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.
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)?