Tube ratings - what do they mean?

Discussion in 'Amps and Cabs' started by pete12string, Apr 17, 2016.

  1. pete12string

    pete12string Supporting Member

    Mar 1, 2012
    New Jersey
    I've been looking around at NOS/ANOS tubes. Specifically 12AX7. I saw a couple from one place and it was stated:

    "Both sides test 84 where new is 80."

    What does that mean? How are tubes rated? What is good and what is bad? I'm just learning about tubes and can't find too much on their ratings...

  2. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

    Oct 14, 2005
    Berkeley, CA
    Tube testers vary in their ranges of values, and even in what they actually measure. Usually tester readings are expressions of transconductance, which kind of tests how lively the tube is (a ratio of the change in plate current to the change in grid potential, with units of micromhos). Most testers have a "nominal new" setting that is approximately 1.5 times as much as the level below which the tube is considered "bad." So if your tester had new at 100, a "bad" tube would be testing below 66. Between 66 - 100 (or more) would be "good."

    But here's the thing - you have to ask the person testing what the exact parameters of the tester are, and make sure they've got a well-maintained and calibrated tester, and that they've used it properly. And those tests won't tell you anything about microphony, and they won't help you match power tube pairs either. It's just the beginning of the process of assessing whether a tube is truly "good" or not.

    - Thom
    pete12string and Stratobuc like this.

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