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3-500z filament voltage Too high

Discussion in 'Amateur Radio Amplifiers' started by N4FMG, Feb 27, 2012.

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  1. AG6K

    AG6K Guest

     The XL of a 100Ω WW resistor is considerable. even at 1.8MHz.

  2. G0HZU

    G0HZU QRZ Member

    Doesn't it depend on the way the 100R-1000R WW resistor is constructed?

    I would expect that some WW resistors that are ballpark 100R to 560R would only exhibit a few hundred ohms inductive reactance at 3.5MHz.

    Such a resistor would form a divider with the 1nF cap and this would leak RF into the detector (not good) and the resistor would also get hot. I would avoid WW resistors unless proven OK on a VNA at the frequency of use and even then I'd much rather use a non WW resistor technology.
  3. W1BR

    W1BR Premium Subscriber QRZ Page

    And it continues.... If the new Chinese tubes require 5.25 volts to meet spec, why is this horse still being beaten to death?
    Does anyone have concrete measurement data to refute JI's data?? Last I looked, the supply of NOS Eimac 3-500Z
    tubes has long been depleted. How many 3-500Z failures have been linked to OPEN filaments?

  4. AG6K

    AG6K Guest

     In all of the filament-grid shorted 3-400Z and 3-500Z tubes that I've autopsied I have only found one tube that had a broken filament Pete.
  5. G0HZU

    G0HZU QRZ Member

    Slightly off topic but I would suggest that a typical SMD resistor (eg 0603 or 0805) in the range of a few hundred ohms would appear non inductive at VHF/UHF frequencies.
  6. AG6K

    AG6K Guest

     The MOF resistors we use in our suppressor retrofit kits are 20mm long. At 150MHz they have c. 11Ωs of reactance.
    Last edited by a moderator: Mar 6, 2012
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    Wouldn't that be compensated for by adjusting C2 on the input tuned network for the cathode?
    Even at 30 MHz C2 is usually at least 110 pF for a Q of 1... I would think internal stray capacitance would
    have minimal effect, while any inductance would only increase the impedance above the R value?

  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    If you use carbon, or non-inductive carbon film resistors it's not a problem.
  9. W1BR

    W1BR Premium Subscriber QRZ Page

    Hi Sue

    I guess the question is "how much capacitance"?

    Grid to Fiament C between diferent 3-500Z tube varies. Is it really an issue?
    I'd suspect the internal cap of the tube, combined with the C2 pi-net cap, would
    swamp any residual C value in the resistor.

    Are starting to split hairs, like the filament voltage debate... :)

  10. VK4TUX

    VK4TUX Ham Member QRZ Page

    Carl, KM1H, has a suppressor design that cancels resistor inductance, and quite clever too.

    Adrian ... vk4tux
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