    Luhan Monat Guest


    I used to fix TV and Stereo's all running on tubes. Tubes burn out (the
    filiment) so you just plug in any that dont light up. The 12AX7 is pin
    compatable with the 12AU7 but has a higher gain. As stated in other
    postings here, the gain of any tube is never used to set the gain of the

    So put the tube in and see if that fixes it. Unlike transistors, tubes
    do not 'short out' and take other ciruit elements with it.
  18. So, you've never seen a burnt cathode resistor? How about a smoked
    plate resistor.
  19. Don

    Don Guest

    That's cool! So I can use 12au7 in my phono preamp instead of 12ax7?
    And 12ax7 in my HHScott tube tuner where 12at7 are used? Changing the µ doesn't
    effect the gain. Hmmm, how about oscillation? Why did they make more than one
    tube type?
  20. Baphomet

    Baphomet Guest

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