It was a moment that signaled everything had changed. Like John Coltrane's dizzying sheets of sound on "Giant Steps" or the acid fuzz attack of Jimi Hendrix's "Purple Haze," this musical moment was a line in the sand.

It was called "Eruption," not quite two minutes of six-string pyrotechnics two tracks into Van Halen's self-titled 1978 debut album — a hard rockin' fanfare before a blistering version of the Kinks' "You Really Got Me."

With its Hendrix-inspired dive-bomb runs, blues bends copped from Jimmy Page, the sexy swagger of Tommy Bolan and a neo-Baroque technique known as fret tapping that virtually no one had heard before, "Eruption" defined rock guitar for the next half generation.

And to think a composition of such monumental influence was conceived and performed on an instrument that cost less than $200 and was built in a garage on borrowed tools.

To say it is one of the most recognized and influential instruments in rock 'n' roll is putting it mildly.

But as in the case of James Burton's Telecaster or Roger McGuinn's Rickenbacker 12-string, it was the young Van Halen's spirit and technique that made what came out of the instrument sound like bottled fire.

With that guitar, plugged into a wall of 100-watt Marshall amplifiers played through a somewhat dangerous device known as a variac, which starves the amplifier's tubes of proper voltage, Eddie defined his searing, full-bodied tone. He called it the "brown sound."

Van Halen built the guitar in 1974. The body, a cheap Fender Stratocaster knockoff machined by San Dimas, Calif.,-based Charvel, was unfinished bare wood when he bought it. The neck, also from a Charvel pile of "seconds," had been deemed unsuitable for sale.

He took apart a Gibson humbucking pickup, rewound the hundreds of copper wire turns around the magnet by hand for a bigger tone, then dipped it in paraffin wax to keep it from becoming microphonic, a phenomenon that occurs at high volumes when the pickup begins to oscillate and sound generally out of control — the kind of thing Neil Young goes for on purpose.

The pickguard was cut from a blank using tin snips, the edges rounded out with a soldering iron. (What a stink that must've made.) After assembling the instrument at home, Eddie painted it glossy black, then added several overcoats of white atop masking tape laid out in laser-stripe patterns over the body to preserve portions of the original black underneath.

The resulting geometric image was a harbinger of 1980s design. It also looked killer in photographs and in the ever-important realm of music videos. He played it on the albums "Van Halen" and "Van Halen II," on memorable tracks such as "Ain't Talkin' 'Bout Love" and "Running With the Devil."

And though by the early 1980s, Eddie was insanely rich and able to afford any guitar he wanted, he would return to his homemade axe time and again.

He painted it with an overcoat of red, added garish truck reflectors, removed the pickguard altogether to reveal the instrument's electronic guts and added an innovative device called a locking tremolo system, designed by guitar whiz Floyd Rose to prevent the strings from sliding out of tune every time the whammy bar is used.

In the ensuing years, fans dubbed the instrument "Frankenstrat," or simply "Frankenstein." It can be seen in countless videos, publicity photos and stage shots taken throughout the band's history.

Eddie Van Halen helped to launch the do-it-yourself guitar ethic. Today, the Internet is flooded with sites devoted to modifying guitars. Gnomelike sound effects gurus find expensive old echoes, fuzzes and phasers, trace their signal paths and decode part specs, then post them online for others to rebuild and tweak.

In the 1970s, though, the circle of garage guitar builders was small: basically Van Halen, engineer Craig Anderton, who published the first and still-definitive book on the subject, Pete Cornish and a couple of West Coast roadies in between jobs. Now, it's everywhere.

Aaron Nelson, a well-known effects guru and administrator of diystompboxes.com, one of the most heavily trafficked music electronics Web sites, said Van Halen's influence is everywhere in the DIY community.

"Eddie was an innovator," Nelson said. "He was able to experiment and think outside the box at that time. His tone and his feel still remain some of the best recorded tone to this day. His use of the Floyd Rose and the modifications he made to his guitar caused an entire generation to try and copy him."

Guitar magazines now carry ads from companies that sell preassembled guitars and aftermarket parts like the ones Eddie connived to obtain a generation ago.

"Eddie has a great ear," Nelson said, "and he knew enough to get himself a great tone."

When Van Halen launched its first tour with David Lee Roth, Fender's custom shop released a limited edition reproduction of Eddie's homemade guitar. The reflectors, the cigarette burn marks on the headstock, the unused screw holes, the nicks, the dings, the worn finish, the grime accumulated on the controls, all were simulated from high-resolution digital photographs.

When Eddie got his first copy for approval, he had to write on the back of the neck so he could tell it from the original.

Replicas of the guitar Eddie built as an impoverished youngster sold for $25,000.