Do tubes "burn in"?

Davemitchell, thanks for a very cogent explanation of dielectric material forming process -- I agree that this is the way I normally think of "burn in." A different word is needed then, because whatever may be happening, in my experience the sound of a tube will change over the first 10-30 hours of playing time.
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tubes have no 'bell curve'

I believe the question is excellent, but the listening comparison behind the question is flawed. You were comparing two different tubes from two different manufacturers. These will inherently sound different. The same tube from different manufacturers will sound different, and the fact that these were not the same tube types exacerbates the situation.

Tubes do a slow burn-out during their lifespan, so it stands to reason they "burn in", but the change occurs gradually over time. It will be impossible for most listeners to tell a difference from day to day, or week to week, but if one was able to compare the sound today with the sound several months from now a difference could very possible be heard.

I'm certain the goal for most premium tubes was to make any change imperceptible over the tube's lifespan.

IMO.

I've found that tubes don't really benefit much from burn-in. The first few hours might have a slight bump up in emission, but otherwise they don't seem to change much. And like the other guys said, it's a long road downhill from there. Fortunately, this can be very slow.

You'll hear much more effect by removing the tube and plugging it back in. In other words, cleaned contacts are significant.

jh

My concern was not that after burn in the tubes would sound the same. I was just seeing if I liked the 12SX7's at all. They sounded different than the 12SN7's, as expected, but I did not expect them to have such poor clarity, especially noticable with jazz, acoustic, and vocals. But, as stated, they need to warm up ... really warm up. Maybe obvious to most people, but it really was not to me. My stock equipment sounds pretty much the same whether warming up for 10 minutes or 1 hour, so I was not used to allowing for more warm up time. After warming up the 12SX7's for a complete hour, they sounded way better than before. Now I like them, although they are not as clear as the 12SN7's, after proper warm up their clarity is acceptable, which is good because I like their other attributes: better imaging; a little tubier in the upper and lower freqs. Jimi Hendrix and Rory Gallagher never sounded so good. Thanks guys for all of your explanations and insight.