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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

    Can you explain why it has to be at rest?

    The Jennings method would attempt to tell you which orientation make the gap worse. You aren't trying to determine the actual spacing with this test. i.e. you are just finding the correct orientation for the centrifuge.

    However, by slowing down and speeding up you could see if there was a permanent shift in frequency indicating that something had permanently 'moved'

    I guess you have used the centrifuge successfully but I'm nonetheless a bit surprised it is able to straighten the filament. You won't get much force with 11g on such a small structure (a few grams in mass?)

    Are the filaments quite pliable?
    Last edited: Feb 2, 2012
  2. AG6K

    AG6K Guest

     Since the filament is a double-helix suspended from the ends, my guess is that the grid/filament spacing is affected by 11G-lateral.

     Correct. Once the direction of the bend is determined, there is no reason to backtrack. At this point I'm thinking X-ray for finding the direction of the bend.

     at 1900ºK and 11G most metals can be unbent.

     Sometimes we overshot the mark and had to do a 180 in the centrifuge to bend it back to straight.

     At 1900ºK they can be bent but I would never rate Th-W as pliable since W alloys are apparently pretty tough.
  3. AG6K

    AG6K Guest

     I'm presently seeking an out-of-warranty 3-500ZG with a bent filament (filament-grid short) to see if I can determine the direction of the bend with an x-ray machine and possibly straighten the bent filament with a centrifuge.
  4. AG6K

    AG6K Guest

     Bad news: We 3 could not see which way the filament is bent in either 3-500Z s/n AMK 096 of s/n AMK 085. I subsequently measured the filament/grid C at c. 12pf in each tube. Since normal filament/grid C is c. 8.5pF, 12pF confirms that the filament helices are not in the center of the grid cage. Running these tubes in a factory-stock SB-220 or TL-922 would be risky since the filaments get even closer to the grounded-grids when they are hot, increasing the chance of a filament to grounded-grid short also shorting out the+110v PS - which would burn out unfused T-2, the combo filament- xfmr/+110vdc PS-xfmr. The next step is to try running the tubes with bent filaments in a SB-220 that has been modified from the original V-cutoff bias to R-cutoff bias.
  5. AG6K

    AG6K Guest

     UPDATE: The X-ray image was sharp enough but the view of the bent 3-500Z filament was blocked by the shadow of the chrome-plated brass sleeve over the anode lead. The set screw for the brass sleeve is not a Torx, not an Allen and not a Bristol. Does anyone know where to buy a wrench that will fit? Tnx
  6. AF6LJ

    AF6LJ Ham Member QRZ Page

    I would think it would have to be one of the above...
    Has anybody looked at the socket (set screw head) with a powerful magnifying glass?
  7. N3JBH

    N3JBH Ham Member QRZ Page

    Rich there is several types of tools used to drive various bolt's , screws etc. Could you post a picture of it and possibly get some measurement's?
  8. AG6K

    AG6K Guest

     it takes a Bristol wrench. I have the right size.
  9. AG6K

    AG6K Guest

     X-ray of a 3-500Z with filament to grid short: 3-500_x-ray.jpg
    Last edited by a moderator: Feb 24, 2012
  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    You can actually see it....
    Glad you found out what wrench was needed to remove the anode sleeve.
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