Imin2u: I can explain it. Tube gear using small signal tubes such as 12AX7's or 6922's is generally best left on 24/7. Quoting from the "TIPS & ADVICE" section of the owner's manual to my VAC Rennaisance 140/140 Mk. III tube amps:
"How long should tubes last? It has long been known in professional circles (and probably now forgotten) that a tube such as the 12AX7 will display BETTER performance characteristics after TWO YEARS of CONTINUAL operation than when it was new. In normal use it is not unusual for a low level tube to last 5 years or longer. Output tubes [i.e., power tubes used in tube power amps] are another story, as they are continuously providing significant amounts of current." (Emphasis original).
The expansion and contraction small signal tubes undergo as gear heats up and cools down upon being turned on and then turned off takes its toll over time. In addition, the voltage rush at turn-on is especially hard on small signal tubes. Tube gear with tube rectification is a lot easier on tubes at start-up, and some tube gear has a soft-start feature to soften the blow, but with a few exceptions, it's just better, for both tube life and for sound quality, to leave tube preamps on 24/7.
After three to five years, the tubes should be replaced, as they do start to "lose their luster", but they won't fail if used this way.
"How long should tubes last? It has long been known in professional circles (and probably now forgotten) that a tube such as the 12AX7 will display BETTER performance characteristics after TWO YEARS of CONTINUAL operation than when it was new. In normal use it is not unusual for a low level tube to last 5 years or longer. Output tubes [i.e., power tubes used in tube power amps] are another story, as they are continuously providing significant amounts of current." (Emphasis original).
The expansion and contraction small signal tubes undergo as gear heats up and cools down upon being turned on and then turned off takes its toll over time. In addition, the voltage rush at turn-on is especially hard on small signal tubes. Tube gear with tube rectification is a lot easier on tubes at start-up, and some tube gear has a soft-start feature to soften the blow, but with a few exceptions, it's just better, for both tube life and for sound quality, to leave tube preamps on 24/7.
After three to five years, the tubes should be replaced, as they do start to "lose their luster", but they won't fail if used this way.