Chicago instrumental rock institution Tortoise’s first album of original music in five years offers “a lot of variety,” according to bassist Doug McCombs. The as-yet-untitled set is due April 21 from Thrill Jockey.

Drummer John McEntire’s “Prepare Your Coffin” has what might constitute an actual verse, chorus and bridge, but several pieces “don’t adhere to any traditional song forms. They move through different parts, but the lines are blurred.”

Meanwhile, the band’s trademark mallet instruments take a backseat this time to layers of synthesizers (“there might be two or three of them harmonizing, taking more melodic roles,” says McCombs) and “unconventional drumming and percussion.”

Several cuts were road-tested last summer, a “luxury” Tortoise usually never has. “Normally we’re making an album and then trying to figure out how to play it, and realizing that they’re some real problems,” says McCombs.

Early in the process, Tortoise was considering one long piece to constitute the whole album, “but that ended up not really being possible with the material we had,” McCombs says. “I think it’s something we want to put on the back-burner.”

Since the release of 2004’s “It’s All Around You,” Tortoise recorded a covers album with Will Oldham (“That made us aware of how we were dealing with melodies,” McCombs says) and toured performing its 1996 album “Millions Now Living Will Never Die” in its entirety (“It really wasn’t that fun to play something we made 12 years ago,” he admits).

The goal is to be back on the road this spring and summer. “Waiting until the fall would have made us a little antsy,” McCombs says.

Meanwhile, McCombs is working on a duo album with guitarist David Daniel, spawned from occasional shows they’d play in Chicago. “What he and I do is improvise, so it was really great and easy for us to just get gigs and not have to rehearse or anything,” he says. “To record it is a different proposition, though. It will take us awhile. We recorded way too much material, so now we have to sift through it.”