I use a matched set of 5751 NOS RCA from 1959 in my Belles 21A preamp. I went to them from Mullard 12AX7. I found no audible difference in gain with my system. I found the RCA's to provide a little more air and detail between instruments, a little more depth of soundstage and very nice extention and tightening of bass. I got these from Vintage Tubes and have been very happy with the upgrade.
12AX7 vs. 5751
I'm getting my first tube pre (Counterpoint 7.1) this week or early next week, and of course I have start fiddling with it ;-) I've been reading Joe's Tube Lore (http://www.audioasylum.com/audio/faq/joes-tubes.html) and the 5751 caught my eye, however I see statements like "lower gain that the 12AX7" and lower "mu" (what is mu?). Anyways could I use the 5751 tubes in my Counterpoint without causing any damage? altavistaaudio.com mentions nothing about the 5751.
thanks
thanks