Guess who’s coming to jam?

As anyone who’s ever had the good fortune of hanging out with a great rock ‘n’ roll band when they’re just goofing around, fiddling with riffs and busting out their favorite tunes amid chatter, laughs and maybe a few brewskies knows: When each player is trying to connect with the rock gods and the stars align just so, a jam can be a magical thing.

Unfortunately these mythic moments are usually reserved to the studio, and those who get to experience the serious knock-your-socks-off rock-offs often fall into one of three categories: groupies, roadies and other musicians. OK, maybe blood relatives too.

But the fact is, it’s possible to see some of the biggest names in the business jamming on any number of stages around Los Angeles. Not long ago, Keith Richards picked up a guitar at the Joint, and on another night, Brian Johnson of AC/DC picked up the mike. Just last month, Stevie Wonder stepped onstage at Zanzibar in Santa Monica.

At the Cat Club, this kind of devil-may-care atmosphere makes for a wild time at two weekly events. On Thursdays, bar owner Slim Jim Phantom (Stray Cats) and guitarist Gilby Clarke (Guns N’ Roses) rein in their pals for some cover tune action, and thanks to Axl Rose’s drop-in a couple of years ago, it’s become a high-profile jam spot.

For a less self-conscious mishmash of both undiscovered talent and established players from the worlds of pop, punk and especially metal, the humdinger is Happenin’ Harry’s Hellhole on Sundays, where ringleader Harry and his band, the Haptones (including two mesmerizing female Japanese dynamos, Mayuko and Tzusumi Okai, on bass and guitar), provide a backdrop of solid musicianship on familiar faves from the likes of AC/DC, GNR and Black Sabbath.

On any given Sunday at around midnight, Harry can be found looking into the tiny, narrow bar and summoning friends from bands including Iron Maiden, Sugarcult and Cinderella to join him on whatever cut strikes their fancy. Guitars and drumsticks are passed quite seamlessly, with the lineup rotating throughout the night.

He usually has some idea of who’ll be stopping by (so does the crowd via a weekly e-mail announcement), but sometimes it’s a surprise -- for everyone.

The set is never rehearsed or planned, even when he does know who’ll show. “We’re the most unorganized jam band there is,” says the promoter, whose been doing his night at the Cat Club for nearly four years.

His promotion was a mainstay back in the Sunset Strip hair metal heydays of the late ‘80s, then at the Central (now the Viper Room).

Harry does most of the singing, and while he has let other vocalists join him onstage (Disturbed’s Dave Draiman, Leif Garrett, regular CeCe Deville of Poison fame), this unstructured party would be total chaos if he didn’t stay onstage and make sure things ran smoothly.

“I need to orchestrate,” he explains. “I invite people up, but I have to know from prior experience that they know the material....

“You could be the bass player from Nine Inch Nails, but if you don’t know ‘War Pigs,’ and you just start bumping around on an ‘e’ string, it’s going to sound [awful] and everybody’s going to look bad.”

With a more planned (but no less exciting) presentation, nobody ever looks bad at guitarist Waddy Wachtel’s “Big Mondays.” Wachtel, a revered axeman and producer who has played for and with the likes of Keith Richards, Stevie Nicks, Warren Zevon and Adam Sandler, has been noodling up a storm at the Joint for almost five years. Famous friends including Richards, Robert Plant, Roger Daltrey, George Clinton and Joe Walsh have all stopped by to join in.

Wachtel’s core band, consisting of veteran British vocalist Terry Reid, Stones touring backup singers Bernard Fowler and Blondie Chaplin, plus bassist Rick Rosas (Joe Walsh), vocalist/guitarist Stacy Michelle and drummer Phil Jones (Tom Petty), have a more methodical style -- they do rehearse and Wachtel always writes a set list. Of course, that goes right out the window when a big name stops by.

“I certainly don’t need to make Keith Richards come to a rehearsal,” says Wachtel, who was one of the X-pensive Winos, Richards’ side project. Still, other rockers have agreed to a little practice session before the big night.

“Fred Durst came down, and we asked him to get onstage with us,” Wachtel recalls. “I knew he did the Who’s ‘Behind Blue Eyes,’ so I asked him to do it with us, but he kept hemming and hawing, so I said he could come down to our Sunday night rehearsal first and then do it the next Monday. That’s what we did and it turned out great.”

Even when celebrities don’t stop by, the night is a rip-roaring rock fest full of impressive performances. On a recent Monday, Wachtel’s rapturous fret work and a dreadlocked Fowler’s rhythmic croons and swoons made for a marvelous, soulful version of the Stones’ “She Was Hot.” Michelle, Reid and Chaplin also showed off their potent pipes during the set of, what Wachtel called, “the best songs in the world.”

But Mondays are by no means a free-for-all; in fact Wachtel even takes issue with the word “jam.” “It may look loose, but don’t kid yourself,” he says. “The people who join us onstage are heavily screened.”

Like the Cat Club, the Joint is an unassuming, small room that’s decidedly disproportional to the star power that shines within. But that seems to be the point. With a few exceptions over the years, egos are left at the door and talent reigns supreme.

Still, problems can arise in this world of big marquee names and nights. Harry’s other jam night, called Hollyweird, was at the Cat Club on Wednesdays until he moved it a year ago to the Joint, where it is thriving.

So well, in fact, that AC/DC singer Brian Johnson recently made an appearance. The night actually saw the coming together of both Wachtel’s and Harry’s happenings, as Waddy and the gang made a special appearance playing backup for Johnson, who shook the crowd all night long with his scorching wails on an arsenal of AC/DC hits.

It was fitting that the bold music of Australia’s finest would bring two of L.A.’s best, if decidedly different, jam nights together. After all, standards such as “Highway to Hell” and “Back in Black” are often the first songs a would-be musician learns. Along with gems from the Stones, Zeppelin and maybe a couple more modern favorites like GNR and Nirvana, this is the material that inspires rockers of the future -- and fuels the masters.

Notably, there’s not a lot of new material covered at either of these gatherings. “Nobody does rock ‘n’ roll in the spirit of what it was. It’s watered down now,” Wachtel muses. “What makes nights like ours special is, you get to hear your favorite songs done as good as they can be done, played by a bunch of guys who really mean it.”

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More surprises

Funk

* Afro Funke at Zanzibar: This Thursday night groove-fest often inspires soulful surprises, like last month when Stevie Wonder joined world funksters Zap Mama on congas and vocals. 1301 5th St., Santa Monica. (310) 451-2221.

* Greasy Beats at Temple Bar: Boogie band featuring members of Macy Gray’s group, the Black Eyed Peas and Breakestra plays the second and fourth Wednesdays of the month. 1026 Wilshire Blvd., Santa Monica. (310) 393-6611.

Country

* Ronnie Mack’s Barn Dance: Grand Ol’ Opry-style show features up-and-coming artists, with the Hillbilly Jam closing the night. Tuesdays at El Cid, 4212 Sunset Blvd., Silver Lake. (323) 668-0318.

Rock

* Cubensis: At the tribute to one of the ultimate jam bands, the Grateful Dead, the set lists are as free and frolicky as the crowd. Sundays at 14 Below, 1348 14th St., Santa Monica. (310) 451-5040.

* Metal Skool: It’s a tribute band, not a jam band, but you never know who might jump on stage with the wigged wonders. Past pop-ins include Steven Tyler and Gene Simmons. Mondays at the Roxy, 9009 Sunset Blvd., West Hollywood. (310) 276-2222.

* The Cocktail Shakers: Rockabilly, blues and indie gems from a rotating ensemble. Mondays at the Lava Lounge, 1533 N. La Brea Ave., Hollywood. (323) 876-6612.

Blues

* All-Star Pro Jam: Hosted by Walter Davis. Mondays at Harvelle’s, 1432 4th St., Santa Monica. (310) 395-1676.

* Babe’s & Ricky’s Open Blues Jam: Hosted by Tony Ibarra. Mondays at Babe’s & Ricky’s Inn, 4339 Leimert Blvd., Leimert Park (323) 295-9112.

* Cozy’s Pro Blues Jam: John Marx & the Blues Patrol host. Mondays at Cozy’s, 14058 Ventura Blvd., Sherman Oaks. (818) 986-6000.

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Jam picks

Big Mondays, the Joint, 8771 Pico Blvd., L.A.; (310) 275-2619.

Happenin’ Harry’s Hellhole, Sundays at the Cat Club, 8911 Sunset Blvd., West Hollywood; (310) 657-0888.

Lina Lecaro can be reached at weekend@latimes.com