I'm confused by the description "Mullard" which I assume is either a design or manufacturer... also the "long plate"... what's the meaning sound wise in that?

Mullard was a British manufacturer of vacuum tubes for many, many years, with a history going extending all the way from the 1920's to the late 70's. http://en.wikipedia.org/wiki/Mullard

Can't believe there's such a cachet about the 12AX7. If I remember right, it was the audio output stage on the KnightKit shortwave receiver I built as a 10 year-old kid back about 1955.

We called them "valves" not "vaccuum tubes" in case the wikipedia article is not clear.

Around these parts, the term "tube" (pronounced "chube") is a mild derogatory now, so youtube.com caused some amusement at first.
Apologies for jumping in with useless information.

Mullard was a manufacturer, as already noted, so you're talking "NOS" or new old stock tubes of substantial age, which doesn't hurt anything if they haven't lost vacuum and are unused.

"plate" refers to one of the internal parts of the tube, and since it's visible, no doubt it became a lay nomeclature to call a specific version of a tube (as designs could change over production cycles and time) by a visible physical characteristic. For example, "Silver specials" happened to have bright silver internal components, easily visible.

Tubes are funny animals, while old and outmoded in theory and general SS practice, they process sounds differently from fine tuned sand (silicon chips), thus they have a mystique about them. When I really got into amp tweaking, I found that you COULD get much better tonality through careful tweaking and choice of components, IF you knew what you were doing.

I also found that as tubes aged (which happens fairly rapidly as they are high voltage devices running at high temperatures, not to mention heating/cooling cycles and there are electrons physicaly moving between components) you begin to lose the magic. Preamps last longer than power tubes, but still lose something over time. So in some respects, it became a frustrating process to keep amps in tip top shape and sounding their best. OTOH, for studio use and critical applications, the difference was well worth it.

Tube gear is sort of like those cool old muscle cars - if you like tinkering all the time and staying on top of upkeep, it can be VERY satisfying... but it's not for everyone!

For whatever reason the 12AX7 is the one (preamp) tube design that has survived - with perhaps the 12AT7 (although I recall those becoming nearly impossible to acquire). Probably has to do with it's use in so many preamp sections of amplifiers, and the "rock and roll" cachet that goes with names like Marshall Fender and Vox as well as many others. I scratch my head when I hear about tubes in full spectrum audio as I consider a tube to be a rather "lo-fi" device as tubes distinctly color the sound, but then again... sometimes lower fidelity and tonal coloration are what it's all about, and actually sound better/smoother/less harsh to the listener!

Another "important" use of phase reversal is to create karaoke versions of recordings. The vocal track is usually positioned at the center of the stereo image. Reversing the phase of one channel causes the central information to be fully cancelled leaving just the instruments.

Thanks all... helpful replies, every one... and the Karaoke trick is one I'll have to experiment with!

If you have two mic's on a snare drum. One above and one below both alined vertically but in different directions... one pointing up and one down, both aimed at the drum heads you will hear a difference by inverting the phase of the bottom (pointing up), mic. That's one use of the switch. Double micing from opposing directions. The phase switch swaps the electrical polarity.