Tube Life

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...

The rate at which tubes wear out and how they sound as they do varies with the circuit they're installed in. The general tips above (and any that follow) are useful but if the OP wants the most applicable guidance he should post the preamp he's using these tubes in.

I appreciate all your help and your suggestions. I was slow about posting the actual unit due to the unlikely chance that anyone would have one or have heard of it. It’s a handmade unit “Sheer Audio MM-77” that I purchased off ebay from Vintage Audio Labs.