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. G0HZU

    G0HZU QRZ Member

    Hi Sue
    Thanks for the info :)
  2. AG6K

    AG6K Guest

     In a factory-stock SB-220 or TL-922 there's 600pF from the grid and the grid-choke to gnd. At 120MHz, 600pF has 2.4Ω of XC. This means there would be 120v/2.4Ω = 50Amperes through the capacitors @120MHz. What's wrong with this picture?
    Rich, ag6k.

  3. AG6K

    AG6K Guest

     Correctomundo. A floating I-beam resonates at a half-wave, not at a quarter-wave - so the dipper needs to have a low-C output connection through a few pF cap in order to measure the dip at the end - or one could measure the dip with a plug-in coil at the center of the I-beam.
    • Rich, ag6k
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    I know that :)

    I'll have more information on the amplifier tomorrow morning.
    I should have answers for the questions up by ten or so if not before.

    As for the floating I beam;
    If an I beam is resonating and there is nobody around to measure it,...
    is it really resonant. ;)
    Have a good day Rich.
  5. G0HZU

    G0HZU QRZ Member

    The picture taker did a simple low frequency analysis that summed three 200pF capacitors together and assumed the same analysis applied to real components and layout at VHF?
  6. W8JI

    W8JI Ham Member QRZ Page

    That's absolutely correct, and the picture taker does that with everything. Over simplifying the system and ignoring limits, like emission current limits, allows some very wild and impossible theories like bending elements from oscillations.

    While the multilayer stack inside the capacitor case is pretty broad in bandwidth, the leads and multiple paths add another dimension.

    73 Tom
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    To answer people's questions regarding the SB-220.....
    The grids of said 3-500Zs are connected to ground through 27 Ohm quarter watt resistors which are bypassed to ground.
    The amplifier has a Harbach keying interface board, soft start, and high voltage board.
    The caps are original.
    The tube shorted from the sudden stop not from the initial act of moving the amplifier.
    Sorry Rich I forgot to ask about the tube but I shall do that in the morning....
    And I forgot to ask which tube it was....
    The only items that were replaced were the tube and the twenty seven ohm resistor.
  8. AG6K

    AG6K Guest

     I did not compose the picture. The three 200pF capacitors are not in parallel?
    • Rich, AG6K
  9. AG6K

    AG6K Guest

     The osc. does not bend the filament/cathode, it's the accompanying D.C. current pulse. .

     Where's the error Tom?
    • Rich, ag6k.
  10. G0HZU

    G0HZU QRZ Member

    If you look at how long the 200p cap leads are (were) in this SB-220 they are over an inch long.


    I was quite shocked at how long and skinny the cap connections were. So there could be 30nH in the cap leads? This inductance has 23 ohms
    reactance at 120MHz. The 200pF part of the cap has about -7 ohms reactance so the net reactance of each leaded 200pF cap is about 16 ohms (inductive) at

    There's even more inductance in series with each cap if you include the socket tabs and the socket pins and the tube internal grid wire.
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