Tube Life

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.

Thorman's advice is good--with an extra "new" set, you can switch out tubes to determine if the sound has started to go bad. Weak tubes sound dead and lifeless compared to newer tubes. The only problem with this approach is that you have to pay for an extra set that you may never need.

There are some line and phonostages that run tubes so gently that they may not go bad in your lifetime. This would particularly be the case with the Telefunken 12AX7 and 12AT7 (reputation for long life). I have the other Telefunken 12AX7 equivalent (ECC803S) and I am hoping that that is the case (REALLY expensive variant).

On the other hand, there are designs that run tubes pretty hard. I recall that a dealer who kept Counterpoint linestages on for 12 hours a day had to change tubes every 6 months.

Having an extra set of tubes lets you gauge the sonic deterioration of your main tubes. Such is good practice. Also, tubes do fairly often catastrophically fail (one channel goes silent... that kind of thing) --- and, by having an extra set, you can use those tubes to assess which of your tubes died (that is, if, for example you have 4 pre-amp tubes per channel and a channel died, you can readily swap out systematically the tubes to discern which tube died).