Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jan 11, 2019.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    There's an advantage to so much filter capacitance.

    If in the middle of a transmission the power goes out, you have time to say "73" before the caps discharge.
    WG8Z and WA7PRC like this.
  2. AI3V

    AI3V Ham Member QRZ Page

    Indeed, you only need enough c to supply energy at the lowest expected modulation frequency.

  3. AI3V

    AI3V Ham Member QRZ Page

    The 4cx35000 hf transmitters I worked on had a crowbar test that wanted us to put a small diameter wire on the end of the "chicken stick" and touch the b+ lead to see if the thyratron fired.

    None of us were brave (stupid?) Enough to do it.

    Knowing what I know now about "arc flash" that was a very very good thing.

  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    I prefer to refer to them as "Jesus" sticks because:
    • when the discharge "BANG" is really loud, that's what you yell.
    • it prevents a premature meeting w/ him.
    WA9SVD, N2SR and N2EY like this.
  5. N2EY

    N2EY XML Subscriber QRZ Page

    Actually, you don't - because the antenna changeover relay will drop out almost immediately.
    N2SR likes this.
  6. G0GSR

    G0GSR Ham Member QRZ Page

    I don't blame you! I wouldn't do that either.
    We had a metal box with the fuse-wire and a vacuum switch. We just stood back and pressed the close switch.
    The thyratron crowbar firing made a dull "thud" like someone banging on the other side of the wall with a large hammer.
    The TX manufacturers test was to wire the fuse-wire across the HV, then switch on but we were not convinced that this was a fair test.

    Last edited: Jun 6, 2019
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    The jig that Telefunken provided resembled a quite large plastic rat-trap where the fuse-wire was held by a fixture above a grounded metal plate and a well-insulated HV lead was affixed to the holder for the wire.

    When the operator "pulled the string" from a safe distance the holder tripped and brought the wire into contact with the plate.

    I found the datasheet for the ignitron used in the 100 kW amplifier, and it would stand a DC potential of at least 20 kV, a current of 3000 A during the turn-off process and a typical turn-on time of less than 5 µs.

    The energy dissipated in the fuse wire would then be around 500 A at 14 kV during a time of 5 µs or about 35 J, which was not sufficient to melt the wire.

    The first time I read about of ignitron protection of power tubes was in a
    mid-50s article in Electronics written by Vince deLong, then head of transmitter engineering at Collins. A protection circuit for a 30 kW transmitter fast enough to trip without melting when short-circuited by a small piece of tin-foil was described.

    Regarding the "Jesus stick" it was one stated by a Collins field service engineer that the name could have two interpretations;
    • "If they ever become used for their intended purpose of discharging a filter capacitor, the user will scream out "Jesus!"

    • If they are not used as a precaution, sooner or later the user will prematurely meet Jesus"

    Last edited: Jun 6, 2019
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  8. N2EY

    N2EY XML Subscriber QRZ Page

    Anyone besides Karl-Arne and myself know what an ignitron is without looking it up?
    WA9UAA and AF7XT like this.
  9. G0GSR

    G0GSR Ham Member QRZ Page

    Yes... a mercury vapour filled vacuum tube which only conducts when a high voltage, low current pulse is applied to its trigger electrode.
    Works like a thyristor. Once triggered, stays conducting until the supply is removed.

    I did use one in earnest one day. The week previous, we replaced all the HV capacitors as they contained PCB. Unknown to me, the HV cable to the earthing switch from one capacitor had been put back on the wrong terminal. That capacitor remained charged at about 22kV when I entered the enclosure.
    Fortunately, I did what I was trained to do and used the earthing wand on all HV components.

    I don't know what I uttered when I found the live capacitor as the enormous bang obliterated anything I may have said but at least I'm still here.
    We had old fashioned earthing wands about a metre long. Modern ones seem to be far shorter.

    N2EY likes this.
  10. SM0AOM

    SM0AOM Ham Member QRZ Page

    I would say that you had "a close shave".

    What I can recall there were no incidents at least with the HF transmitter crews I was involved with.

    The possible etymology of the "Jesus Stick" was told to me during a coffee break at a meeting at the Collins HQ in 1996 I think.

    We talked about the opened 208U-10A that was in a corner of the lab, and the engineer spoke of a fatal accident with a US Coast Guard transmitter in the 70s where the ground strap to an HV capacitor had broken off, leaving the capacitor "floating" and not discharged.

    A technician had reached into the PS compartment relying on the grounding switch interlock and got the full charge of 25 µF at 6000 V through him. He was violently thrown into the opposite wall and broke his neck.


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