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. AF6LJ

    AF6LJ Ham Member QRZ Page

    The SB-220 My Ex had didn't look like that at all.
    The caps were soldered to the tube pins and ground lugsunder the socket mounting screws. The leads were quite short. There also wasn't all that buss wire running around.....
     
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    I would also add that amplifier is a poor example of proper constructions practice.
     
  3. G0HZU

    G0HZU QRZ Member

    Assuming these amps were built as kits then I suppose there will be variability in the way the caps are fitted. If some have long leads on the caps and some short then it will means some amps will be more unstable than others in the 'usual' range of frequencies they go unstable.

    I found that amp (and one or two others) by googling images of the amp and the caps leads were quite long on all of them.

    I am spending far too much time looking at this issue :) and really should be doing other things but I've found Tom's web pages and also pages written by Rich and whilst there LOTS of good stuff on their pages I do think both are a bit dated and vague in their instability analysis and there's some things I don't agree with.

    It looks like Tom has an HP4396A/B VNA and this VNA would be my VNA of choice for analysing these amplifiers because it supports an external coupler and it is also capable of exporting S parameter data. I haven't used one of these old combo VNA/SA for many years but they are very versatile instruments. I can remember the company buying one of the A versions in the early 1990s and they were pretty awesome. The A had the CRT display and I can remember fighting to be the one to get to use it as much as possible!

    I can't remember but you might be able to export the S parameter data direct to floppy but otherwise you would have to do it the same way I do with my old HP8714B via the GPIB interface to a PC.

    From memory, the HP4396A (and B?) are a bit fiddly to set up even to export screen image plots to a floppy and somewhere I have a document outlining how to do it (doc must be 15 years old!)
     
    Last edited: Jan 29, 2012
  4. AG6K

    AG6K Guest

     Good points. However, trying to move grid resonance higher in freq. to hopefully widen the G-G's self-neutralizing area enough to matter is an exercise in futility since going with a grid bypass C that is small enough to substantially move the grid-resonance up in freq. - making it more VHF-stable, makes the amplifier lose too much gain at HF. . . Since we can't do anything to change C-fb, the only practical way to improve VHF stability is to additionally lower VHF- amplification by going with an even lower-Q VHF-suppressor -- and live with the reality that we are going to lose an additional 2% or so at 28MHz. (0.1db or 1/60 of 1 S-Unit)
    • Rich, ag6k
     
  5. G0HZU

    G0HZU QRZ Member

    Hi Rich
    I am still struggling a bit with the (commonly held?) theories about 'grid resonance' and it's impact on stability.

    We all know that instability is an issue if the internal grid and the chassis ground have (typically) inductance between them.
    But what I'm struggling with are your test methods and also Tom's test methods and also the subsequent conclusions about the results.

    However, I do have another question. How close is the cathode filament to the grid structure (gap size?) on a 3-500Z and what is the breakdown voltage for this gap? I know it's in a vacuum but I was wondering what the BDV would be (especially at RF).

    Jeremy
    G0HZU
     
    Last edited: Jan 29, 2012
  6. AG6K

    AG6K Guest

     RE: Autopsy of virtually-unused Eimac 3-500Z serial #s AMK096 and AMK085 (date-codes 8504) - which have bent filaments. Yesterday Helper Girl and I spent most of the day getting the water pump recall taken care of for her Prius and repairing the 8000v hi-pot tester, so we did not autopsy any tubes. However, we discussed trying to centrifuge the two Eimac tubes with bent filaments since they are otherwise in new condition. If the gas-test goes well today we are going to try centrifuging the tubes to straighten out their bent filaments. If we are able to get the Filament/grid BDV safely above 5000v by centrifuging them, we are going to try them at 7128KHz.
    • Rich, ag6k
     
    Last edited by a moderator: Jan 29, 2012
  7. AG6K

    AG6K Guest

     Photos of virtually-unused Eimac 3-500Zs with bent filaments:
    DSC_0061.jpg DSC_0063.jpg
    • Rich, ag6k
     
    Last edited by a moderator: Jan 29, 2012
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    I'm looking forward to hearing the results.
     
  9. AG6K

    AG6K Guest

     However Catwoman, in order to get to the promised land with a useful 3-500 in hand, the light from the hi-pot's fil/grid arc has to be visible between the fins in the anode cooler and the 27-yr old seals have to be wonderful.
     
  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    Stands to reason...
    Besides without an arc how is one to know which way to orientate the tube in the centrifuge which the Hi-Pot will also tell you.
     
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