"vintage 12AX7 and 12AU7 can last for 10000 hours if you are lucky” - it’s pure engineering, nominal usage conditions vs. spec etc.. harder driven devices last less..
Couldn't resist, just bought matched pair of NOS Mullard 12AX7 long narrow plates from Brent of Audiotubes.
I can't find NOS matched three Sylvania 12AU7 long black plates anywhere. I have NOS RCA early 1950s black plates, but the Sylvania are a touch better in this amp.
inna, the problem of matching NOS tubes is a difference in two different devices behavior in the aging process, where theoretically two devices can be only matched one time. lucky matched NOS tubes buyers are those who get tubes from the same production batch/lot, which is the same as winning lottery ticket!
I've had a pair of the same Mullards in line preamp for over two years by now, about 2500 hours, and I don't hear any difference compared to when they were new, even though my system is not what could be called high resolution by our standards. Still, it's very sensitive to minimal changes, and my hearing is not bad.
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