3-500z shorted after shut down

Discussion in 'Amateur Radio Amplifiers' started by K4ATR, Jan 18, 2018.

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  1. K4ATR

    K4ATR Premium Subscriber QRZ Page

    I have been running a set of RF parts 3-500z tubes for about six months I have noticed sometimes the output would vary from time to time yesterday powered up all was fine when I keyed up no output on meter I unkeyed then keyed again and output was fine I talked maybe 6 hours I let it cool down and shut off. tonight I turned it on warmed up no output again this time it didn't fix itself. I pulled the tubes 1 had a filament to grid short replaced with backup set all is good. Has anyone else had a tube short like this I never run it on high always low which is approx. 2200v high is like 2900v. Think maybe just a bad tube or did I do something wrong to kill it?
     
  2. N8VIL

    N8VIL Ham Member QRZ Page

    I would say a bad tube. If only six months old, I would contact RF Parts and see about warranty return.
     
  3. KM3F

    KM3F Ham Member QRZ Page

    Don't worry about the anode voltage at 2900 for 3-500.
    An amp with 2x 3-500 normally on low power is a waist of money in my opinion.
    An AL 80B single 3500 runs at nominal 3000 v.
    I would think your operation does not run the tubes hard enough for proper Gettering.
    Please don't get upset at what I just said. Only trying to bring you to a different perspective.
    There are a lot of people who think running an amplifier low power is saving the tubes.
    As you experience with yours, it has little value.
    If a tube goes bad, it's not how hard you ran it so there is no saving the tubes from low power operation.
    Good luck.
     
    K0OKS likes this.
  4. NQ1B

    NQ1B Ham Member QRZ Page

    I've found that my 3-500Z's won't glow orange with the SB-220 on the low voltage setting, even at 1 kw output. On the high voltage setting, they will, at the same power output. I don't know whether they are gettering properly at ~2800 V, so I make it a point to run the amp for a while at 1 kw SSB PEP on high-voltage setting about once a month to get them nice and orange.

    I suspect far more 3-500Z's have died through not enough good usage leading to lack of gettering than due to being pushed to their normal high-power range.
     
  5. K4ATR

    K4ATR Premium Subscriber QRZ Page

    Mine don't glow orange on low either I never thought about lower plate voltage not allowing it to getter properly but that makes sense. So can not running it to make the tubes glow can cause a tube to short filament to grid?
     
  6. W1BR

    W1BR Premium Subscriber QRZ Page

    Most likely the current surge at turn on caused the filament to fail.
    Just like an old incandescent light was most likely to blow at turn on.
     
  7. K4ATR

    K4ATR Premium Subscriber QRZ Page

    Yes I agree I'm still waiting to see if they will warranty the tube my backup set working fine so I know the amp is ok.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd say "probably not" to that one.

    If they're only six months old, sounds like a simple early life mortality. It happens.

    I'd think the best way to cause this kind of failure would be shock and vibration while the tubes are hot, such as transmitting and moving the amp at the same time. You probably didn't do that.

    If they really are only six months old and you bought them from RFP, they should be under warranty.
     
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Have you measured your filament voltage ?
     
  10. K4ATR

    K4ATR Premium Subscriber QRZ Page

    No but going to this evening didn't think to check
     

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