For any of you guitarists that use tube amps out there... I'm really fortunate that my girlfriend's brother is not only a really cool guy, but he also owns an amplifier repair place. This week he let me borrow some Mullard 12ax7 preamp tubes to try out, from his "private stock." Granted, I'm not the kind of guy with the picky golden ear that can discern the type of tube in an amp by just hearing... I wanted to see how much of a difference these things would make in amps I was totally familiar with. I currently have 2 50w guitar heads, one is a 1978 Marshall master volume with 6550s in the output section and a 1981 Park master volume with EL34s in the output. The Marshall is usually run with Svetlana 6550s and Sovtek 12ax7s, and the Park with Electro Harmonix 12ax7s and either EH EL34s or blue Teslas. I tried the 3 preamp tubes in each position in each amp and swapped the Teslas and the EH tubes in the Park. I ran them through the same 1977 Marshall 4x12 cabinet with 25w Blackbacks and for guitars, used a 59 LP Special, a 1965 Epi Olympic with a P-90, and an 01 58RI LP. As far as how much of a difference they make... They may make the amp a bit sweeter and smoother (both clean and overdriven), but there's no way I'd justify spending $30-$80 a tube for any of the percieved difference I heard or felt. I really wanted to hear a marked difference- but in all honesty, after tinkering all afternoon, I just don't think it makes that much of a difference.