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Thread: Visual signs of a bad tube?

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    Visual signs of a bad tube?

    I am looking for ways to visually identify bad tubes. I am going through boxes of tubes in my basement and doing a preliminary sort.

    I know that a tube that has lost its vacuum will have a white frosting on the inside which means bad tube. I have read that if a tube has "smoke stains" (brown tint) on the inside that the tube has been driven hard and probably not any good. I have also read that if a tube's getter is black, not shiny silvery, that the tube is probably worn out.

    Are there any other visual indications of a bad tube? Thanks.

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    Bent Member Chuck H's Avatar
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    Well I know for certain that the "smoke stains" aren't a good indicator on el84's. I had a box of them, given to me by a guy who was looking for the perfect quad. Maybe twenty tubes. Most were reported to have only a couple of weeks of use on them. Many exhibited "smoke stains" on the glass near openings in the plate structure. Even those tubes played long and well for me in a combo amp.

    I don't think I've ever seen a dulled or blackened getter. Not that it doesn't happen.

    Just my own observation, but it seems to me that the shiny "getter flashing" seems to evaporate with age in some tubes. As I've noticed that many used tubes with a small patch left seem to be at or near the end of their life.

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    Take a look at these two 7951 tubes.

    The tube on the right has a nice shiny silver getter. It tests good.

    The tube on the left has a smokey dark getter. It tests bad.

    Click image for larger version. 

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    I remember for a while brand new Sovtek 12AX7WA's would have a decidedly dark looking getter- not at all silvery like you would expect. Im just saying- use it as an indicator but thats about it. Good tester needs to sort them out. Bob

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    Member HaroldBrooks's Avatar
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    I've had one or two tubes that Red plated on me, and on close inspection, the grey plate has a darkened patch on it, resembling metal that was heated cherry red and cooled. Actually, this takes some time to get that bad, as I've had others that red plated for just a moment until the bias was corrected and they look just fine afterward. So if you have a tube with a noticeable patch on the plate (make sure it's inside and not on the glass), chances are it red plated for a while, and is now that tube is not reliable.

    Burt pins and a cooked looking top or bottom is another possible tipoff to a bad tube (brown, reddish brown, or too black), but you need to differentiate if it's on the inside our outside of the glass before you jump to any conclusions, as some Smokey looking tubes that have been handled a lot, and exposed to grease and smoke residue can look burnt, but in actuality they are good.

    When I buy used tubes, I clean them with a bit of dollar tree hand sanitizer when they are stone cold, and while this takes off almost all of the Painted markings (with the exception of the Octagon on RCA tubes), but if the tube is not rare or expensive you probably should do this. You can clean every part of the tube except the markings, and you will now see very clearly inside the tube, as if were brand new, after it's cleaned with hand sanitizer and a paper towel. https://www.google.com/search?biw=19...=1555208658787

    Once a tube passes visual inspection, Just barely tapping on the tube (very lightly) with a chopstick is also a good way to spot bit micro-phonics, but realize that almost all tube make some noise when you do this and still work fine. It's uncontrolled runaway micro-phonics that are the problem, and you will know soon enough.

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    Member hrbay's Avatar
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    looks like 2 good 7 59 1 tubes, with the left one more hours than the right one. The getter shine diminishes some as a tube gets more hours on it. I've had some T~S 6550's that tested average to good maybe 75%, but had little or very faint getter material left in them. I think that the getter coating attaches to the molecules of any impurities that did not get evacuated when made, and redeposits on the frame structures and wires but not back on the glass. Or maybe after 50 or so years on a shelf some impurity I guess could get through the glass or base joints to the glass envelope. If the tube is found to be leaky on a tester check, it should probably be marked as such and retired.

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