magnetron question

Discussion in 'General Technical Questions and Answers' started by WA4ILH, Mar 20, 2019.

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  1. WA4ILH

    WA4ILH Subscriber QRZ Page

    When I was in the Navy, many years ago, most (all?) of our radars employed magnetrons. Most radars that I saw had timeout timers of several seconds which prevented the operator from placing the radar in the radiate mode until the filament had sufficient time to heat. How is it that microwave ovens can start producing energy almost instantly?
    Tom WA4ILH
     
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  2. W9KEY

    W9KEY Ham Member QRZ Page

    Actually, I'm pretty sure home microwaves do have a filament heater time function. Mine, when started, has a power-on sequence where the internal fan, light, and carousel start immediately, but the "grunt" of applied power is delayed by (perhaps) two seconds. I suppose this could be verified with a few simple neon bulb placed inside, but I always assumed that short delay provided time for the magnitron's filament to heat. Could be wrong, though.
     
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  3. WB5WPA

    WB5WPA Ham Member QRZ Page

    There must not be the big 'danger' of stripping off the oxide layer on the home oven maggy's cathode compared to the mil maggy (which is pounded with a really strong, narrow, low rep rate 'pulse' (compared to 60Hz) by a thyratron (in the old days) fired at the far end of the PFN) ...
     
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  4. K8ERV

    K8ERV Ham Member QRZ Page

    The early microwave ovens did pre-heat the filament. Required an Amperite time delay tube, and a separate filament transformer.

    Advance Transformer (made mostly fluorescent light ballasts) developed the present power supply that among other things combined the filament and plate transformers in one unit, so there could not be a filament pre-heat feature.

    The tube manufacturers would not warranty such use. The conventional wisdom said the filament surface would be damaged as with all xmitting tubes.

    Not knowing any better I did extensive life testing with the "instant on" circuit and proved it did not harm the tubes. Maybe the first time this was done. This allowed a great simplification of the power supply, reducing the weight, cost, and parts count. It also eventually made the 120v countertop oven practical. Advance Transformer deserves much credit for the success of the countertop oven. Their system is now used in all microwave ovens.

    Later I developed (along with Bud Anderson K8CRO (SK) the variable power cycler that cycled the tube on/off MANY more times. My goal was 100,000 cycles. The tubes still survived.

    It takes the magnetron about three seconds to heat up and start emitting power. Works.

    I don't know if this system has been tried/proven with other types of power tubes or not.

    TOM K8ERV Montrose Colo
     
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  5. K1LKP

    K1LKP Premium Subscriber QRZ Page

    HI TOM,
    THANK YOU FOR YOUR OUTSTANDING SERVICE
    IN THE NAVY........ BRAVO.........
    THREE CHEERS FOR THE
    KNIGHTS.
     
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  6. WB5WPA

    WB5WPA Ham Member QRZ Page

    re: "I don't know if this system has been tried/proven with other types of power tubes or not."

    I have an original Litton "Minute Master" uwave oven with the mechanical timer ... still in use, multiple times a day, to heat anything that needs heating or re-heating, like hours-old coffee still sitting in the brew-pot ... don't recall *any* special circuitry inside (this thing is pretty dumb inside) to 'stage' a warm-up cycle ...

    A tube using a direct-heat filament (like a 3-500Z with its Thoriated Tungsten filament) these tubes could be just a couple seconds (or a fraction of a second?) to bring the filament up to working temperature ... I have no knowledge of what is inside a uwave oven magnetron! :)

    Tubes like the 8877 (3CX1500) with an indirectly heated filament takes minutes to bring the oxide-coated cathode up to a temperature where electrons can be depended on to "emit" safely and not let parts of the oxide on the cathode (stripping oxide from the cathode) stream toward the plate ...
     
  7. W9KEY

    W9KEY Ham Member QRZ Page

    Thanks Tom, interesting post. I used to supply components to the Advanced Transformer group (for ballasts) - good people.

    But just to clarify - am I correctly reading above - that indeed the magnetron in a countertop oven takes about 3 seconds to begin producing power, but the delay is not specifically engineered into the circuit, rather it's just the time it takes the tube to heat up and begin operating (although I suppose that delay is due to filament heating time?). I know in mine you can hear something happen (power grunt) some short time after hitting Start.

    Funny - I remember those old Amperite time delay relay tubes.
     
  8. K6CLS

    K6CLS Ham Member QRZ Page

    I would put my Kill-a-Watt meter on the power cord, and watch the amps. Anyways on my microwave oven the carosel and fan start off fast and slow a little after a couple of seconds, maybe a little drop on the AC when that 1300 watt fires up.
     
  9. K8ERV

    K8ERV Ham Member QRZ Page

    KEY- you are correct. They sound is probably the magnetic field of the power transformer vibrating the steel wrapper.

    Did you know Fred Abrahams at Advance?

    TOM K8ERV Montrose Colo
     
  10. K8ERV

    K8ERV Ham Member QRZ Page

    The early magnetrons had cathodes and took as I recall 3 minutes to preheat.. Amperex 115NO180 delay tube. Later tubes used filament emitters.

    TOM K8ERV Montrose Colo
     
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