Tube Life

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).

my experience has been as tubes begin to "poop out" you lose a little something sonically. I have noticed this after tubes become and noisy and before as well. I have noticed on both my line preamp and phone preamp.

I see no reason whatever to leave a preamp on all the time, unless the unit is solid state. if you have expensive rare tubes (e.g., the TFKs you guys are discussing), why let them go to waste when you are not even getting the fun out of them? It is true that there is some stress on a tube at turn-on, but that can be ameliorated by a "soft-start" circuit and by only applying plate voltage after the filaments have warmed up (which takes only about 10-20 seconds for 9-pin miniature triodes like 12AX7). Both of these goals are easily achieved by any thoughtful designer. Most tube units will be in "full song" within 20 minutes or so after turn-on. (So will I.) When a tube ages, it loses "gain" and input sensitivity. Thus the report that the sound gets flabby or dull or lifeless makes perfect sense and is a good guide to tube replacement.

In contrast, SS units are not at their best unless or until operating temperatures have stabilized, which can take hours. Since SS devices are at even greater risk than tubes at turn-on, and since their lifespan is much longer than that of a tube. it does make sense to leave SS gear on at all times.

Lewn, I aqree totally with you. I don't see any reason to leave tube gear on all the time. Most sound very good within a pretty short time after being turned on, while solid state gear can take hours to come up to full song.

Concerns about the thermal shock at turn on are over exaggerated. When cold, the filament will conduct more readily so that there is a quick in-rush of current and rapid heating at turn on. But, with tube filaments, this does not happen with the kind of speed and severity of something like a light bulb.

In any case, all my tube gear use tube rectification, so the signal tubes are protected from instant on conditions--it takes a while before the rectifier is putting out full current so it acts like a soft-start circuit.