As one of the lesser sages of rock 'n' roll once sang, only the good die young. Ponder that in light of recent releases by Jimi Hendrix and the Grateful Dead, two of the searing musical phenomena of the late '60s. There's one thing nice about early departure: When the good die young, they don't have a chance to get bad.

The Grateful Dead, of course, are still quite alive. Immorality, though, has its drawbacks. True, in concert performances, the Dead have largely maitained their interstellar quality -- and if things get slow, it's off to the Great Pyramids for a musical confab with the spirit of Ra. But in the studio, problems abound.

After literally dozens of group and solo recordings, the Dead are pressed to come up with something new, something that will please old-time Dead Heads as well as their new legions of teen-age fans. On "Go to Heaven" (Arista A19508) they say to heaven with it -- and wind up with a cheerful potpourri that pales in comparison with previous efforts. Pales so much that it's almost transparent, like the smoky haze and the white suits (suits!) the Dead boys wear on the album cover.

There are a few great moments in "Heaven," and they come in the two songs co-written by Jerry Garcia. He might look like some old acidhead who runs a T-shirt booth in a flea market but Captain Trips still handles his guitar like a master. He is truly the Hoyt Wilhem of rock -- his fastball long gone, he dazzles by position and varying speeds. His contributions here, with lyrics by the wonderfully cryptic Robert Hunter, include a peppy but lightweight "Alabama Getaway" and the record's high spot, "Althea," where Garcia croons, "Nobody's messing with you but you," a characteristically Dead sentiment.

The rest of the record strikes a compromise with the times, and doesn't sit right at all. Bob Weir, who gets more hairblown and macho as he ages, chips in four songs that bog down in excessive buoyancy. Worst of all is the newest Dead member, keybordist Brent Mydland. His two songs are wellcrafted in the Dan Fogelberg mold; his voice hits a castrato that Don Henley would be proud to claim. On a Grateful Dead album, this fluffy tripe is embarassing. Meanwhile, the original keyboardist, Pigpen, is whirling dervishly in his grave.

Which brings us to Jimi Hendrix (1942-1970). He has released more albums posthumously than he did when he was among us. (We will disregard the small but insistent group which insists that Hendrix did not die, but was borrowed by interplanetary invaders seeking a good guitar teacher.) Most useful were the two "best-of" albums released by Hendrix's active musical executor, Alan Douglas, under the titles of "The Essential Jimi Hendrix." eThe current record is called "Nine to the Universe" (Reprise HS 2299). A better title would be "The UNessential Jimi Hendrix."

The liner notes to "Nine" inform us that it is made of selections edited from off-hour sessions Hendrix did with various musicians in 1969. They were never intended to be released, and if they sound rough, or aimless, well, that's because Jimi was JAMMING, stupid! Needless to say, the artist was not consulted on the decision to market these five selections.

Is the music valuable? Yes -- to Hendrix scholars and stone fanatics. Even to an uninformed ear, "Nine" makes it obvious that Jimi Hendrix could play guitar like no one else: He bent his notes until they scraped the universe and then he slowed them down and made his Telecaster sing voodoo incantations. Uninformed ears, though, would do better to listen to the more controlled excitement of "Are You Experienced?" or "Electric Ladyland," where his solos are structured into statements, and his underrated vocals add to his still-growing mystique.

Unfortunately, the call for new products takes precedence in these cases, and instead of listening once more to the certified masterpieces, we must consider new music from even long-departed friends. The real value of "Nine to the Universe" is not, as the liner notes presume, and indication of what Hendrix might have done had he lived -- it is to send us back to what he did do. We need look no further than the Grateful Dead's current effort to see that as musicians get older, they do not necessarily get better.