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. W8JI

    W8JI Ham Member QRZ Page

    When an off-center or off-balance object is spun on axis, it gets worse.

    When placed in a centrifuge, you'd have to know which way something was off and how much to spin it. I think more than likely, if a tube ever really does get fixed, there is just something loose in the tube (like a broken piece of grid) that gets dislodged.

    Most case, you can just gently tap on tubes and make the shorts come and go.
  2. AG6K

    AG6K Guest

     Correct. The tube is held by rubber bands in a padded cradle with the filament bulge toward the axis of the centrifuge. The cradle is fixed the to the arm of the centrifuge. When the arm spins, G-force pushes the filament bulge the other way to straighten it out

     Which is the trickiest part of the process Tom. Since the anode obstructs the view, it takes someone with extraordinary low-light vision to determine the direction of the indirect light coming from the arc between the filament and the grid. . . Since graphite-anode 3-500s do not have hollow cooling fins that allows one to view the grid & filament, we have never attempted to straighten the bent filament in one.

     The molybdenum grid is tough. We have never damaged a grid at 11G.

     True, but in order to be useful there needs to be at least 5000v of fil/grid clearance -and that means one or more trips to the centrifuge.
    • Rich, ag6k
  3. G0HZU

    G0HZU QRZ Member

    Apologies in advance if this suggestion is wacky but could you fit current sensors in line with each of the three grid pins and capture the (arc discharge) current pulses (on a 3 channel DSO?) and maybe 'DF' where it arced based on the current sharing between the three grid pin results?

    You wouldn't need to see inside the tube with this test?

    You would be looking for very tiny differences but you could maybe get a reading with the right test setup.
    Last edited: Feb 1, 2012
  4. G0HZU

    G0HZU QRZ Member

    I guess the above method would get less accurate the higher up the grid it arced. The other wacky method would be to see if you could stick external sensors around the tube exterior and see if there is an assymetry in any external field set up by the arc. But again, the difference would be very tiny and hard to quantify. (hard because the grid acts a bit like a Faraday cage?)
    Last edited: Feb 1, 2012
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    I don't think that would work since the grid is terminated in a circular peace of stamped metal the pins are welded to.
  6. AG6K

    AG6K Guest

     Good point Sue. Another way to locate the bulge is the gravity-utilitizing technique invented by Jennings for checking the concentricity of the Cu plates in vacuum variable capacitors: Connect the C in parallel with the L of a variable frequency oscillator / freq.-counter, with the tuning shaft horizontal. Rotate the C. When the freq. reading is lowest, the bulge side is down --- or when the freq. reading is highest, the bulge side is up. . . Jo E. Jennings, W6EGV, was definitely a sharp guy.
    •• Rich, ag6k
  7. G0HZU

    G0HZU QRZ Member

    Oh I see. I don't really know much about the internal construction so maybe that was a bad idea...

    I suppose you could use the Jennings method in the centrifuge if you fitted a JFET osc to a socket that used the tube filament/grid as part of the tuned circuit.

    Then run it with a tiny battery and read the frequency on a nearby spectrum analyser (or maybe even look at FM on a demod meter) ?

    Run the centrifuge at medium speed and try various rotation angles of the tube and pick the one that gives the biggest expected shift in freq.
    Then turn up the angular velocity to get max g

    You might even be able to monitor progress with this method? :)
  8. AG6K

    AG6K Guest

     The tube has to be at rest to determine the filament/grid spacing.

    Last edited by a moderator: Feb 2, 2012
  9. VK4TUX

    VK4TUX Ham Member QRZ Page

    A system using an electronic balance system similar in principle to the method tyres are balanced could be used.
    If a identical new tube is used in in a tight pin socket, and then balanced with weights on the socket plate for perfect balance spun on the centre axis of the filament helix then this could provide the test platform.

    Once the perfectly weighted socket is setup, the suspect tube is fitted in place of the new perfect tube, and the electronic balance machine weight data/location required for balance would tell you what you need to know.

    Adrian ... vk4tux
  10. AG6K

    AG6K Guest

     A clever method - but one would likely need a milligram-resolution scale to do it Adrian. Maybe an x-ray machine would do the job?
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