Tube Life

Preamp tube usually go out with a wimper. And not a bang.
So, unlike amp power tubes.. you can just use preamp tubes until they annoy you.
And I leave my VAC Standard tube preamp on 24/7.
I bought it used a year and a half ago, left it on 24/7 so far.. still no problem. (with used old stock tubes)

Do it the right way,,When in doubt,buy another set and have them on hand..If anything changes ( in sound ) dump the old set for the new...Always try to stay one step ahead of trouble.......

I have a friend using 6sn7tubes in the amp input and they are 15 years old. Working well, no noise or problems. Use until noise is an issue. Life ends with noise for me. Jallen

Telefunkens are known for their durability so they may be chuggin' along just fine. Or there may be a decrease in sound quality that you haven't noticed because it occurred gradually in imperceptible increments.

If you plan on keeping the phono pre you will eventually need new tubes so you might as well get them while you've got time to look for a deal. When you get the new ones plug them in and see if they sound better. When the new ones sound better, it's time to retire the old ones for emergency duty only.

if your stereo sounds 'sleepy' time for new tubes - which will be more defined and vibrant I have found...