3-500Z Home Brew problem almost solved and I need ideas.

Discussion in 'Amateur Radio Amplifiers' started by KM6CND, Jul 21, 2018.

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  1. KM6CND

    KM6CND Ham Member QRZ Page

    I was looking at the meter the whole time. Can't say. That was the last band the amp actually worked on. BTW, I went from key up to collapse of the cross needles in less than 10 seconds.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Watch everything when that happens, and there's no reason to "STOP" just because the output evidently falls to zero.

    A 3-500Z is a very rugged tube, and it can dissipate a kilowatt internally for some time before you damage it, provided the grid current doesn't soar. Its "500W plate dissipation" rating is based on continuous transmission.

    Repeat the test (be sure that dummy load can handle about a kilowatt for some period of time -- you didn't mention which model it was) and if the output falls to zero, report back what the HV and plate current indicate when that happens.

    Tube drawing normal current and operating at normal voltage, and producing no measurable output power means the power is all being dissipated in the tube and maybe a few lossy, resistive components. Can be an indication of an open connection anywhere in the plate tank circuit (inductor, bandswitch, etc) or an open or bad T/R relay contact -- or other things. But usually not difficult to figure out what happened.
  3. KM6CND

    KM6CND Ham Member QRZ Page

    I'll have a report later today. MFJ-267 is the dummy load.
  4. K6BRN

    K6BRN XML Subscriber QRZ Page


    Be careful. The MFJ-267 dummy is rated at 1500 watts for only 10 seconds and this is probably an optimistic rating by MFJ. As far as I can tell its based on a simple air cooled carbon rod/resistor. This type of dummy load is easily overheated and damaged or destroyed. So if you are pumping 500 watts into it, it may be good for 30 seconds, then need a cool down period of several minutes.

    Brian - K6BRN
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    It comes with a derating curve. A single 3-500Z can output about 1 kW if the power supply is good, and the MFJ-267 is rated 1 kW for 20 seconds.

    Longer than that and I suspect it will become two MFJ-133.5s.:):p
  6. KM6CND

    KM6CND Ham Member QRZ Page

    Here's the latest...

    I set up for a new test.

    • FT-DX1200 on 80M, 3900.00, CW, 5 watts
    • Band switch on 80
    • Caps nulled
    I key up and the cross needles are flat, as I turn the top dial, I see activity. I continue to turn the dial and then I hear arching! It was coming from somewhere near the coil.

    Time for a critical exam

    See that 1/4 inch nut, holding the "Curily Q"? That one braided cable was loose and arching. Fired up the gun and fixed it. You can't see it, but the next two clips on the coil were almost touching, another possible arch area.

    I started the test again and no more arching!

    I can't make sense of the high scale on the MFJ dummy... on the low scale with 5 watts in, the needles are off the charts. On the high scale,

    I see 15 watts in and "20" (200 watts) on the high scale.

    Cathode - 20
    Plate - 15

    30 in - 300 out

    Cathode - 22
    Plate - 18

    Any additional drive has no effect.

    Making progress :)
  7. KM6CND

    KM6CND Ham Member QRZ Page

    Just moved to 40 to run the test again. So far...

    5 in, 90 out

    Now what was that tune up phrase? Peak this and Dip that?
  8. KM6CND

    KM6CND Ham Member QRZ Page

    Spoke too soon on the arching.

    I moved to 20 tuned up on low power, moved to 40 watts and the very end blade of the top cap arched. Time to shut it down.
  9. KM6CND

    KM6CND Ham Member QRZ Page

    OK Ham Bros, what's the cause of the arching in the air capacitor?
  10. WG7X

    WG7X Premium Subscriber QRZ Page

    Not to be an arch enemy, but the term you're looking for is arc as in:

    An electric arc, or arc discharge, is an electrical breakdown of a gas that produces an ongoing electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. Wikipedia

    Now back to your trouble shooting...

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