pre amp tubes and gain

It is my understanding that as pre-amp tubes age, they lose gain. So one sign of tubes that need replacing is that you have to turn the volume control higher than you used to for similar listening levels.

If this understanding is correct -- which I guess is a question in itself -- then my next question is whether the reduced gain happens gradually or if it only starts to happen as the tubes near the end of their life.

So as an example: I have a tubed pre-amp (my first one, obviously) and I notice I am turning up the volume to higher levels than I used to. But the tubes are probably only halfway through their likely lifespan. So this would argue for the gradual reduction in gain theory. Or, it could just be that as I've tuned my system a bit, it is more comfortable to listen to it at higher volume levels....

Your insights are most appreciated.


A simple answer is to test your tubes to see what condition they are in. Modern tubes seem not to last as long as some NOS. Depending on the circuit NOS may be a better alternative. I had an brand new 12AX7 tube simply go bad in less than 3 months. I've also had some NOS that got noisey in less than 6 months time. I always try and keep or obtain a couple NOS and recent manufactured tubes to swap out when I need to trobleshoot. Some circuits actually run tubes at a much higher bias than need to. Some recordings also are recorded at a much lower volume too. Which are you experiencing?
interesting. i just had a tube go bad (matched pair, but rarely do things go down in synch), which resulted in one channel having microphonics & lower gain than the other.

id suspect my experiences are not unique. if you install a matched pair, one may (will?) go down before the other. and you may find one channel louder than the other, i did.

goofy tubes. if they didn't sound so good i would be happy to be without them.
As you mentioned; tuning, tweaking and lowering the distortion levels in your system can also lead to enjoying more realistic(louder) SPLs.
In my experience, it's not so much a gain issue, but more of a lifeless, soft, undynamic sound. It happens very gradually, so I think our ears get used to it. Of course, I've had a tube go microphonic, which I noticed right away, but the aging is a different matter.
That's true, as the gain is pretty much set by the tube's design parameters. As tubes age though, they lose many other attributes that engage us musically.
this is very helpful. thanks all.