After having the brand new replacement phono stage on for about a day, incredibly, the right channel MC stage 6922 went popcorn noisy! This is the fifth time in a row! I swapped tubes, and as before, the noisy sound went to the left channel. For just a moment, put aside this specific case and consider flipping a coin five times and it lands on heads each time, or tails each time. The probability of that happening is only 3 percent. Now, take away from that whatever likelihood that a new tested tube goes noisy, say within 24 hours. Afterall, this is an electronic device and we're not expecting it to fail so soon. Further subtract that's it's just one side. I don't know what that chance is, but combined, it must be very low. But, it happened to me. Now, in spite of it being very tempting to more heavily favor the idea that there is a problem with this circuit design or execution (like a bad resistor or something else, such as the circuit design driving the tube too hard), and considering the advice I got above, especially from @atmasphere and @mulveling , I must now give more weight to the idea that I simply got incredibly unlucky. That is because I took the left channel tube and put it in the right socket, and I took one from my good stash of growing 6922s of a different brand and stated specs and put it in the left side. I've had the unit on for a about a week straight, and... it's remained quiet! Also, the first time I had this issue, it wasn't untiI I owned the preamp for about six months that the right tube went noisy. Also, I can't find anyone else who has had this problem. One thing I'm tempted to conclude, though, is that all of these bad tubes of various brands came from the same vendor, who I wonder may not be testing them correctly. They're almost certainly not running each unit that is sold for say, a week straight to make sure it's still good. Going forward, since I desire a precisely matched pair, upon Andy Bowman's recommendation for a set of very tough and good sounding tubes, I've ordered a set of Matsushita 6922s.
I'll be damned if that right side goes noisy again any time soon!
Furthermore, this all sounds like it may be an indictment of the 6922. But plenty of gear runs these tubes. My PS Audio BHK 600 pair is just one example. I haven't had any problems. Granted, this is in the driver stage of an amp, not a highly sensitive to noise MC gain stage.
