Tubes Asylum

Output tubes have glowing red spots - Why ?

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What would cause the plates on two out of 4 output tubes ( 1 in each channel ) to have a 1/2 inch glowing red spot on them? The tubes are not that old and the bias setting is right on the money for all 4 tubes.. Are these tubes bad?? - Should they be replaced or do some tubes run normally with slight red spots on them ?
JimmyT

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Re: Output tubes have glowing red spots - Why ? Sans Nom 15:09:26 02/22/00 (3)
It just means that DC is getting into the imput grid of your power tubes and that will gradually leads to a meltdown.
Replace all 4 coupling capacitors.
Jacques.
- Is there anything else it could be ?? JimmyT 16:44:22 02/22/00 (2)
  Is there any thing else it could be ?? - are my tubes still good ?? When I turn DOWN the bias BELOW NORMAL the red glow goes away ..
  JimmyT
  - Re: Is there anything else it could be ?? GregB-KC 17:53:56 02/22/00 (1)
    It's just about certain that what the gentleman who answered you before said is correct and the following scenario is not likely, but... There is another possible reason for increased current through a tube in addition to increased bias voltage - increased plate voltage. When you lowered your bias voltage, it could have offset this affect, but the downside to that is you lose your AC range into the output tube and start clipping at a lower amplitude. Is there anything that could be causing greater than the specified maximum voltage across the tube (cathode to plate)? shorted through power supply resistors? shorted turns in your output tranny (I sublimely hope not).
    Greg B.
    - Re: Is there anything else it could be ?? Sans Nom 10:39:27 02/23/00 (0)
      Povided that all of the following conditions applied:
      (1) replace/repair all bad parts necessary to bring it back to specs.;
      (2) check all operating points (B+ voltage, PSU filtering, biasing esp. the older type using selenium diodes) to verify that they are all working fine;
      (3) provided that the red spots were small and localized and you didn't left it running like that for too long;
      (4) tubes are from reputable manufacturers with good quality materials and construction.
      Then you should be able to salvage the tubes afterall.
      If the tubes failed to stay in specs. or the grid inside is somewhat broken due to meltdown my best advice for you is to replace them all. It's no use crying over spilled milk in this case.
      Jacques.

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