MANSFIELD - When describing an Allman Brothers show, it may be just as useful to start by describing what you won't get.

You won't get a steady stream of decades-old hits, sure to please casual fans looking for a blast of nostalgia. That may be a winning formula for many so-called classic rock bands, but it's not the direction the Allman Brothers take.

What you will get are plenty of lengthy jams, some improvisation, a few surprises and a display of superior musicianship from a talented group of performers, carrying on the spirit of a band that's been around for almost 40 years.

So it was Saturday night at the Comcast Center, where the Allmans made their (more or less) annual stop. It was one of their first shows after a lengthy hiatus, made necessary by Gregg Allman's battle with hepatitis.

The singer/keyboardist/guitarist and founding member seemed to be in fine form Saturday night, as did the rest of the band, as they kicked off the two-hour set with "Statesboro Blues," followed by "Soul Shine." The music throughout the night was a mix of blues, jazz and rock, combined with a backdrop of psychedelic imagery (with appearances by the band's trademark mushroom, as well as a Georgia peach). A one-song appearance by New England blues artist Susan Tedeschi was a pleasant surprise.

The Allmans also like to throw a few covers into the mix. On this night, it was Van Morrison's "Into the Mystic."

Gregg Allman shared the spotlight with guitarist/vocalist Warren Haynes, who was on fire this night, as was young guitar ace Derek Trucks. Two percussionists and a guest saxophonist also complemented the sound.

Gregg grabbed an acoustic guitar for a slowed-down version of "Melissa." But the real jamfest, giving each member of the band some time in the spotlight, was the instrumental classic "In Memory of Elizabeth Reed."

The hard-rocking "Whipping Post," wrapped things up as the encore.

The Allmans' jam-heavy shows may not be everyone's cup of tea. But the band manages to keep things fresh for themselves and their dedicated fans, probably the reason the band enjoys a dedicated, cult-like following after so many years of touring.

Bob Weir and RatDog

Speaking of band's with cult-like followings, the opening act was RatDog, the band fronted by former Grateful Dead guitarist Bob Weir.

It might be more accurate to call Ratdog a co-headliner, as they played a set equal in length to the Allman Brothers. A substantial portion of the audience was there to see Ratdog, judging by the number of tie-dye clad fans swaying to their music.

Ratdog was just as jam-heavy as the Allmans - no surprise for a Grateful Dead spinoff band. The band also put its own spin on a couple of Bob Dylan songs: "A Hard Rain's A-Gonna Fall" and "Maggie's Farm." And one of the highlights of the set was a rousing cover of The Beatles' "Dear Prudence."

Peter Noll can be reach at pnoll@thesunchronicle.com.