Correction: An earlier version of this article incorrectly reported that Mr. Auldridge was a 1967 graduate of Wheaton High School. He was a 1967 graduate of the University of Maryland.

Mike Auldridge, a bluegrass musician whose broad knowledge of many musical forms helped redefine and modernize the steel guitar known as the Dobro, died Dec. 29 at his home in Silver Spring. He died a day before his 74th birthday.

He had prostate cancer, said a daughter, Michele Auldridge.

Mr. Auldridge was a founding member of the Washington-based bluegrass group the Seldom Scene and, in a career spanning six decades, he recorded with Linda Ronstadt, Lyle Lovett and Emmylou Harris, among others.

He was renowned for his mastery of the Dobro, a guitar with a metal resonator instead of a sound hole. The Dobro, a trademarked name for a resophonic guitar, is held flat and played with a slide over the strings. Unlike other types of steel guitars, it does not rely on electric amplification. The resonator functions as an amplifier and gives the instrument a distinctively warm tone.

Although the instrument was popular in bluegrass music — Dobro player Josh Graves was a featured soloist in the group Flatt and Scruggs — Nashville musicians regarded its sound as clunky and archaic.

Mr. Auldridge’s work in bluegrass helped change that perception as he borrowed ideas from other musical idioms, including blues, jazz and rock, and helped design and pioneer a model of the instrument with eight strings instead of the usual six.

“He phrased differently,” Jerry Douglas, a Dobro player with Alison Krauss & Union Station, said of Mr. Auldridge in a 2011 Washington Post interview. “He was the first guy to use the Dobro in a more modern way, to phrase it more like a saxophone or some other instrument.”

In 1971, Mr. Auldridge formed the Seldom Scene with banjoist Ben Eldridge, guitarist John Starling and two former members of the Country Gentlemen, mandolin player John Duffey and bassist Tom Gray.

The name Seldom Scene was an inside joke, reflecting the fact that all the members were working day jobs. Mr. Auldridge was a graphic artist for the old Washington Star newspaper.

“We initially had three restrictions on what we would do with the Seldom Scene,” recalled Gray, the bass player. “We would only play one night a week, festivals on the weekends and would only record when we were ready. We would not tour, we would not have a band vehicle. It worked well for those of us who had to keep our day jobs.”

With a style often described as “newgrass,” the group broadened the standard bluegrass repertoire with selections from contemporary folk singer-songwriters Bob Dylan and Steve Goodman, rockers such as Eric Clapton and the group’s songwriter, Starling.

“We liked James Taylor as much as we liked Ralph Stanley,” Mr. Auldridge once said. “And we attracted an audience of like-minded people. We were contemporary and urban. We weren’t singing about mother and log cabins because that’s not where we came from.”

The group first held a Tuesday night residence at the Red Fox Inn, a rowdy neighborhood bar in Bethesda. As their following increased, they started work at the Birchmere in Alexandria, a restaurant with a sound system and posted signs that discouraged talking during performances.

“Any artist who came through town came by to see them,” said Birchmere owner Gary Oelze. “John Prine, Emmylou Harris, Vince Gill — they would all stop by and sit in.”

Harris in particular was taken with the group’s musicianship and their blend of voices.

“You had two incredibly distinct voices that you’d never think would go together,” she told The Post in 2006. “Duffey, one the great classic bluegrass tenors and John [Starling], who is one of the most subtle, soulful, almost like a pop voice. But their voices are so different, but then you’ve got that cement of Mike’s voice, which has a beautiful tone to it but a certain invisible quality that ties it all together with just enough texture — it’s just like no other sound.

“You know, like bluegrass has that really hard sound,” she added, “but somehow they softened it but it still had all that power because it was just so unique.”

Mr. Auldridge recorded several solo albums. “Blues and Bluegrass” (1974) featured guest appearances by Ronstadt and guitarist Lowell George of Little Feat, and included Mr. Auldridge’s instrumental interpretation of “Killing Me Softly,” a song popularized by Roberta Flack.

Mr. Auldridge’s “Eight String Swing” (1981) featured such swing standards as “Stompin’ at the Savoy” and “Caravan” performed with bluegrass instrumentation.

Michael Dennis Auldridge was born Dec. 30, 1938, in Washington and grew up in Kensington. He was a graduate of Wheaton High School and attended art classes at the Corcoran College of Art and Design while in his teens. He received a bachelor’s degree from the University of Maryland in 1967.

Although his father was a banker, Mr. Auldridge could claim a family connection to the music profession. His uncle, Ellsworth T. Cozzens, a steel guitarist who performed and recorded in the 1920s with country singer Jimmie Rodgers, inspired Mr. Auldridge’s interest in music. Mr. Auldridge started out on guitar and later banjo before taking up the resophonic guitar.

As teenagers, Mr. Auldridge and his older brother Dave,now deceased, a guitarist and mandolist, formed a bluegrass group, the South Mountain Boys, and performed on the Washington area radio station WDON in the mid-1950s. Michael Auldridge first recorded in 1969 with the group Emerson and Waldron and the New Shades of Grass.

In the mid-1990s, when the Seldom Scene’s leader Duffey decided to limit the group’s touring, Mr. Auldridge formed another group, Chesapeake, with bassist T. Michael Coleman and guitarist Moondi Klein — then current members of the Seldom Scene — and mandolist Jimmy Gaudreau. Mr. Auldridge left the Seldom Scene in 1996, although he sometimes performed in reunion shows.

In more recent years, he played with Harris, Lovett and bluegrass singer Darren Beachley. Mr. Auldridge, Gray and Starling were also reunited in the past decade under the band name John Starling and Carolina Star.

This year, Mr. Auldridge completed a trio album with fellow Dobro players Douglas and Rob Ickes and was a recipient of the National Endowment for the Arts National Heritage Fellowship.

Survivors include his wife of 51 years, Elise Fox Auldridge of Silver Spring; two daughters, Michele Auldridge of Madison, Wis., and Laura Auldridge of Malibu, Calif.; two brothers, Gene Auldridge of South Carolina and steel guitarist Thomas Auldridge of Wheaton; and one granddaughter.

In addition to the Dobro, Michael Auldridge was also accomplished on the lap steel and pedal steel guitars. Although he often came to Nashville and Los Angeles to record, he never became a studio musician, preferring to stay in Maryland.

“We converted a lot of people who thought they didn’t like bluegrass,” he told The Post in 1981, “but they liked what we were doing. That was our greatest contribution.”