High Pitched Beeping sound - SP8/ST70

Hello I am running an SP8 into a low gain ST70. I am getting a high pitched beeping/screeching sound out of both speakers after warm up, maybe 10 minutes after turning everything on. The sound would be something like 1% of the sound I am hearing, so 99% music, 1% beeping. It is barely noticeable until you get to quiet passages, and then it is extremely annoying.

I have a large number of 12au7s, and it does this with all of them unless it is cold.

I basically only listen to vinyl, so this evening I switched from the vinyl input to the next one on my preamp. It still beeped. I turned the preamp off, and it still beeped. I pulled all the 12au7 tubes on the st70, and it went away! I replaced the outer two and it did not return. I replaced the middle 12au7 with two different ones from the stash, and it was present with both of them.

This makes me think it is in the circuitry for that middle tube.

The sound is in time, and is basically BEEP two three four, BEEP two three four. It is a quiet sound, but is definitely there and is very distracting. Probably at about 10khz or so. the level is constant regardless of where the preamp is set.

I am thinking the timing it has is a clue. Oscillation? Could it be a stage of the cap can going bad?

Thanks for the help. This setup sounds amazing and I need it to be completely silent again ASAP! I cant go back!!!

I'm sure no expert on anything here but I don't know of anything inherent in a tube amp or a tube that would produce a rhythmic, steady beep sound but I could sure be wrong. I wonder if it isn't in proximity to another newer electronic device and is picking up some weird signal or modern wireless device, maybe even security systems or a router or other stuff that might produce a pulsing sweep or something. I know I often wake up hearing a beep and can't find the source without a lot of searching...oven timer, phone needs charging, smoke alarm needs battery...in todays world there are a million devices competing for wavelength and space. This theory might not be valid since it starts after warmup. Just my 2 cents, could you change the location and try again to see if it is in an electronic field of some kind? Ok experts....weigh in....

I had thought this was tube microphonics, and have been busy so I hadnt had time to do much troubleshooting until yesterday. I just played it for two hours though, and it never happened.

I am thinking I need to reflow the solder joints in the power supply since it is both channels...

that's definitely a very unusual problem, and as many of those types of problems that mysteriously appear and disappear, it's not the amp itself.
WildIowa may have had a good thought there, or maybe some aliens are trying to figure out why your tube amp sounds so good?!

I didn't want to believe you guys, but now I think I am...

When it comes up again, I will try unplugging things. Power line noise would make sense, I do have a turntable, wii, playstation 3, tv and cable box plugged into the same outlet.

Since I have both Roy and Bob's ears, I would like to thank you both for this excellent stereo. It is truly awesome and has given me many hours of enjoyment and before and after work stress reduction. It was a journey getting everything set up perfectly, and I discovered the replacement tweeters I bought for my chorus II speakers were pieces of crap (I spent a month or so trying to figure out how to tame the harsh, grating treble!)

I bought some stock Klipsch Fortes for a steal ($200) and that fixed any and all issues I was having. It just shows how important your entire set up is when dealing with such fine pieces of equipment. Even with little problems like the above, it is very much worth the effort.

For the benefit of others, I wanted to post that this problem was related to the power tubes in the ST70. I was using original mullard XF3, and under the impression that because it sounded good to me, the tubes were good. Also, the problem must have only presented itself in the tubes when they were being driven, and by removing the driver tubes while troubleshooting I mistakenly thought I had ruled them out.

Thanks for the tips, hope this may help someone else...