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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

Showing 2 responses by rushton

rushton's avatar
rushton
2,445 posts
 
A brand new tube will "burn in" over the first 10-30 hours of use and its sound will change over that time. Thereafter, Pawlowski6132 is exactly correct about the warm up time with each listening session. Whether the burn in will resolve your concerns with this particular tube is an unknown -- my guess is that the general characteristic you've noted will not change much.
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rushton's avatar
rushton
2,445 posts
 
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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