I'd like to go back / To that dear little tumbledown shack, / Yes, I'd like to spend the day / Where in childhood I used to play. / But time won't stand still, / We must travel till Jesus will call, / Then we'll be happy, / In land where no cabins fall ...

The words and music drift out of the magic little box by the bed. The mind sputters to life. It must be the weekend. It must be morning. It must be Sunday. It must be bluegrass.

Thousands of radios from Fredericksburg to Martinsburg, W. Va., Washington to Gettysburg, are similarly lighting up. People are giving up the right to their weekend sleep so they can celebrate mountain music with that local guru of bluegrass, Gary Henderson.

Talk about folksy. Henderson raises folksy to high art. A day never passes when he doesn't manage to start a few records at the wrong speed, leave a half-minute of dead space between songs or dissolve into gales of laughter after inserting the wrong tape into the wrong deck and getting an advertisement for "All Things Considered" instead of Jim and Jesse singing "I Wish You Knew."

"Heck," says Henderson, "they don't pay me enough to worry about things like that. I'm having a good time and I think it comes across on the air."

The portly Henderson, good-naturedly nicknamed "the round mound of sound," by his listeners, first started broadcasting bluegrass on WAMU-FM 12 years ago. Since then it's been nothing but burgeoning success.

In 1967 he and Dick Spotswood talked American University into giving up half an hour a week at 11 p.m. for a dignified bluegrass show. "The station was very serious in those days," said Henderson. "No fooling around."

One day the station managers wondered how response to the bluegrass show was going. Henderson said he didn't know. The next show he put out an appeal for mail. "We were buried," he said.

The half-hour was expanded to three hours on Saturday morning. Then it went to 8 to noon Saturday. Then a 1 1/2-hour gospel show, "Stained Glass Bluegrass," was added on Sunday mornings.

Now WAMU-FM, buoyed by tremendous bluegrass support when it makes its annual appeal for money donations -- in the area of a third of the total, according to Henderson -- is the bluegrass station in the mid-Atlantic states, running more than 20 hours of the high, lonesome sound a week.

The bottom line in all of this remains Henderson, whose weekend morning shows from 8 to noon at 88.5 on the dial are required listening for bluegrass followers.

One reason his show is popular is theat he has the music to make it work. He lives alone in a small brick house in Silver Spring. In that house is the most colossal collection of recorded bluegrass imaginable. There are records and tapes piled high against the walls, scattered on the floor, feet deep in the basement.

Henderson, whose weekday job is as the No. 1 remote transmission technician for National Public Radio here, is a recording freak. He has tapes, cassettes, 45s, 78s, wires and anything else that will record sound and play it back.

And he's been a bluegrass fan and a picker for more than 20 years. In his spare time he plays the bass with some local bands.

But even more appealing than the broad range of material Henderson plays is the slapdash, come-what-may style of his live shows.

Fans call the Saturday-morning production "the show where anything can happen." And often does.

A few weeks ago Henerson was puttering along merrily when Akira Otsuka, the mandolin player from the Grass Menagerie, showed up.

Then Buck White, of Buck White and the Downhome Folks, arrived with his singing daughters, Cheryl and Sharon. Jerry Douglas from the Country Gentlemen materialized, as did Ricky Skaggs, Emmylou Harris' wildman fiddler.

Anyone who has seen Henderson's closetsize studio can imagine the chaos that reigned when they all started chattering away on the air. Finally Henderson carted off the Whites, Skaggs and someone with a guitar to "Studio B" and got them singing hymns -- just voices, mandolin and guitar.

Then he turned on the Studio B mike.

It was spectacular.

Henderson never knows who will drop by on Saturdays, or when. Sometimes people bring tapes. Sometimes they bring records. Sometimes they bring instruments. "Sometime it's just me and Lulu," he says.

There no lock on the door to the studio. The only thing between Henderson and whoever blunders along is Louise Burke (Lulu), who dropped by one day months ago to find out what goes on in a radio studio.

She's been coming every Saturday since, settling in for the full four hours, answering the phones, making coffee.

"I just like having something to do," she said. "If I sleep in I feel bad all day."

Does Henderson get paid for all this?

"Gas money, that's all," he says.

For gas money he goes through some agony. Last month he had a show slated with fellow old-music collector Joe Bussard from Frederick. Bussard had some tapes Henderson wanted to use on the air.

The only trouble was that Bussard was afraid to drive into Washington. So Henderson got up at 3 a.m. Saturday and drove to Frederick to pick his guest up.

They had breakfast along the way, got to the studio about 6, and Henderson spent the next two hours fighting off Bussard, who loves to talk, so he could get his show prepared.

He never succeeded, and at 8 he was still sorting through announcements and news flashes. The show began in a flurry of confusion.

Things shook down along the way. Henderson leaped from tape machine to turntable to microphone all morning, cueing Bussard, tearing through new clips, sorting through records and generally taking on the appearance like a hyperactive gymnast.

At noon signoff he breathed a sigh, gathered his gear, shooed Bussard out the door and drove him back to Frederick. He got home sometime that evening, having survived two days on one hour of sleep.

Sundays aren't like that. Bluegrass fans take their gospel seriously, and the 8-to-noon "Stained Glass" show is appropriately dignified.

Except a couple of weeks ago, when Henderson had a rare Sunday guest, Ed Selflinger.

In addition to his "Roound Mound" nickname, some folks call Henderson "Poppin' Fresh -- the Pillsbury Doughboy."

He mentioned that to Selflinger.

"Good," Selfinger said over the air. "Then I've got just the song."

He switched on the gospel "Hallelujah, We Shall Rise."

"For that," said Henderson, "we should have been thrown off the air." BLUEGRASS FESTIVAL

Sunday afternoon at 3, at the Capital Centre, there'll be what's billed as the First Annual Indoor Bluegrass Spectacular, starring Bill Monroe & the Bluegrass Boys, Doc Waston, the Osborne Brothers, Ralph Stanley, Wilma Lee Cooper, Jimmy Martin, Jim & Jesse & the Virginia Boys, Mac Wiseman, the Country Gentlemen, the Seldom Scene, Bill Harrell, the Stonemans, Fiddlin Chubby Wise and the Bluegrass Cardinals. Tickets are $8.50 and $7.50.