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3-500 Tube Life

Discussion in 'Amateur Radio Amplifiers' started by VK4TUX, Dec 2, 2011.

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  1. AG6K

    AG6K Guest

     Bad spotwelds were a mfg defect with Eimac 3-500Zs in the 1990s. The problem was not using enough current to get a melt. Since I had a taken a college course that covered spotwelding I was simply amazed that Eimac-Salt Lake City dropped the ball on something as simle as this.

     Bad soldering of one of the tube's filament pins is more common mfg defect - but that's an easy fix with rosin flux, solder-wick, and Ag-Sn solder.

     The Chinese tubes I have seen had sound spotwelding and excellent vacuums. The early problem with the glass recipe's D-factor have been corrected. . . Chinese Gigavac vacuum relays have so much OC withstanding V ability that they sometimes leak I externally before they leak I internally across open contacts. Kilovac and Jennings vacuum relays don't have vacuums that hard.
    cheers Susan
    • Rich, ag6k
  2. W1BR

    W1BR Premium Subscriber QRZ Page


    I'll play devil's advocate here :)

    At this point I question whether the Eimac 3-500 data sheets are still valid, except for any remaining new, old stock
    tubes that are still in existence. Where are the data sheets for the new Chinese tubes, or other manufacturers?
    Looking the IMD/saturated power data for a sampling of several different manufacturers 3-500Z tubes on Tom's
    website shows a variation of a minimum of 5.25 volts for Chinese tubes down to 4.5 volts for Amperex tubes. The
    Chinese samples suggest the premise that the QST article was based on is flawed, while the Amperex tube sample data
    might support it. :)

    Last edited: Dec 24, 2011
  3. W8JI

    W8JI Ham Member QRZ Page

    Anyone with common sense would measure distortion before setting voltage abnormally low for the particular tube they actually have in their hands. It would have to be tested on the worse case band, because peak emission demand varies with the band.

    This would include Amperex and any other manufacturer, because emission changes from tube-to-tube, and over the life of the tube.

    The bottom line is this quote repeated directly, word-for-word, from Eimac's notes on filament voltage life:

    That answers it all.

    73 Tom
  4. AA5CT

    AA5CT Ham Member QRZ Page

    "At this point I question whether the Eimac 3-500 data sheets are still valid, "

    Then we would have to go back to the early half in the 1900's where the 'research' was down with Thoriated Tungsten et al to get a 'feel' for the technology:

    Thermionic emission, by Arnold L. Reimann ... Main Author: Reimann, Arnold Lührs, 1898-
    Language(s): English
    Published: New York, J. Wiley & sons, inc., 1934.
    Note: Printed in Great Britain.

    Viewable page by page here:;size=100;view=image;page=root;seq=1

    Note: The Thorium only exists (according to texts) about an atom thick on the surface of the Tungsten, and there is a square law relationship between Temperature (T squared) and emissivity (available amps/cm^2). Boil off the Thorium too soon (I have done this accidentally to a pair of 254W tubes) and lose substantial output capability from the tube, unless more Thorium can be 'brought to the surface' (actually done during manufacture!) from within the 1% Thorium/99% Tungsten metal of the directly heated filament ...

    Jim de WB5WPA
  5. AG6K

    AG6K Guest

     "All"? According to "Care and Feeding of Power Grid Tubes" one can measure any of three things in determining where to set Th-W filament-V. Page 163: "This is best done by measuring some important parameter of performance such as anode-current, power output, or distortion,"

     It does not answer the question: Why isn't this information in Section 6.11 Tube Life in Eimac's book "Care and Feeding of Power Grid Tubes"?

    "Faced with the choice between changing one's mind and proving that there is no need to do so, almost everyone gets busy on the proof. "
    - John Kenneth Galbraith -

    • Rich, ag6k
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