LOS ANGELES — Phil Lesh, the classically trained fiddler and jazz trumpeter who, as a founding member of the Grateful Dead, found his true calling in reinventing the role of the rock bass guitar, died Friday at the age of 84.

Lesh’s death was announced on his Instagram account. Lesh was one of the oldest and longest-surviving members of the band that defined the acid rock sound that emanated from San Francisco in the 1960s.

“Phil Lesh, bassist and founding member of The Grateful Dead, passed away peacefully this morning. His family was with him and full of love. Phil brought great joy to everyone around him and leaves behind a legacy of music and love,” the Instagram description reads in part.

The statement did not give a specific cause of death, and attempts to reach representatives for additional details were not immediately successful. Lesh had previously survived bouts with prostate cancer, bladder cancer and, in 1998, a liver transplant necessitated by the debilitating effects of a hepatitis C infection and years of heavy drinking.

Lesh’s death comes two days after MusiCares named the Grateful Dead Person of the Year. MusiCares, which assists music professionals in need of financial or other assistance, also cited Lesh’s Unbroken Chain Foundation among other philanthropic endeavors. The Dead will be honored at a charity gala ahead of the Grammy Awards in Los Angeles in January.

Although he maintained a relatively low public profile, rarely giving interviews or speaking to audiences, fans and band members recognized Lesh as a critical member of the Grateful Dead, whose booming lines on the six-string electric bass provided the perfect counterpoint to the lead guitarist. It foreshadowed Jerry Garcia’s soaring solos and the band’s famous marathon songs.

“When there’s Phil, there’s the band,” Garcia once said.

Drummer Mickey Hart described him as the intellectual of the band, bringing the mentality and skills of a Classical composer to a five-chord rock ‘n’ roll band.

Lesh credited Garcia with teaching him to play bass in an unorthodox lead guitar style, mixing booming arpeggios with spontaneously composed orchestral passages.

Fellow bassist Rob Wasserman once said that Lesh’s style sets him apart from other bassists he has known. While most people are happy to take their time and play the occasional solo, Wasserman said Lesh is both good enough and confident enough to lead fellow musicians through the melody of a song.

“He plays bass, but he’s more like a horn player, doing all these arpeggios — and there’s this counterpoint going on all the time,” he said.

Lesh began his long musical journey as a classically trained violinist with third grade lessons. He started playing the trumpet at the age of 14 and eventually won the second chair in the Oakland Symphony Orchestra of California while still in his teens.

But he had largely abandoned both instruments, and in 1965 he was driving a mail truck and working as a sound engineer at a small radio station when Garcia recruited him to play bass in a fledgling rock band called The Warlocks.

When Lesh told Garcia he didn’t play bass, the musician asked, “Didn’t you use to play violin?” he asked. When he said yes, Garcia told him, “That’s it, man.”

Armed with an inexpensive four-string instrument his girlfriend had bought him, Lesh sat down with Garcia for a seven-hour lesson, following Garcia’s advice to tune his instrument’s strings an octave lower than the lower four strings on Garcia’s guitar. Garcia then released him, allowing Lesh to develop the spontaneous style of play that he would adopt for the rest of his life.

While Lesh and Garcia would often spontaneously exchange leads, the band as a whole would often break into long experimental, jazz-influenced songs during concerts. The result was that even well-known Grateful Dead songs like “Truckin'” or “Sugar Magnolia” were rarely performed the same two performances in a row; this was something that would inspire loyal fans to tune in after show.

“It’s always fluid, we figure it out pretty much on the fly,” Lesh said with a chuckle during a rare 2009 interview with The Associated Press. “You can’t fix these in the rehearsal room.”

Phillip Chapman Lesh was born on March 15, 1940, in Berkeley, California, the only child of office equipment repairman Frank Lesh and his wife, Barbara.

In later years, he would say that his love of music came from listening to broadcasts of the New York Philharmonic on his grandmother’s radio. One of his earliest memories was listening to the great German composer Bruno Walter conduct this orchestra in Brahms’s First Symphony.

The musical influences he often cited were not rock musicians, but composers such as Bach and Edgard Varèse, as well as jazz greats such as John Coltrane and Miles Davis.

When he arrived at the College of San Mateo, Lesh switched from classical music to cool jazz, eventually becoming the first trumpeter in the school’s big orchestra and the composer of many of the orchestral works performed by the group.

But after college, he put the trumpet aside and concluded that he did not have the lung strength to be an elite musician.

Shortly after taking up bass, The Warlocks renamed themselves the Grateful Dead, and Lesh began wowing audiences with his dexterity. Crowds gathered in what became known as the “Phil Zone” just in front of his position on the stage.

Although he was never a prolific songwriter, Lesh also composed and sometimes sang the music for some of the band’s best-loved songs. These included the upbeat country-rocker “Pride of Cucamonga,” the jazz-inspired “Unbroken Chain” and the ethereally beautiful “Box of Rain.”

Lesh composed the latter on guitar as a gift to his dying father, and recalled that Grateful Dead songwriter Robert Hunter approached him the next day with a lyric sheet after hearing the instrumental recording. He said that the paper contained “some of the most touching and heartfelt lyrics I’ve ever had the chance to sing.”

The group often closed its concerts with a song.

After the band disbanded following Garcia’s death in 1995, Lesh often skipped joining the other surviving members when they got together to perform concerts.

He joined the Grateful Dead tour in 2009, and in 2015 he attended a handful of “Fare Thee Well” concerts celebrating the band’s 50th anniversary, as well as concerts that Lesh said would be the last time he played with the others.

But he continued to play frequently with a rotating lineup of musicians he called Phil Lesh and Friends.

In later years, he often performed these performances at Terrapin Crossroads, a restaurant and nightclub he opened near his Northern California home in 2012, named after the Grateful Dead song and album “Terrapin Station”.

Lesh is survived by his wife Jill and sons Brian and Grahame.