3-500Z Filament To Grid Shorts, The Other Cause

Discussion in 'Amateur Radio Amplifiers' started by AF6LJ, Jan 22, 2012.

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  1. AF6LJ

    AF6LJ Ham Member QRZ Page

    yes dear I know the strap is self resonant and you will see it if your dip meter will go high enough.
    What I was saying....
    in response to

    [​IMG] Originally Posted by G0HZU [​IMG]
    I think that by putting the dip meter at a copper grid strap you will be measuring for a resonant dip that will be affected by whatever reactance is in the anode path to chassis and also whatever is in the cathode path to chassis.
    I don't think enough energy can be coupled into that strap to see any other resonance other than the strap.

    I forgot....
    Yes quarter wave verticals stubs or an I beam falling through the air will resonate. :)
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    I'm sure you do.
    I don't expect to see that resonance by coupling into the ground strap grounding the grid in question....
  3. AG6K

    AG6K Guest

     Anyone with a dipmeter and a little curiosity can see it. However, there are other dips there between the three ground straps, so temporarily disconnecting all but one ground strap will narrow things down to the grid itself. In a G-G amplifier, grid resonance is an important thing to know because it's above this frequency where squirrels can become legion -- provided that the tubes have enough gain in that region.
    • Rich, ag6k
  4. W8JI

    W8JI Ham Member QRZ Page

    Anyone can find dips and spurious resonances all over the place in any multiband HF PA.

    NONE of that means it will have parasitic, and it is easy to show there is not enough energy in a parasitic to damage a filament. It can only damage a grid or anode if the oscillation occurs over a long enough steady time to allow heating.

    That's the way things behave in the real world.

    73 Tom
  5. AG6K

    AG6K Guest

     The grid grounding conductor is the only place where one can see the grid resonance dip.
    Rich, ag6k
  6. AF6LJ

    AF6LJ Ham Member QRZ Page

    I would like to see how that is done sometime....
    Although I suppose it's much easier with a stock SB-220 or TL-922.
  7. AG6K

    AG6K Guest

     Since AC current exerts zero net EMF on conductors, it is not RF (AC) energy that pushes the 1800ºK, 3-500Z filament sideways, it is a brief burst of almost unbelievably high DC current. The high burst of DC grid-I – all of which comes from the filter-C through the cathode/filament, so the filament is subjected to high EMF. The condition that allows a through the roof burst of grid-I with a LPF tank is a VHF oscillation.
    • So how do we know that the grid-I was sky high during big-bang events? Hmmm. . . Grid chokes that are wound with #30 Cu wire melt Cu and burn out. . . . . What is the fusing-I rating of #30 Cu?
    • How do we know VHF energy was passing through the neighborhood? Hmmm . . . VHF-suppressor R-supps that are virtually shorted out at HF by 0.06µH or so get zapped during big-bang events.
    • Rocket science this is not boys and girl.

    • Grids and anodes in 3-500s are not what gets damaged Tom. The filament is already hot and it's the filament that gets bent/damaged by the not small EMF from a current-source that we know has a >1kA-peak I capability with modern low ESR filter caps.

     In the "real world" does AC or DC exert a net EMF?
    • Rich, ag6k
  8. G0HZU

    G0HZU QRZ Member

    Remember that I've never been near one of these amplifiers but even a basic analysis will show that if the amplifier oscillates strongly at VHF (i.e. lots of voltage swing at the anode) then there could be sufficient RF voltage across the grid choke to cause it to seriously overheat. This would be most likely if the resistor in the parasitic suppressor blew open circuit initially, then allowing LOTS of anode swing after the initial oscillation event that took out the suppressor resistor.

    I would expect the 1mH grid choke to look like a very small capacitor in series with a highish resistance that varies across VHF. So if that resistance was ballpark 500R at the relevant VHF frequency you could get quite a few watts dissipating in the grid choke.

    For example, if you had 120V rms across the choke and it looked like 2pF in series with 500R at 120MHz ,then the parallel equivalent resistance is 1400R meaning you get about 10W dissipated in the choke. That's going to kill it very quickly...

    The other thing is that the 200pF grid caps will be subjected to very high stress as well and I'd expect these could break down if the VHF hoot was at the worst case frequency for the caps.

    Do these caps ever show signs of damage or changes in characteristics over time?
    Last edited: Jan 27, 2012
  9. G0HZU

    G0HZU QRZ Member

    What would worry me would be what happens once the grid caps or the grid choke blow open circuit.

    I don't know enough about tube theory but isn't there then the case for something nasty to happen in terms of a destructive transient due to uncontrolled cathode to grid voltages?
  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    What happens when the grid circuit of a vacuum tube is open circuited it floats to a negative value and the tube is cut off. Without a DC return enough negative bias will develop to not only stop amplification but oscillation should also should be impossible.
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