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Thread: telling unmarked preamp tubes apart

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    Supporting Member Randall's Avatar
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    telling unmarked preamp tubes apart

    I have about 7 or 8 12AX? tubes with no detectable markings. I have always wondered if there is a way to determine how much gain is there and maybe decide if it is a 12AX7, 12AY7, 12AU7, etc?

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by Randall View Post
    I have about 7 or 8 12AX? tubes with no detectable markings. I have always wondered if there is a way to determine how much gain is there and maybe decide if it is a 12AX7, 12AY7, 12AU7, etc?
    Without a tube tester it can be tricky.... Since visually the plate structure is usually identical for both halves of these types of tubes... just a change in grid wire geometry such as turn-per-turn spacing and turn angle is all that separates a U from and X and T ..
    You can install them into an amplifier and measure the bias and operating voltages and possible signal gain at the anode.. Then compare these numbers against a good known "labeled" tube of that same type...
    Then take into consideration the variances ....just to get you into the ballpark...

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    Best amp for this purpose is a Champ - only one preamp tube. Easy to sort for gain.
    Assuming that you know they are indeed 12A_7 types; my issue is, I have 6EU7s and such floating around...

    Justin

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    Supporting Member Jazz P Bass's Avatar
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    Set up an amp with 12AX7's that you know works.

    Feed it a signal, set it for clean & scope the output of the amp.

    Don't move anything.
    Install the unknown tubes and measure the output on the scope.

    Here is a nice gain comparison chart.: http://www.300guitars.com/articles/p...itution-chart/

    Be aware that this cheap ass test is a guide.
    Basically you are guessing.
    I have seen 12AT7 tubes with the same gain as a 12AX7.

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    There are some tricks to see faded markings that you can't see otherwise. I know holding them over a steaming pot can make lettering visible, there are a couple other ways as well.

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    6EU7 you can tell by filaments on pins 1 and 2 with a ohm meter. I have a hundred or so tubes with no markings. I was gonna start a thread with pictures trying to narrow them down.

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    Bent Member Chuck H's Avatar
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    gelbert is onto it. I've bought many UOS tubes at a salvage warehouse I use to haunt and if you wear good readers and tilt the tube in good light you will usually see markings that seem to be sort of etched in the glass. You may find yourself looking up crossover numbers to determine what something like 7729 or 5814 mean. Otherwise do as Jazz said. I have known good examples of x7, u7, y7 and t7 I could use as base lines for such a test. Hopefully you do to.

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    Those of us who've seen a lot of them or have marked examples handy can sometimes identify a tube brand if there are distinctive structures.

    For example, GE and Sylvania tended to use "box" plates for 12AX7s. Telefunkens look like Telefunkens. Amperex and Mullard have identifying details. RCA and vintage Tung-Sol can be a little tricky to tell apart unless you have examples.

    12AY7s are relatively rare. If there's doubt about whether something is a 12AX7 or a 6EU7, you can look inside to see where the heaters are connected.

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