cool info on EVH tuning..
Though rarely discussed, one of the most distinctive aspects of Van Halen's sound was Eddie Van Halen's tuning of the guitar. Before Van Halen, most distorted, metal-oriented rock consciously avoided the use of the major third interval in guitar chords, creating instead the signature power chord of the genre. When run through a distorted amplifier, the rapid beating of the major third on a conventionally tuned guitar is distracting and somewhat dissonant. Van Halen developed a technique of flatting his B string slightly so that the interval between the open G and B is a perfect, beatless third. This consonant third was almost unheard of in distorted-guitar rock, and allowed Van Halen to use major chords in a way that mixed classic hard rock power with "happy" pop. The effect is pronounced on songs such as "Runnin' With the Devil", "Unchained", and "Where Have All the Good Times Gone?".
With the B string flatted the correct amount, chords in some positions on the guitar have perfect thirds, but in other positions the flat B string creates terribly out of tune intervals. Van Halen is quoted as saying, "...the guitar... is just theoretically built wrong. Because every string — the intervals are fourths, except for from G to B, which is a third, and it's always that damn B string that fucks it up. So I always tune it a little bit flat, and then when I need it in tune, I just bend it up. Because once it's sharp, you can't make it flat! Over the years, you know, it's just a feel thing, you develop a feel for when you hit a certain chord, you know how to manipulate the string to make it in tune."
Despite his wording above, Van Halen does not flat the B string for everything. "The B string is always [difficult] to keep in tune all the time! So I have to retune for certain songs. And when I use the Floyd onstage, I have to unclamp it and do it real quick. But with a standard-vibrato guitar, I can tune it while I'm playing." (Here he was referring to an early version of the Floyd Rose system, which had no fine tuners on the bridge.)