Tube Question - Significance of Tube Noise at Listening Position

I love the word "life" when discussing tubes because they are just as variable as "life" in the metaphysical sense can be. That is what is so alluring about tubes.

may be an obvious point, but the circuit design of tube components majorly affects tube ’life’ in use

examples

c-j’s et5 linestage, as lovely as it sounds, runs a single 6dj8 type tube full out, at (or actually slightly above) the tubes max recommended 6.3v plate voltage -- so standard 6dj8/ecc88 get tired and noisy after about only 6 mos of daily use even when you start with a true nos tube.... so over time they started suggesting 7dj8 / pcc88 tubes in that spot instead as customers were getting frustrated

audible illusions pre’s also were famous for running their 12ax7 tubes into the ground fast by running them so hot 

kevin hayes (vac) is another designer who infamously runs tubes aggressively in his gear, at the detriment of life span, for the sake of superior sonics he desires

otoh arc linestages like the ref3 ref5 and so on use more robust tubes, run them at quite conservative operating points, and those tubes last much much longer in daily use

so as usual, things are never equal, and one’s experience with tube life is highly case specific 

   Funny you should ask as I swapped out my 12AU7’s two days ago in my line stage pre. It got to the point I was hearing an intermittent hiss at my listening position. I switched the mute on and off over the last few months and it would stop but began to return more frequently and is now constant. The offending tube is of a matched pair of Tekefunken G-73 R’s that I absolutely love. I replaced them with a pair of standard nos Telefunken’s and problem solved. Except they don’t SOUND as good as the noisy pair, sound familiar? My solution is that I have about a dozen pairs of various sets of nos 12AU7’s to play with. Fortunately for me I acquired them about 10 years ago before dynamic pricing was involved. NOS tube rolling can become addictive so be carful. A couple of random thoughts are that I run high efficiency speakers and tube noise is more apparent. I prefer nos tubes over new production tubes but they are expensive and  although called NEW have usually seen a few owners so no telling until they’re tested and tried. Also I don’t agree that new gear is voiced to a specific 12AU7 if it ships with JJ’s. They are inexpensive, available, inexpensive, reliable , inexpensive, dependable, and inexpensive. 😆 Occasionally someone actually likes them , unfortunately that wasn’t my experience. However I did find the new production Tung Sol 5751 gold pin very nice , but unfortunately it’s a low mu 12AX7. Try a few pairs of different tubes and see what you like. The three directions for nos are Tekefunken, Mullard , and RCA. That’s a generalization but fairly accurate. FWIT if you go to the Brent Jesse site and read the sound description page for 12AU7’s it’s very accurate. Happy Hunting and I’ll look for you at the Tube Addicts Anonymous meeting. Cheers , Mike B. 

In my experience, I've had nothing but trouble with the 12AU7 in my CAT SL-1 preamp. Never could find tubes that lasted more than a couple of months before the hissing and spitting returned. Frustrated, I modified the preamp to use the 6922 in its place. Quiet ever since.

A good pair of 12au7 Bugle Boys has always been my savior.