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Turned off my system today at the end of a listening session and heard a troubling rumbling static noise in my speakers. Historically this has meant the end is near for one or more of the KT88 tubes my VAC PA80/80 amp employs. So I put in some old tubes to bide me over until I could order some new ones. With the system still off I turned the bias adjustment screws for all four tubes counter-clockwise. I turned the system back on and slowly turned the first screw clockwise and waited to see the LED indicator light turn from green to red, at which point I would normally slowly back off to restore green. Unfortunately, I was unable to get red even after turning the screw all the way clockwise. I backed off on that first screw and went to the second. Again, couldn't see any red, even after going all the way clockwise. Too worried to test the last two tubes, I just shut the system off and pulled the plug.Is anyone here knowledgeable as to whether this indicates a failure of the bias adjustment on the amp unit? Or is it possible my old set of interim tubes have all suddenly failed at the same time?
can anyone recommend a good tube repair shop in the Rochester, NY area? I used to go to Kelsey-Pape, but sadly they shut their doors a few years back and I haven't had the occasion to go to a repair shop in the time since.
Follow Ups:
Does your amp have a rectifier tube such as 5AR4 or GZ34 or is it SS rectification? Sometimes when the rectifier tube starts to go, the voltages start to go south resulting in low bias draw from the KT-88. Be careful if you decide to open it up as the voltages can be lethal.
If you didn't allow for around 10 minutes for the tubes to heat up really well, that could account for the tubes not reaching their bias points. Some tubes take quite a bit of time to heat up enough to reach an acceptable bias range.Turning your bias adjustments down before attempting to bias should keep the tubes from cherrying the plates while you wait for them to reach proper temperature. I would make another attempt to bias the tubes while keeping a very close eye on them for glowing plates while they heat up. If any of them start to glow shut her down. But, if they don't get red I would let the amp run for 10 minutes and try again.
As usual, disconnect interconnects, leave speaker cables connected and let them heat up while watching for red plates. Some tubes take longer than others to heat. If that doesn't work out, I would suspect a bias supply problem before multiple bad tubes.
when the new tubes arrive.
I would only send them back to VAC. Yeah it's more expensive to pack/ship them, etc. But if there are updates/revisions VAC will probably brings yours to new spec.
It helps protect the resale value also.
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