From: none@ntlworld.com   
      
   On 08/09/2010 17:56, Richard wrote:   
      
   >> I believe I can use a megger to tell me about shorts between electrodes.   
   >>   
   >> Now, it seems to me that to really test for shorts we ought to put   
   >> serious voltages on the electrodes. And from what I gather the tests   
   >> should be for a KT88:   
   >>   
   >> a) c to h 250V   
   >>   
   >> b) g1 to all 200V   
   >>   
   >> c) g2 to all 600V   
   >>   
   >> d) a to all 800V   
   >>   
   >> I can motorise my megger and set these test voltages.   
   >>   
   >> I can test cold clearly, but what gets me though is testing for shorts   
   >> hot. Can I actually test for short with a megger when I'm running the   
   >> tube hot?   
   >   
   > Actually I can reduce some of those voltages.   
   >   
   > Also, if the tube is tested for shorts cold, is the only reasonable test   
   > for shorts hot between cathode and heater?   
      
   When I think of "short circuits" I think in terms of testing insulation,   
   such as insulation between a tag-strip and earth, or a cable and earth.   
   That's in my minds eye when I think of testing for shorts.   
      
   The AVO tube testers test for shorts when cold.   
      
   I see tube data and see maximum voltages table and I see these voltages   
   as possible test voltages for shorts between electrodes.   
      
   So I quote for KT88:   
      
   c to h 250V   
      
   g1 to all 200V   
      
   g2 to all 600V   
      
   a to all 800V   
      
   So, I'm minded to test the tube at these voltages with my megger when   
   tube cold.   
      
   Then I think the same tests should be made when tube is hot. Because the   
   AVO tests for hot as well as cold, although I don't have an AVO tube   
   tester, so I'm not sure what the hot tests are.   
      
   Of course, when hot there is a potential problem, can you test the tube   
   for shorts when hot?   
      
   I believe you can, but it's clear that the test polarity is important.   
      
   I'm thinking, that if the polarity is right, I can test the tube hot as   
   cold with my megger at the above voltages.   
      
   Now when I've seen some circuit for shorts there has been nowhere near   
   the voltages I quote above.   
      
   Take the Eico 667 shorts tests, page 10  b), plate to all leakage test -   
   is that -70v the actual test voltage for all tubes, or what?   
      
   Is the Eico testing for shorts/leakage using -70v for all tubes? I sorta   
   doubt it but don't know for sure.   
      
   Is leakage tests cold a completely different animal from leakage tests hot?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)