12ax7 vs 5751

I agree with Rccc that the only way to really know is to experiment--tube performance is very specific to the particular application, and in addition, there is the matter of individual taste. This can be, unfortunately, quite expensive. The advice of any one user, no matter how many types of tubes were tried, only apply to that particular user's gear and taste.

That said, there are certain tubes that "tend" to be acceptable in a broad range of applications and to a broad range of listener taste. The 5751s discussed above fall in that category. I would also put in that category Amperex Bugle Boy 12AX7s.

I have found, for my particular application and taste, the Telefunken ECC803S to be the best, but, that tube can be excessively detailed, lean and "analytical" in the wrong system (it has also become crazy expensive). I took a big gamble with getting those, because I did not have an opportunity to hear them before purchasing (a friend lent me a BIG selection of 5751s and 12AX7s to try, including Telefunken ECC83s, but no ECC803S).

Have fun, just don't go broke in the process.

If clarity, 3d soundstaging, and dynamic punch is your thing, the ultimate tube for those qualities, to me, is the Telefunken ECC803S. But, it can be, in the wrong application, quite lean and analytical sounding (bass is very tight and fast, but not full sounding). It also has a reputation for having an extremely long usable life (something I am counting on).

Also, because of the expense, there are fakes floating around. Supposedly, the fakes are tubes with similar looking internal structures that have been given new markings on the glass. I have not heard of any fakes having the correct embossed in the glass Telefunken diamond on the bottom of the tube, so that might be the way to tell.

I have heard that some Tesla ECC803S tubes are also quite good, but, there are some newer Tesla ECC803S that are not that good and have a much shorter life than the Telefunken tubes (I have no personal experience with the Tesla tubes).

My phonostage came from the manufacturer with 1960's RCA 12AX7s that are also on the punchy and clear side of the spectrum (as opposed to warm and rich sounding). They were decent, but I prefer both the Telefunken 12AX7s and the ECC803S.

Yes, as a general observation, British tubes do deliver a warmer sound. There are makes of other nationalities that will also do the same. For a warmer sound, I like Amperex Bugle Boys (Netherlands). These tend to work reasonably well in all applications.