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In Reply to: Help Needed...1 Tube won't light up on Atma-Sphere M50 posted by speedracerucr on July 18, 2005 at 21:25:43:
Update,I checked on the socket as per 'tsup's suggestion and I found that the socket was fine. The tube was gripped tightly by the socket. Pushing down on the tube did not remedy the problem. Any other thoughts or ideas? Thanks
Follow Ups:
-it sounds like you have several that have failed. It also sounds like there may be an issue with the socket in question.
Ralph,I had just recently retubed both amps not too long ago (about 1 month ago) and I did not notice any problems with the tubes on the other amp. Could it be that the grid resistor(s) have failed? I understand that the M-50s are over 15 years old and I don't believe they have been checked over since the original owner purchased them new, so I would probably like to send the problematic amp back to the factory for service and repairs.
I would examine the tube or tubes in question as per Ralph's suggestion. You might want to check out the Atma-sphere website for a guide on how to visually examine the power tubes for flaws. As a precaution, I would visually inspect all 8 powertubes in the problematic amp. If one or more of them fails and you still keep the bias setting at 550ma, you are forcing the remaining good tubes to work too hard, thus worsening the issue with further tube failure. If all tubes 'appear' fine, and the issue is with the socket, then in the short term, you can simply set the bias setting at 480ma (7/8x550)and be good to go.On a separate note, if you retubed everyuthing recently, it might be a good idea to check out all 16 powertubes anyway, since serious flaws would likely show up soon after being pressed into service.
That would be 6/8X550ma. Each 6as7 has two triodes in it. Each bank of 4 tubes (8 triodes) is biased at 550ma. If one tube in one bank is not lighting up then pull it and pull one tube from the other bank and bias for 6/8x550ma.Tre'
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