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

    One of the members of our weekend group on seventy five meters owns an SB-220, it works well and is lightly modified as he put it. The first weekend in January he had been doing some cleaning had every peace of gear in it's place save for the SB-220. One evening while in QSO while the ampli9fier was sitting idle he gave the amplifier a gentle push back to it's normal resting place and at that moment. BANG!!
    He shut down the amplifier and proceeded to troubleshoot it the next day.
    Besides the fact the grid shut was blown off the board one of the tubes had developed a filament to grid short.

    This brings up the question; how many of those tube failures are caused by moving or even bumping the amplifier while the filaments are hot?

    never know but given the conversations that have been raised on the subject I had to pass this along as just one more data-point in the discussion.

    I've always taken it for granted that you don't move an amplifier with hot directly heated cathode tubes in it and I have always rolled my eyes when I hear of people in the old days running those Heath compact KW amplifiers mobile.
     
  2. VK4TUX

    VK4TUX Ham Member QRZ Page

    Sue, What do you think happened first, the "bang"or the bent filament?

    A 50 ohm 50w ohmite glitch resistor would have saved the tube. A forward biased 6A10 across R3 would have been beneficial along with the usual
    meter diode protection etc etc.

    If you think a hot filament touching the grid during amp movement caused this event, then please tell me what you think caused the big bang?

    I imagine the grid choke and P-supp R are not too healthy either.


    Adrian ... vk4tux
     
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    First came the bent filament, the big bang was the grid resistor going up in smoke.
    The amplifier had a Harbach replacement board in it and had the meter protection.
     
  4. W8JI

    W8JI Ham Member QRZ Page

    I would say none, or nearly none.

    I regularly move hot-filament amplifiers, pick them up and inch or two and drop them, bang on tubes with a long insulated rod, and so on.

    Unless the tube already has an intermittent short or bad weld, nothing bad happens. If it does short, the only thing that happens is the plate current increases to the level of zero bias plate current. Why would anything else happen?

    As someone who regularly has hammered on tubes to locate intermittent shorts, it wouldn't bother me in the least to operate a tube mobile unless it was constant subjected to vibrations of significant magnitude. The filament doesn't get soft, weak, or mushy from being hot.

    Ask him what he had in the amplifier for filter cap size and ESR, and what surge limiting resistor he had.

    I see shorted grid to filaments dozens of times a year, and they never cause a bang. On the other hand an anode to grid discharge, without a suitable fault current limiting resistance, is known to cause damaged grids.

    And naturally this would come down to some thinking an oscillation can cause such an arc. Never mind common sense, that even if we drove the tube with 1000 volts positive on the grid, current could not exceed the emission limitation of the filament unless the tube had gas. :)

    73 Tom
     
  5. AG6K

    AG6K Ham Member QRZ Page

     RE: the SB220 :
    1. Was there a glitch-R in the +HV lead - and if so how many Ωs and Joules?
    2. was the shorted tube V2 or V1?
    3. was either grid RFC damaged?
    4. what was the measured resistance of the 47Ω R-supps?
    Would the owner like to have the shorted tube autopsied, photographed, and the JPGs posted? If so, I will pay the postage.

     When Karina A. and I were in the filament straightening business we found that straightening a bent 3-500 filament required 5.85v (1910ºK) on the filament and 30 to 40-seconds at 11G, so it seems that the force that bends 3-500 filaments is nothing trivial.

     Catwoman: As long as it's not an aerobatic stunt plane mobile there should be no problem since on a bench test I determined that one month operating a 3-500Z horizontal made little change in grid-filament BDV.
    • Here's a puzzler: TL-922s have a well deserved reputation for shorting the inboard tube while simultaneously blowing R-supp, collapsing or otherwise damaging the inboard tube's 1A grid RFC, and making a big band - all of which happens when the ZSAC drops to Zero.

    • Rich, ag6k


    "Almost nothing is as simple as it first appeard/" -- Mr. Murphy
     
  6. VK4TUX

    VK4TUX Ham Member QRZ Page

    Sue, What would the voltage be across the ~30 ohm grid resistor you think to make it cause the bang heard "going up in smoke". ~2.25 - 5.1v would be the voltage available from the cathode in contact.The amp is near zero biased so plate current increases, not sure if this would effect the IR = E across the grid resistor though? The filament is extremely difficult to bend at emission temperature.

    I have an open 3-500zg that had the glass broken in transit, and I did a check a month back tapping the base hard with cathode lit 5v @ 14amp . The grid was connected to a fluke 87 with record min/max set, there was no voltage recorded between grid and FT CT connection during the test which involved wacking the base mounted in a socket with a small mallet. I also had a Sampo 60mhz scope connected visually looking for spikes with display retain settings. Nothing seen.

    I seems something else happened to your friend besides his moving the amp, which may have initiated the event, but by causing a filament - grid short, I think not..


    Adrian ... vk4tux
     
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    I can answer some of this....

    The SB-220 in question was stock for the Harbach replacement HV diode board.
    The tubes were original tubes from the early seventies.
    As I remember that board has no provisions for a glitch resistor.

    Next time I talk I will ask him about the filter caps....
    The tubes in question had a lot of hours on them so it wouldn't but out of the relm of possibility for mechanical weakness to be the cause.

    The ham in question has an electronics background...

    So we shall see I'll ask a few more questions and get back to you.

    For the record;
    I've seen RCA conduction UHF power amplifier tuves shaken apart in high power (MST series) Motrack radios. and those tubes are metal ceramic with indirectly heated cathodes.
     
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    Catwoman :D
    I love it. :)

    I'll talk to him about the tube.
     
  9. WG7X

    WG7X Ham Member QRZ Page

    ...And its just that easy!

    Well... Speaking only in my semi-official capacity as a routine critter catcher, I can state unequivocally that your friend obviously, "accidentally", unlatched the critter catcher located somewhere in the lower bowel region of his amp, there by loosing the dreaded "banger cat" which then exited the amp at supersonic speed, causing the small "sonic boom" which was heard.

    The sonic boom in turn caused various wires and thingies to become discombobulated and making with the arcky-sparky and flashy-washy which then rendered the amp unfit for use.

    There!

    Simple, no?

    73 Gary

    PS: For the humor impaired::D
     
  10. W8JI

    W8JI Ham Member QRZ Page

    I thought it was because the movement made the amplifier cross the flux lines of the earth, generating a small current that triggered a parasitic.

    This can be cured by positing the amplifier so the front panel width runs north-south, or by contra-winding the suppressors.

    Or perhaps it was because the reflection of his diamond ring diverted a photon into the case, where it hit something in the tube and made it break into uncontrolled oscillation. This is why we should never wear rings when working on live circuits, especially in solar storms.
     
  11. N4BBQ

    N4BBQ Ham Member QRZ Page

    That was darn funny there Tom.
     
  12. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    What is a good value/wattage for a Glitch resistor on the SB-220 and Drake L7? I probably should install one of those while rebuilding the Power Supply. It just goes in series with the HV lead, correct?
     
  13. AF6LJ

    AF6LJ Ham Member QRZ Page

    Regardless of the cause;
    I don't believe the movement shock excited the amplifier into oscillation.

    The only other possibility I can see would be a loose grid wire touching the anode.

    From something I remember or think I do the grids in the amplifier are directly grounded.
    I will know more Saturday morning.

    If it was a stray grid wire the evidence will be inside the tube.
     
  14. AF6LJ

    AF6LJ Ham Member QRZ Page

    I always make a point of taking off my earrings while operating or working on electronic gear.
     
  15. W8JI

    W8JI Ham Member QRZ Page

    A good resistor value, that will make a significant difference, is 20 ohms.

    It has to be capable of handling shorts without arcing internally or exploding. The 175 series P rated RCD resistors are good. Ameritron uses two ten ohm in series, and has virtually no resistor failures in tube arcs.

    For larger capacitance values, and higher voltage, it takes a large carbon glo-bar type, like a Stackpole or Carborundum style a few inches long.

    Upsizing the electrolytics in the SB220 could cause problems. With stock electrolytics, tube grid damage is not as likely as with higher capacitance low-ESR capacitors.

    Some upgrades, like better capacitors, come with hidden downgrades, like increased fault currents.

    73 Tom
     
  16. AG6K

    AG6K Ham Member QRZ Page

     Tube arcs? Big-bangs are from arcs made in the atmosphere, not in a near-vacuum.

     agreed - or one can put two Ohmite 120J surge resistors in series to handle more Joules. In a SB220, two Ohmite 15Ω, 120J surge resistors will safely discharge 400uF filter caps and limit peak I to only 100 Amperes. .

     it's not the grid that gets bent Tom, it's the filament helices:
    3-500.f-g.short.JPG
    p.15 September,1990, QST Magazine

     Good point. However, without a suitable glitch-R, during its intermittent parasite near 110MHz, the original el-cheapo SB-220 filter caps bent many 3-500Z filaments, resulting in fil-grid shorts and T2 meltdowns.
    • Rich, ag6k
     
  17. AG6K

    AG6K Ham Member QRZ Page

     Correct For 3-500Zs, limiting peak discharge current to 200A seems to prevent filament bending during glitches that cause a large burst of grid-I. At 3000v this means that 15Ω will suffice. For a SB-220 with a stock C-filter = 25uF, an Ohmite 120J surge resistor will do the job. - note - If u need one, we buy these in bulk for our suppressor retrofit kits and we sell them separately for $1.77.
    • Rich, ag6k
     
  18. VK4TUX

    VK4TUX Ham Member QRZ Page

    Picture of RFP 3-500zg tested;

    IMG_0424.jpg

    The grid filament clearance is substantial on these.

    I'm curious if the floor has carpet where this occurred Sue?

    Adrian ... vk4tux
     
  19. AF6LJ

    AF6LJ Ham Member QRZ Page

    [​IMG]

    That's interesting looks like the filament doesn't go all the way up to the top of the grid structure.
     
  20. VK4TUX

    VK4TUX Ham Member QRZ Page

    That's right, and helps to explain the intensity of color (hot-orange) on the bottom third of 3-500zg RFP anodes seen when operating. The color slowly rises as the heavy usage continues to make the whole graphite anode bright. On low-medium use you just see some color at the base.


    Adrian ... vk4tux
     
    Last edited: Jan 24, 2012
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