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. VK4TUX

    VK4TUX Ham Member QRZ Page

    Directly strapping to the chassis seems to provide better gain > amplication of the input signal, and a noticable smoother tuning process.

    I dont dispute the strap inductance, however does your model show this additional gain achieved?

    Adrian... vk4tux
     
  2. VK4TUX

    VK4TUX Ham Member QRZ Page

    3-500zg_fil_drp.jpg
    Looking at the base it seems the centre pin filament support is crimped to one side cathode electrode input and has slipped down through the crimp ?

    Adrian ... vk4tux
     
  3. W8JI

    W8JI Ham Member QRZ Page

    They are welded, not crimped. That should never happen, so unless the tube was subject to many dozens or hundreds of G's from a drop on pins which would have shattered the grid, glass, and everything else, or unless the weld was pried on and broken after the tube was apart, it was a poor weld.
     
  4. AG6K

    AG6K Guest

     True enough, but at 3.5MHz more grid bypass C is needed.

     why is a dipmeter measurement of freq. flawed?

     What's more direct than a dipmeter -- and what could br less invasive?

     Thorium (Th) is mildly radioactive. Th can be alloyed with U233 to generate electricity.
    • Rich, ag6k
     
    Last edited by a moderator: Jan 25, 2012
  5. G0HZU

    G0HZU QRZ Member

    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 suppose you can call this 'grid resonance' but to me you are just measuring a dip that can be affected by factors external to the tube. So I'm not sure you can attribute this dip to any kind of internal 'grid resonance'.
     
  6. AF6LJ

    AF6LJ Ham Member QRZ Page



    You won't be able to couple enough energy into the grid itself and into the tube by coupling through a copper strap between the grid connections and ground.
     
  7. AG6K

    AG6K Guest

     Valid points but the affect will be minimal. The anode to gnd path is 5pF in series with C-Tune's 30 to 350pF, which varies C from 4.3pF to 4.9pF. which moves the grid resonance about 4%. As for the cathode path to chassis gnd, the relatively larger values of C2 in in the input Pi-network would probably not much move grid resonance from band to band.

    QUOTE]
    I suppose you can call this 'grid resonance' but to me you are just measuring a dip that can be affected by factors external to the tube. So I'm not sure you can attribute this dip to any kind of internal 'grid resonance'.[/QUOTE]

     No matter which contributes how much on which band, grid-resonance is important in a g-g amp because below this frequency the g-g circuit is self-neutralizing --- but above this frequency all bets are off since there is no method to achieve stabilization other than to artificially decrease VHF gain with a L/R VHF amplification suppressor. .
    • Rich, ag6k
     
  8. AG6K

    AG6K Guest

     I see a sharp dip c. 90MHz on 3-400Z and 3-500Z grids. Carl H., KM1H says 811As and 572B grids dip c. 75MHz.
    • Rich, ag6k
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    Yes but...
    Do you actually see a sharp dip by coupling to the ground strap from the grid pin of a 3-500Z to ground?
    I don't think so.
     
  10. AG6K

    AG6K Guest

     Yes Catwoman. . . . It has L and C. Do 1/4 wave verticals resonante ?
    • Rich, ag8k
     
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