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  • February 24, 2017 at 6:19 am #4090
    PC Load Saucer
    Participant

    Hi all.

    Just remembered that back on 4HV I documented a partial fix for “Bad” tubes.
    http://4hv.org/e107_plugins/forum/forum_viewtopic.php?50094.post
    http://4hv.org/e107_plugins/forum/forum_viewtopic.php?150298.post

    The fix is based on a well known effect called a Zobel network, in this case a series resistor and HV
    capacitor allows counting even if the tube is prone to oscillation and AFAIK is used in high end counters to reduce saturation at abnormal radiation levels by minimizing dead-time.
    A neon in parallel with a resistor also works, interestingly (tested a week ago)

    I’ve tested it with the SBM21 and 20, as well as CBT11A (ebay special, supposedly faulty) and all three work fine
    notably the smaller tubes work far better if this is done.

    It seems that adding this circuit allows the discharge pulses to bypass the resistor but also adds a measure of damping so
    sub-threshold pulses which would normally be ignored are counted.

    An additional modification is to add a radio frequency sensor around the tube, in the form of an AM RX coil.
    This is tuned to around 65 KHz and can do double duty as an extra pulse detector as well as serve as an MSF/whistler antenna.

    April 15, 2017 at 8:38 am #4256
    SaltoUYStation
    Moderator

    Is it possible that “bad tubes” would explain why some units show as consistently offline on the map? ~ Pia

    April 17, 2017 at 6:22 am #4285
    PC Load Saucer
    Participant

    Unlikely to be tube(s), as these are practically impossible to damage.
    It might be tin whiskers, I’ve seen this once or twice on HV stuff.

    Usually if a tube starts to go bad it will oscillate and false count, in
    my experience with SBMxx tubes.
    A good test is to connect a red LED and 1K resistor in parallel, connected
    in series with the tube (on the Hot side!) with wire spirals so as not to
    damage the heat sensitive tube interconnect and this will at least visually
    show when the tube counts particles.
    If the LED is on all the time then the HV might be too high, or other problem.

    With alpha tubes such as LND712 if there is a faint red glow inside then the
    tube is bad. Intriguingly this does not always apply to all the electrodes
    and on multi-electrode tubes ie CBT11A this can be handy to know.

    April 17, 2017 at 8:43 am #4286
    SaltoUYStation
    Moderator

    Ah, thanks for the tech details, Conundrum. I’m not a techie, so, a lot of that goes over my head, but should be helpful for those trying to figure out the offline and other issues.

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