From: none@ntlworld.com   
      
   On 09/09/2010 18:39, Richard wrote:   
   > 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?   
      
      
   To answer my own question - correct.   
      
   But, this is not the test really required. Should be tesying leakage at   
   say 500V.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)