From: none@ntlworld.com   
      
   On 08/09/2010 18:45, Richard wrote:   
   > 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?   
      
   This is what I think on shorts:   
      
   The maximum voltage ratings of tubes are NOT meant to be taken as test   
   voltages for leakage/insulation tests/shorts.   
      
   As to leakage tests when cold, it's not meant to be clear what test   
   voltages you would use. But, you could use the maximum voltage ratings   
   if you wish. And these tests could be made with a megger.   
      
   But in fact, you could ignore shorts test when cold and just rely on   
   shorts test hot. And in that case, you can use a circuit employing a low   
   voltage of -70 volts as per page 10 of Eico 667 manual.   
      
   So, you can test for electrode leakage for tubes using -70v.   
      
   Correct?   
      
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