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Recently, I have experienced two separate noise issues with my MP-3. Both occurred with the volume off but amps (M60s) on. The first was with the Trim controls generating a moderate amount of static/noise when turned back and forth, sounding like an old volume pot that has been unused for a while. By itself the noise is not particularly problematic as it only happened when the trim control was turned. I raise the issue as it may have some relationship to the second noise problem I am having.
The second issue is more of a concern. After the preamp had been on for about two hours I began to hear a noise that rapidly increased in volume through the speakers despite the volume on the pre being off. It became alarmingly loud and I had to shut off the MP-3. I removed the top and restarted it. As it warmed up the noise began again and upon inspection seemed to be related to one of the 12AU7s. Moving the tube in the socket seemed to affect the noise and it stopped for a while. Next day it was back even though I had changed both 12AU7s and cleaned the tube sockets. Only now it was coming from (being affected by) the other 12AU7 socket/tube than the previous day. Further tube swaps only showed that despite additional tube changes (3 tubes tried in each socket) that the noise would intermittently appear and that jiggling the tubes in one or the other socket seemed to temporarily silence the noise. The problem could not be localized to either a specific tube or socket and it has continued to occur sporadically and when it does, it sounds like a smoke alarm has gone off. Today, I was in the workshop after having turned the system on and all of a sudden I heard the noise from two floors away... really loud.
I have inspected the solder connections on both tube sockets and they look perfect so I am at a loss as to what may be causing the issue. Thoughts and suggestions are welcomed. Thanks.
Follow Ups:
So the first step is to swap the 12AU7s left for right and see if the problem migrates to the other channel.
If yes, replace the tube.
If no, then proceed to the 6SN7s and swap them left for right and see if the problem migrates to the other channel. More than likely you will find the culprit pretty fast this way.
Its not likely that the gain trim controls have anything to do with what sounds like the primary issue.
The gain trim controls can be noisy if the sections of the 12AU7s associated are not matched. If the mismatch is severe, the gain trim control could be damaged. We have also seen them get dirty, in which case a small shot of DeOxit into the notch on the control and vigorously operating the control can sort out the problem.
Thanks for the reply. I have currently changed out 12AU7s and I think there were two that were bad (between the ones that were originally in and the ones I used for replacement) which complicated the troubleshooting. At this point I think the problem is solved, but I am presently running the pre and waiting to see if this issue returns. Am using Shuguang tubes that are almost new.
Ralph will be all over this, but did you also check the circlotron tubes? It does sound like a bad tube, somewhere. I don't own an MP3, but I believe it has one 6SN7 per channel in the circlotron.
I have not changed the 6SN7s but will try some new ones. I did tap and jiggle them and they seemed not to affect the issue.
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