vintage '50s 12AX7: Mullard, Amperex BB, Brimar

Would appreciate general characterizations of sound character compare/contrast of these tubes in a preamp. Thanks!

Generally speaking the Amperex would be more lively and closer to neutral than the Mullard/Brimar...The Mullard /Brimar tubes would have more Bloom and a bit warmer all around than the B/Boys....Depends on what sound your looking for..Personally I like the Amperex ( Herleen ) BB best of the three.......
My favorite 12AX7 is the Amperex BB Long plate G-getter. For details regarding its sonic virtues take a look at my GON review on the AA reference Hybrid Tube DAC. My second favorite is the Mazda which I also reviewed, so you can get the information on this wonderful sounding 12AX7 also.
You might find this helpful. I have experimented with a number of 12ax7's and have settled into the Brimar CV4004's. As you'll read through many of these threads it really ends up being your ear that drives your final decision. Best of luck.
I really like the Mullard CV4004's. Indeed, all of this is personal preference and what tube works best in that piece of gear's circuit design.
Theses are very helpful. Much appreciated.
I have never tried the Brimars, but have various iterations of the Mullard and Amperex 12AX7. I am currently enjoying a pair of RCA 7058, which is a ruggedized 12AX7 variant designed for automotive use! The heaters are set up a bit differently, with pin 9 to ground, so they will not work in all circuits. The sound is very dynamic and quite smooth, with a nice balance of detail and drive.