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Thread: Valve help - stuck with reading

  1. #1
    Join Date: Nov 2014

    Location: London

    Posts: 55
    I'm Tim.

    Default Valve help - stuck with reading

    Iím a bit stuck trying to understand how closely a bunch of valves I got as singles are. I'd like to create a matched pair. Can you help me translate the ratings given? All tubes are the same type a 12A

    valve 1
    TESTED gm1600

    valve 2
    Tested on a TV 7
    Results = 72

    valve 3
    Tested on a Hickok 6000A :
    Nominal New = 7800
    Min = 4680
    Results = 9200

    valve 4
    Tested on TV-7/B. The tubes test very strong with a reading of 75. Minimum is 42.

    Hope this makes sense!
    Thanks, Tim

  2. #2
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 5,691
    I'm Alan.

    Default

    Not a lot!

    If you have one at 72 and another at 75 that's good. However have they been tested at the same conditions?

    One says tested on TV 7, the other TV-7/B, without knowing what these are it's difficult to be sure.

    Is valve 1 gm=1600? Gm values are usually small. Looking at the data sheet the typical gm = 1.65.

  3. #3
    Join Date: Nov 2014

    Location: London

    Posts: 55
    I'm Tim.

    Default

    All valves are 12A type. I don’t know any more than that I’m afraid!

  4. #4
    Join Date: Nov 2014

    Location: Denbigh, N. Wales

    Posts: 106
    I'm Mark.

    Default

    The Gm is just being quoted in Microsieverts rather than millisieverts, so the value is 1000 times larger.
    Aka Micromhos, see for example the RCA sheet for the 112A ( = 12A )
    https://frank.pocnet.net/sheets/021/1/112A.pdf
    If they have been tested the same way, the figures sound fine to be used as a pair.

  5. #5
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 95
    I'm Dan.

    Default

    If you bought them with those numbers, they are totally useless. You have no way of knowing the conditions, calibrations of the testers.
    Last edited by Blackdog; 01-03-2021 at 00:31.

  6. #6
    Join Date: Nov 2014

    Location: Denbigh, N. Wales

    Posts: 106
    I'm Mark.

    Default

    They're not useless. I am assuming this is a DIY project, with 12A's ( ! )
    Get some sort of circuit set up, with a B+, anode load, cathode resistor and bias. Get some voltage readings and compare current for that Va, and Vg to the datasheet. If it's above about 85% of the datasheet numbers you can use. Select the closest two as a pair.

  7. #7
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 95
    I'm Dan.

    Default

    You misunderstand. The numbers are useless for matching. They weren't tested on the same machine.
    Your idea of setting up a circuit is a good one. If you want to match that's the best way.

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