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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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