3-500Z gassing?

Discussion in 'Amateur Radio Amplifiers' started by K6BSU, Feb 3, 2018.

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  1. AI3V

    AI3V Ham Member QRZ Page

    At various times in my professional life I have worked either making vacuums all day, or making the equipment to make vacuums, and the various gauges to test high vacuum.

    Just about everything in the Sammy hammy world about "gettering" tubes is rubbish.

    Gettering is about "cleaning up" the all most perfect vacuum when a tube is made, not some kind of miracle sponge to soak up leaks, that can somehow be rejuvenated by random heating.

    It's really a fascinating study in crowd behavior.

    You might notice that you never read a post that describes the before and after idle current, tube gain, or output power when stations describe with great enthusiasm how they go about gettering.

    Especially considering current and vacuum quality are directly related.

    Rege
     
    VK6APZ, KP4SX and K6CLS like this.
  2. WB2GCR

    WB2GCR Ham Member QRZ Page

    I think that's a bit harsh.

    Probably what we're talking about is either plugging in a tube that has sat around for
    a long time and hitting it immediately with HV shortly after the filaments have warmed up
    and see what happens, or alternately doing something that at least allegedly will remove
    residual gas from the tube...

    Since iirc, Eimac recommends some things in this regard and there have been numerous
    articles published in well regarded publications, it seems likely that there is something to this.

    The final step in most tube mfr is to fire the getter, and in the case of some tubes that consists
    of heating the plate itself to a red hot or higher temperature. Tubes do out-gas internally over
    time, so we're not talking about metal-glass seals leaking in all instances.

    The real question is where is the "extra gas" coming from and what to do about it?
    Throw the tube away without at least trying?

    Dunno...
     
  3. K6BSU

    K6BSU Ham Member QRZ Page

    The universal problem with most of the suggestions is: Run the amp at high power and get the plate some sort of redness. Others advise using a separate special testing rig just to dissipate power for degassing.

    So, without building a special test rig, the original amp must be used. But, some caution that a gassy tube will suddenly draw tons of power and wipe out the remainder of the amp.

    I'm not in a mood to rebuild my amp after a bad tube destroys it.
     
  4. G0GSR

    G0GSR Ham Member QRZ Page

    Yes, I'm sure there is.

    Some years ago, I had to replace the valves in 6x 1kW Marconi FM transmitters, each using a pair of QY4-400.
    When first powered with the new valves, 4 out of the 6 suffered internal flash-overs within the 4-400's.
    After the first "splat"all were subsequently OK, protected by the TX's fast over-current trip.
    Amateur amps don't seem to have the luxury of a proper, anode over-current trip.

    TV amplifier klystrons, 10-40kW, we had a dedicated "gas tester" which powered the filament and applied reduced voltages to the other electrodes. This device had a "gas current" meter which visibly dropped as the tubes conditioned.

    Higher power valves (500kW) were subjected to "spot knocking" by applying an anode voltage far in excess of the norm, limited to a very small current so that the flash-overs caused no damage. Over time, the flash-overs reduced or stopped altogether and the valve was then ready for service.

    We had a few 1o and 20kW Siemens TV transmitters. The manufacturer supplied a dedicated valve conditioning unit in which the spare valve was operated for a specified number of hours prior to putting into service.

    Many other large valves, we took no precautions or did no "conditioning" and they just worked.

    So, Yes, there is something to this conditioning business for some types of valves at least.

    Frank
     
    KD2ACO likes this.
  5. AI3V

    AI3V Ham Member QRZ Page

    My experience with high power tubes, including 40kw hf tubes, aircraft carrier radar tubes, (peak power up to one million watts)and intelsat earth station 6kw 6ghz twt's is that we did exactly nothing to "getter" any of them.

    Put them in the transmitter, let the filaments warm up and get on the air.


    Rege
     
    K8PG and VK6APZ like this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A bad tube can't destroy your amplifier if you have a glitch resistor between the power supply and the tube anode.

    The resistor sacrifices itself and opens the circuit. You may lose a $3 resistor, but you don't lose anything else.

    Re conditioning and gettering and such: Very high power tubes mostly have indirectly heated cathodes, and are external anode tubes that are mostly metal-ceramic and don't have any glass components.

    Huge difference between those and a 3-500Z, which is a cheap, directly-heated, glass tube having its gettering on the internal anode.
     
    WA7PRC likes this.
  7. G3YRO

    G3YRO Ham Member QRZ Page

    What sort of resistor would you use for a pair of 3-500s?

    Obviously, you don't want something that can handle high current surges.


    Roger G3YRO
     
  8. W1BR

    W1BR Premium Subscriber QRZ Page

  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    The concept is to limit the short circuit current to a value that can be handled, until the mains fuse/breaker opens. Tom W8JI discusses that here: http://www.w8ji.com/fault_protection.htm
     
  10. K6BSU

    K6BSU Ham Member QRZ Page

    I guess I was worrying for nothing. My 3-500Z amp has been silent for over 20 years. After blowing out the dust and cobwebs, and changing the AC input to two power cords and shack wiring to two outlet boxes from a 240V service, the amp fired right up. Momentarily drove it to 800W, but backed down to 600W, and worked an east coast stn. on 20M.

    That Eimac 3-500 probably dates from before 1980.
     

    Attached Files:

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