JOHN DUFFEY -- Thursdays, 9 to 1, with the Seldom Scene at the Birchmere Restaurant, 2723 South Wakefield Street, Arlington.

In the last couple of years John Duffey's fans have notice a change in the big man with the huge singing voice.

Duffey, blessed in adulthood with a startling range of four octaves, from low E in the bass range to a high E in tenor, is growing conservative.

For two decades he had thumbed his nose at the scientific presumption that human tissue abused would grow unless in time. He kept singing in smoky barrooms four and five hours a night. He kept smoking two to 2 1/2 packs of Camels a day and he kept greasing his throat with his own peculiar elixir, gin sours.

Finally two years ago he and fate had a showdown and Duffey lost. he realized he had to stop smoking Camels.

He switched to Camel Lights.

John Duffey does not exactly pamper himself. His voice was a gift and he's always treated it like a gift, using it hard and often and sometimes mercilessly.

Once Duffey had a voice lesson. His father, John Sr., was for 25 years a tenor with the Metropolitan Opera Company's touring production company. He was 54 and finished with singing when John Jr. was born in Washington.

In the '40s, when his son was still a boy, the retired opera star would sit him in front of the radio on Saturday afternoons for the Met's weekly show. Young John understood not a word of it.

"I guess he used to sing some around the house. He still gave lessons. But I never heard him. Whenever a student came they shooed me out of the house."

In the '50s young John Duffey discovered bluegrass music and was thoroughly taken by it.He wandered around the house singing songs. One day his father heard him.

"Look," said the elder Duffey, "I don't like that stuff but if you're going to sing it at least sing it right."

That, said John Jr., was when he got his singing lesson.

"He told me to sing from down here," said Duffey, pointing to his broad midsection. "From the diaphragm."

End of lesson.

Duffey has been singing from the diaphragm ever since, belting out the high, lonesome parts of the high lonesome music with such stalwarts as Bill Clifton, Charlie Waller and, until recently, John Starling.

In the old days bluegrass used to be performed around a single microphone, the trios and quartets doing an amusing dance as they scurried to gather around the mike for each chorus. Duffey would wait until the circle was complete, then take a stance about three feet behind the rearmost singer and blast away. Even at that his tenor cut a clear path through the mash.

Nowadays Duffey's band, the popular Seldom Scene, operates with three mikes. They turn his voice mike way down. Sometimes it seems they could cut it off altogether and he'd still predominate. Duffey can sing.

The world knows that, even if Duffey himself still demurs. "I was recording for six years before I could stand to listen to myself," he said the other night as he waited for showtime at the Birchmere Restaurant in Arlington, where the Seldom Scene plays to sellout crowds every Thursday night.

"It was 1962 before I could hear my own records and say, 'Well, it's okay.'"

Now Duffey thinks the voice is beginning to desert him. "It's apparently a medical fact that as you get older your voice goes down a little bit. The high notes start to dwindle. Age I guess, and the effects of body abuse.

"Frankly, though, I think my voice has more quality now than it did when I was screaming four octaves. After I could stand to hear to myself sing I'd listen to a recording and I'd think to myself, 'Sounds like you're reading it off paper.' I realized there were ways of getting into a note, besides just blasting it out.

"This may sound conceited, but I think phrasing really separates the men from the boys. I like it when it sounds like I just swallowed a fifth of whiskey."

Indeed, Duffey's voice will probably never sound like he just swallowed a fifth, even if he really had, which he might.

The idea of John Duffey mellowing is hard to take. He still carries himself like a cop in a bad neighborhood. He can remember the days when he played at the Harmony Club near Union Station, and they had a steel cage around the bandstand to protect the musicians from flying debris.

He still wears his hair in a forbidding combination flat-top/D.A. and wears formfit shortsleeve shirts that his biceps threaten to burst at any moment. And white shoes.

And the voice still comes out, 25 years later, clear and strong and true.

One day a non-believer took to criticizing Duffey at a club in Bethesda. "Look at him," said the naysayer. "He acts like he's the cat's meow."

From the next table came the voice of a bluegrass addict.

"He can afford to," said the voice. "He is the cat's meow."