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Hi,I am having some problems with one of my Atma-Sphere M50 amps. I would like to see what other members think of the situation. (perhaps Ralph or Legendre can chime in as well before I send it back to the factory for service) When I fired up my amp this evening to standby mode, I noticed that one of my tubes did not light up. I tried switching out that tube for another one, but that too failed to light up. I did not switch the amp to full power after that. From my understanding, the standby mode supply heater current to the tubes to warm them up before full power up. Before this episode, I had recent problems with this amp with tube plates glowing orange. I tried replacing the glowing tube with a new one but I noticed that another tube in a different socket would start to glow orange a few hours later. During this whole time, the amp held steady bias at the factory recommended 0.55 amps. What do you guys think? Thanks
Follow Ups:
Update,After talking to Bill at Atma-Sphere, I was able to get the tube to light up after tightening the "grip" of the tube socket, especially at pin positions 7 and 8 by using a small flat screwdriver and adjusting the pin holders. I am glad that this simple fix was a able to do the trick. Now in terms of the glowing plates on the tube(s), I think I will order a new set of 6AS7G's and see if that will fix the problem. Thanks for everyone's suggestions.
Hey, glad to hear you solved the issue, but out of curiosity, isn't this pretty much exactly what I suggested (down to the screwdriver cure)? I've copied my orginal post below.
If the tubes are both good, then the issue appears to be with the socket. Before you dig any deeper, do you find the connection between tubes and the socket in question to be firm or jiggly? Have you tried holding down a tube in the suspect socket with a little bit of force, or perhaps applying slight lateral pressure - just to see if the tube lights up in standby mode? I've noticed something similar in the past with where the contact between the tube pins and the receptacles in the socket loosens over time. If applying pressure downwards or laterally gets the tube to light up, then your problem is likely a contact issue. I suggest taking a small screwdriver and prying the receptacles in the tube sockets closer to each other such that they really grip the tube pins fairly firmly.
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
Peter
-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'
Have Fun and Enjoy the Music
"Still working the problem"
If the tubes are both good, then the issue appears to be with the socket. Before you dig any deeper, do you find the connection between tubes and the socket in question to be firm or jiggly? Have you tried holding down a tube in the suspect socket with a little bit of force, or perhaps applying slight lateral pressure - just to see if the tube lights up in standby mode? I've noticed something similar in the past with where the contact between the tube pins and the receptacles in the socket loosens over time. If applying pressure downwards or laterally gets the tube to light up, then your problem is likely a contact issue. I suggest taking a small screwdriver and prying the receptacles in the tube sockets closer to each other such that they really grip the tube pins fairly firmly.
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