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SB-200 over current at power up - help please

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  • jpnery
    I have a SB-200 with all Harbach mods. Two tubes replaced about two weeks ago to restore full output. Suddenly the amp burned the 1 ohm resistor on the
    Message 1 of 7 , Apr 1, 2014
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      I have a SB-200 with all Harbach mods.  Two tubes replaced about two weeks ago to restore full output.

       

      Suddenly the amp burned the 1 ohm resistor on the Harbach HV board on power up.  It seems the plate current is excessive on power up or within a second or so after the amp is powered up.

       

      The 1 ohm resistor has been replaced and a fuse was put between the HV board and RF section.  With the tube plates disconnected the fuse does not blow.  

       

      With the plates disconnected the negative supply is providing negative 120VDC.  The same negative 120VDC is measured at the tube grids.

       

      With the plates back in circuit the fuse blows immediately.

       

      Have tried to meter check all resistors, and diodes and found nothing else defective.

       

      Any help regarding things to check would be very much appreciated. 

       

      TNX & 73

       

      John WA1ESO   WA1ESO(AT)JUNO(DOT)COM

       

    • WA6FWF
      Sounds like you have a shorted tube, pull the tubes out and measure from the plate to the pins and find the one that is shorted. 73 Kevin WA6FWF
      Message 2 of 7 , Apr 1, 2014
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        Sounds like you have a shorted tube, pull the tubes out and measure from the plate to the pins and find the one that is shorted.

        73 Kevin WA6FWF

        On 4/1/2014 5:23 AM, jpnery@... wrote:

        I have a SB-200 with all Harbach mods.  Two tubes replaced about two weeks ago to restore full output.

         

        Suddenly the amp burned the 1 ohm resistor on the Harbach HV board on power up.  It seems the plate current is excessive on power up or within a second or so after the amp is powered up.

         

        The 1 ohm resistor has been replaced and a fuse was put between the HV board and RF section.  With the tube plates disconnected the fuse does not blow.  

         

        With the plates disconnected the negative supply is providing negative 120VDC.  The same negative 120VDC is measured at the tube grids.

         

        With the plates back in circuit the fuse blows immediately.

         

        Have tried to meter check all resistors, and diodes and found nothing else defective.

         

        Any help regarding things to check would be very much appreciated. 

         

        TNX & 73

         

        John WA1ESO   WA1ESO(AT)JUNO(DOT)COM

         


      • jpnery
        Hi Kevin, Two other OM s also say shorted tubes. They don t read short when cold and tested with an ohmmeter but this is not to say a short does not appear
        Message 3 of 7 , Apr 1, 2014
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          Hi Kevin,

          Two other OM's also say shorted tubes.  They don't read short when cold and tested with an ohmmeter but this is not to say a short does not appear when they are heated and HV applied.

          One suggested putting one tube back at a time.  The one that blows my test fuse will be the bad one.

          The amp owner tossed the old tubes when he noticed the new ones brought the power back up to the normal output.  Too bad.

          TNX & 73

          John WA1ESO
        • kj4ws
          You have a Bad Tube my Friend !!!!!!!! We can only hope that you purchased your tubes from someone like RF Parts. Their Warranty ... is hard to beat. RF Parts
          Message 4 of 7 , Apr 2, 2014
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            You have a Bad Tube my Friend !!!!!!!!  We can only hope that you purchased your tubes from someone like RF Parts. Their Warranty ... is hard to beat. RF Parts has always been great to deal with and I actually did need the warranty once on a Matched Pair of 572B Tubes. I was asked some detailed questions about the tubes and their failure. Then several days later my new tubes arrived.... If, you purchased from one of the Auction Places ?? Good Luck! With any type Warranty.
          • jpnery
            Kevin and the group, One of the two tubes turned out to be shorted. It seems earlier on it was an intermittent short and at times it would not short on power
            Message 5 of 7 , Apr 2, 2014
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              Kevin and the group,

              One of the two tubes turned out to be shorted.  It seems earlier on it was an intermittent short and at times it would not short on power up.  However when this happened a slight tap of the tube got it to short, flash and take out the test fuse I put in circuit.

              In the end the short remained so I could see the short with an ohmmeter and while testing with the power off.

              Owner said he will order a pair of tubes rather than just one.

              TNX & 73

              John WA1ESO 
            • WA6FWF
              John, Glad you found it, out of curiosity what brand of tube are we talking here? I have heard stories of Chinese tubes having sagging filament issues, in the
              Message 6 of 7 , Apr 2, 2014
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                John,
                     Glad you found it, out of curiosity what brand of tube are we talking here? I have heard stories of Chinese tubes having sagging filament issues, in the shack here I run the original Cetrons and I have a backup set of Svetlana's, I rotate them every year in hopes of making them last longer and keeping them from going gassy.
                73
                Kevin WA6FWF

                On 4/2/2014 4:31 PM, jpnery@... wrote:
                Kevin and the group,

                One of the two tubes turned out to be shorted.  It seems earlier on it was an intermittent short and at times it would not short on power up.  However when this happened a slight tap of the tube got it to short, flash and take out the test fuse I put in circuit.

                In the end the short remained so I could see the short with an ohmmeter and while testing with the power off.

                Owner said he will order a pair of tubes rather than just one.

                TNX & 73

                John WA1ESO 

              • jpnery
                Hi Kevin and the group, Here is the final report on the amp repair. RF Components was out of stock on the 572B tubes so another used tube was installed to
                Message 7 of 7 , Apr 11, 2014
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                  Hi Kevin and the group,

                  Here is the final report on the amp repair.  RF Components was out of stock on the 572B tubes so another used tube was installed to replace the shorted one.  

                  Several other burned components were replaced as well but the amp is now back to specification.  In CW 1000W in for 600W reading on the Bird watt meter.

                  The defective tube was made by Electron Tube of China.

                  TNX AGN & 73

                  John WA1ESO
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