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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

     Jim: Measuring absolute true RMS filament V is not needed. What is needed is: 1. something that will indicate a 2% increase in filament V above the point where relative P out just begins to decrease. 2. a means of adjusting filament V. . . . However, amplifier mfgs have a negative incentive to have rheostat-adjustable filament-V because allowing the user to optimize emissive life is going to reduce replacement tube sales.

    .
     The most common cause of 3-400Z and 3-500Z tube failure that I have seen in the last 40-yrs is filament to grid shorts from a sudden pulse of EMF.
    3-500.f-g.short.JPG
    QST Magazine Photograph, September, 1990, p.15
    This condition usually comes with a flash of light / big-bang - the energy for which comes from that stored in the filter caps. .. In my opinion, intermittent VHF parasitic oscillation is the cause. Avoiding this problem is not difficult: 1. Retrofit a glitch R in the HV+ to limit peak discharge-I from the filter caps to <200A and 2. decrease VHF amplification by lowering the Q of the VHF parasite suppressor(s).

     The worst quality 3-500Zs and 8877s I have ever tested/autopsied were made by Eimac in Salt Lake City in the 1990s.

     When Louis Pasteur suggested that surgeons in Paris hospitals should scrub before cutting he was ridiculed.

    .
     Amperex made pretty good, pretty high Mu 3-400Zs and 3-500Zs. One of the Amperex tubes in my SB-220 was made in 1967. However, Amperex 3-500Zs are no less subject to bent filament damage from VHF oscillation than any other brand of tubes so you still have to retrofit a glitch-R and lower the Q of the VHF suppressors unless you have a free supply of new tubes.

    It is my opinion that the people who design amplifiers are not infallible.


     
  2. W1BR

    W1BR Premium Subscriber QRZ Page

    QUOTE Jim: Measuring absolute true RMS filament V is not needed. What is needed is: 1. something that will indicate a 2% increase in filament V above the point where relative P out just begins to decrease. 2. a means of adjusting filament V. . . . However, amplifier mfgs have a negative incentive to have rheostat-adjustable filament-V because allowing the user to optimize emissive life is going to reduce replacement tube sales.

    That certainly makes more sense than the recent ARRL QST article that suggested blindly setting the filament voltage
    to the lower end of the recommended operating range using improper, and dangerous, procedures and equipment,
    and without verification of the end results.
     
  3. AG6K

    AG6K Guest

     Setting Th-W filament-V too low can not permanently damage emission. Restoring emission involves operating the filament at normal V for a brief period. However, with an oxide-cathode tube, running the heater-V too low can strip off the barium and strontium oxide emissive material.
     
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