I'm not sure if this tube is exactly the same as the JJ 12AX7A. According to my information it is (same factory, just a name switch a couple of years ago), some state that they do hear differences between the Teslovak and the JJ. Anyway, it's supposed to be a near exact copy of the Telefunken ECC803, a tube I've never seen or heard, but the Teslovak surely looks like the Telefunken's mechanical description I was given (ECC88 alike). And the sound? Well, it has some nice transparent highs, especially when playing lead and clean. The lead has that nasal sound, but just like the Brimar it lacks some gain for playing power riffs. The rhythm sounds a little hollow.
Information from http://www.fbk.eur.nl/BIT/MHM/gear/preamptubes.html
