Disappointing short life of NOS preamp tube

"I actually wonder that. Not sure there’s a guarantee of being "NOS", or whether that would even matter except for power tubes."

mulveling- yeah, the "NOS" thing is roll of the dice. With preamp tubes, it may not be such an issue whether or not you're getting a genuine, unused "NOS" tube.

True NOS is something you actually purchased  from the local hardware or drug store for $1.87, tossed in a box in storage for the next 60 years, and you "discover" it.

I've been using some 50's  RCA 6L6G's (the low voltage coke bottles) also from Brent that were installed just weeks after the 12AUf7's. One just died the other day.

I'm  hoping to get another batch from him, and refresh all 8 needed. 

I had a pair of 1964 Mazda 6dj8 that came from Brent Jessee and they were matched section, matched pair.   They were some of the best sounding tubes I have ever owned.   Unreal tone , unfortunately the preamp they went in is tough on tubes so they didn’t last as long as expected but man did they sound good 

About six months ago I bought a 12at7 from Upscale, from Kevin’s Stash    It was a Mullard 6201 and it sounds incredible.    
 

The above dealers are a safe bet   

I have an amp that is a hybrid, It uses both sides of one small signal tube a 6DJ8 on the input (no need to match tubes) and the output uses power transistors. A ridiculously good sound. In this scenario the tube will and does last a very long time and the amp has been powered up 24/7 for best possible sound. Initially I have tube rolled with different brands that have produced dramatic results. It is nice to be able to tailor the sound for my speakers and for my own own tastes.