I would tell you tubes require no "break in" time. They need to come up to a constant operating temperature and any existing gasses need to burn off. That should take less than an hour in most cases. After that, what you hear is what you get. If the tube is used in a power amplifier, the transformers also need to reach thermal stability, which could take quite a bit longer than the tubes themself.

I'm in the opposite camp. Tubes seem to improve after about 15 hours which, by
break-in time standards is the shortest of any component I've ever used.

Without completely getting into the woods with the why, I do believe it's somewhat measurable. At least
with output tubes. New output tubes usually require having to re-bias an amplifier. I've
had new output tubes that required 20 hours before the bias would remain constant.

Bias is a somewhat different item than the tubes themself. In the op's case we're talking about a 12AX7 as a pre amp tube.

Agreed...my KT88's took 20 hours to get rid of the grain in the upper register.

I guess that's like anything else in audio where you have a subjective judgement being made. I would say, yes, tubes from the same manufacturer with the same tube designation (12AX7, KT66, 6DJ8, etc.) will sound different from one another. There are plenty of reasons why this might be. The same tube type from another manufacturer is likely to sound different also.

There are plenty of tube substitutions you can make which will affect the tone of your component, a 12AX7 has several variants that serve as drop in replacements. The idea of tube rolling is to find the specific tube(s) you prefer in your system. Sometimes I've found one tube to not be the best choice for one location, say a phase splitter or a line tube, in my system but to work well in another location, say, as a driver tube or in the phono stage. One reason here is each tube has its own level of gain, if you change the gain in the circuit by changing tubes, you'll possibly change how everything downstream from that change is affected.

On the other hand, I've worked with repair techs who felt all tubes were the same if they were the same tube type and measured well. Other repair techs were reasonably picky about the tubes they suggested but not nuerotic about it.

The LPS is a really decent tube, regardless of the price. The German Valvo tubes
are my favorite, but they've become almost as expensive as the Telefunkens. You
can still get the Amperex tubes for a bit less and they seem to provide the best
texture next to the Valvos. Mullards are too warm for my tastes and grossly
expensive for their limited virtues.

Well to me, a non-tube user but a person with a technical understanding in electronic circuits, I can believe that tubes could have a breakin period. Since tubes obviously do deteriate over time and eventially fail then they must have a non-linear performance curve. Therefor they must have a period of peak performance. Impurities in their vacuum. Contaminations in their atmosphere caused by the outgassing from the deteriation of their filament.

Quote:
Well to me, a non-tube user but a person with a technical understanding in electronic circuits, I can believe that tubes could have a breakin period. Since tubes obviously do deteriate over time and eventially fail then they must have a non-linear performance curve. Therefor they must have a period of peak performance. Impurities in their vacuum. Contaminations in their atmosphere caused by the outgassing from the deteriation of their filament.

From what you say Cyclebrian ( Cyclebrain? --conjures visions of delicious calf muscles ) it appears you're talking about a gradual break down, not break in. I've never heard mention of tubes breaking in and never experienced it but I'm willing to believe it's possible if someone can explain how it might happen.

Hi Gents,

Years ago I use to use some special oxide coated power tubes and it took some hours before the cathodes "broke in".

By that I mean the characteristics, cathode current actually changed and reached a peak after some few hours. I haven't really scientifically tested audio power tubes, but I think I hear differences over some time just like some others have already mentioned.

Take care and hope this helps.