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Testing 811a

Discussion in '"Boat Anchor" & Classic Equipment' started by N4MTB, Aug 8, 2017.

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  1. N2EY

    N2EY Ham Member QRZ Page

    All of the microwave oven supplies have a pretty ingenious setup. The magnetron is run with the anode grounded and the heater-cathode supplied with negative voltage, There's a diode and capacitor, and the whole mess runs as a sort of voltage doubler.


    By simply rearranging the diode and capacitor, a half-wave supply can result. No good for an amplifier, but perfect for a simple tester. Put a Variac in the primary circuit and a couple of well-insulated meters in the secondary, and you've got something.
    KD0CAC and KD2ACO like this.
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    A hipot test is an arc test, to test the integrity of the vacuum.

    You set the current limit to a few mA or less.

    No bias is required, and the test takes one second, there isn't any dissipation unless an arc occurs and then it's instantaneous Joules, but no actual dissipation.
    KD0CAC likes this.
  3. KD0CAC

    KD0CAC Ham Member QRZ Page

    This has been on my " one of these days " lists for a long time .
    I come across a lot of used gear , mostly not working and seldom have a working amp , let alone enough working amps to " test in unit the tube was meant for " , so would like to build a tester that could test multiple types of power-tubes , but willing to limit to some of the most common so that building the tester does not get too involved .
    Thanks for the info , subject .
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    One problem when testing, Is that you need to test at the operating frequency.

    You can tell if a tube is bad, But you can not tell if it is good. o_O
  5. N2EY

    N2EY Ham Member QRZ Page

    Low frequency/DC testing can tell you a lot, though, without risking damage to an amplifier.
  6. K2XT

    K2XT Subscriber QRZ Page

    With all due respect I would like to request that my post only get one no vote, not three. Here's why:
    Some examples of how a B+ test, (with some current limiting and fusing, using rather low value filter capacitors to hold down the number of joules) rather than a full hipot test can be very useful are,
    1. If one tube in a multi-tube amp has become gassy and is arcing and it needs to be identified a lower voltage, controlled test can find it.
    2. If you are simply trying to find out if a hamfest purchase of some 813s are really shot or might be usable before putting them in an amp.
    3. If you want to verify that some known bad tubes have been improved by gettering them you can run them in a controlled B+ environment.
    ps - I have done all three of the above and found the test to be very useful if not perfect.

    HOWEVER, I feel I do deserve one no vote for failing to acknowledge that a real, true hipot test will show that a tube is perfect, and may be the only way to guarantee that it is perfect, which is a very desirable thing to know, especially if you were, say, to want to guarantee it to a buyer or have a high degree of confidence for a long time, that the tube is going to hold up. No argument.
    N2EY likes this.
  7. WA9SVD

    WA9SVD Ham Member QRZ Page

    Or verse vica, convert the amp into a microwave oven...
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I don't disagree with any of this, but a "real," honest hipot tester is a pretty cheap appliance and most can be set from zero to about 10kV, with current limiting from zero to maybe 10mA max. We have them all over the place, here at work because we manufacture a lot of stuff with power supplies inside and the hipot applied right to the AC line input reveals a bad socket, a pinched wire, and all sorts of stuff with the push of one button that may not be found otherwise -- until the user gets it and uses it a while, which would be a bad thing.:p

    It's one of the first things an NRTL (like U.L.) does on almost any kind of electrical equipment when supplied to them for safety recognition.

    There are portable, handheld (crank-operated!) hipot testers that are very useful and self-powered so you can bring one to a swap meet.:)

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