When to replace pre-amp tube?

If you think its been used for a long time and your not having any problems,thats the time to replace the tube..Much easier ( and less costly ) to just spend the money now and be worry free for a couple more years.......

IIRC, the nuvistor tubes are very long lasting. Yep, here it is, S'phile review of Nuvistor amp says "Typical tube life expectancy is about 100,000 hours, and the failure rate, with almost 1500 Nu-Vista products now in the field, is "about 0.1%." So don't worry. " lets see, 100k hrs = 24/7 for 11.5 yrs.

About once a week.

Swampwalker

that's a simple enough math :) thanks

When marketing the amplifier, as far as I recall, MF stated they were good for 100,000 hours and that they would "reserve" a replacement tube set for every unit produced.

That was then and I don't know what the now position is.