The tension in the air was palpable when the sweep second hand crossed 6 o'clock: The red ''on air'' light flashed on, a taped voice announced, ''This is Minnesota Public Radio. . . ,'' and Butch Thompson hit the opening notes of ''Hello Love'' on the Steinway grand.
Thus began the live radio broadcast of ''A Prairie Home Companion'' from Mead Chapel at Middlebury College, the first stop on the show's spring tour.
One of radio's most popular productions, ''Prairie Home'' is carried by more than 206 public radio stations with an estimated 1.5 to 2 million listeners. Its format harks back to radio's Golden Age, interspersing humor, music, and the storytelling of host Garrison Keillor. It has been called the ''Grand Ole Opry'' of folk and bluegrass, and with some justification: The inspiration for the show came to Keillor after he wrote about the Nashville country music program for The New Yorker magazine. ''Prairie Home'' is a celebration of small-town America. And yet it enjoys popularity in such urban, urbane settings as Manhattan.
Mr. Keillor seemed to take an immediate liking to this rural Vermont town. Perhaps it reminded him of his ''hometown,'' Lake Wobegon, Minn., the fictitious setting of his real-life-dressed-up-as-fiction tales. But still, he missed Lake Wobegon and its familiar scenes: Bertha's Kitty Boutique, Ralph's Pretty Good Grocery, Bob's Bank, and, of course, Powdermilk Bisquits (made from whole wheat raised by Norwegian bachelor farmers). He felt a little sad that he wasn't around for this year's Junior-Senior Prom at Lake Wobegon High. It's a big deal, he said, affectionately recalling his own. Every year it's put on ''by the juniors for the seniors - kind of like social security.''
This was vintage Garrison Keillor - touching, gentle, and at times uproariously funny. But to see him do what usually is only heard is not to be disillusioned. Keillor, who at 6 feet, 4 has been called ''America's tallest radio comedian,'' lumbers around the stage. When he delivers his monologue, he doesn't read from a script or notes. He stands cradling the microphone stand, swaying gently, eyes closed at times.
Besides the regulars - the Butch Thompson Trio, Stoney Lonesome bluegrass band, and Peter Ostroushko - the show's musical entertainment focused on local Vermont talent: ''La Famille Beaudoin,'' who performed French-Canadian folk songs, and the ''Word of Mouth Chorus'' which performs a traditional form of American music called ''shape note.'' The producers aimed to present music not usually available in Minnesota.
The Middlebury broadcast marked a first for the nine-year-old show: the use of a portable satellite uplink (transmitter). Since 1980 the show has been broadcast nationwide via satellite, but has been limited on road tours to broadcasts from cities that have a permanent uplink.
Having the option of using a portable uplink means the show can now be broadcast from more rural locations. But according to the executive producer, Margaret Moos, this does not mean the show will take more road trips, since the cost and amount of planning necessary are prohibitive.
Rumors that ''Prairie Home'' may ''graduate'' to television were effectively quashed by Mr. Keillor and Ms. Moos.
''I think our show would suffer from a jump into televison,'' Keillor explains. ''The sheer mechanics could sink this pretty slim little show, and I don't see that we'd have much to gain from the risk.
''Besides, a lot of people cook while listening to our show, and you can't cook and watch television at the same time and enjoy it. Good television tends to drive out good cooking. Our show is heard in thousands of kitchens amongst fabulous aromas by people dizzy with pleasure at the prospect of dinner, for which we are a lead-in show.
''I'd hate to replace that happy gang with a bunch of people lined up facing one way and eating Kraft or Stouffer's.''
Ms. Moos did suggest, however, that they may some day do a live radio and television simulcast.
Part of the fun - and the risk - of live radio is that anything can happen. And it does. Stage manager Marge Ostroushko (Peter's wife) told with amusement about a reporter who once wrote that the show ran itself. ''Ha!'' she exclaimed. After six years with the show, she's used to crises.
''People sometimes forget to bring their instruments with them on stage,'' Ms. Ostroushko says. ''You can't yell out since it's going live, so you'll see someone mouthing (the word) 'guitar' '' to somebody else backstage. A guitar is passed to the performer. And what reaches the listener's ear sounds smoothly professional.
Another first for the spring tour was a broadcast from New York City. The show is now in Boston, where it has its highest listenership outside Minnesota, for five sellout shows ending tomorrow.