Do tubes "burn in"?


My preamp (octal tubes; cathode-follower outputs) came with two NOS RCA 12SN7GT tubes and I really like the sound I get. Just for grins I picked up a slightly used pair of GE 12SX7GT (U) JAN tubes (Canada) to try. Straight off they were more prolific in the bass and more golden in the mids and highs. I enjoyed these attributes. But the more I listened, the more I noticed the vocals were pushed further forward and the clarity was missing in the mids (vocals, saxophone) when compared to the 12SN7's. I disliked these attributes. The 12SN7's seem to be more evenly balanced throughout. My questions are: Do tubes "burn in" and improve with use? And, can the negative (for me) attributes I described for the 12SX7's improve with burn in (better clarity, less emphasized vocals)?
rockadanny
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They definitely settle and smooth out after maybe 10 hours.. Just probably something with the metal getting heated and or some kinda substance burning off the plates and emiting a stronger more consistent current = sound.. Only thing I have to support that there is a DEFINATE physical change in a tube one way or the other although rather minor is, Many times when I plug a brand new tube in, even for the maybe first 5 to 10 power ups, the Puff, Peak, Pop, And fizzle a little thru the system, and even with no speakers hooked up you can hear this crackling right through the Glass on the tube itself, then all of a sudden on power up and useage they are smooth as butter and dead silent, so something is adjusting due to the applied heat I would assume.
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Just probably something with the metal getting heated and or some kinda substance burning off the plates and emiting a stronger more consistent current = sound.

That is definately plausable Undertow. It could also be that there is some dielectric interaction especially near the base of the tube. I do also agree that whatever burn-in the tube itself might go through is sonically very minor.