You've heard about the comedian who wanted to play Hamlet?

Meet the tuba-or, more specifically, meet Harvey Phillips, who is the Paganini, the Casals, the Segovia and the Landowska of this well-known but sadly misunderstood instrument.

"I don't think the tuba is funny," says Phillips, who is widely considered the most persuasive interpreter who has ever hefted one of the 35-pound behemoths, applied his embouchure to its bell-shaped opening and sent his breath through its 32-foot conical length.

"My children's food and clothing, the roof over their heads, all come out of the tuba," says Phillips. "What's so funny about that?"

Phillips came to Washington yesterday with wind ensemble wizard Frederick Fennell to assemble and lead a group of about 300 tubas in a program of Christmas carols last night on the Ellipse south of the White House. But that modest spectacular was only the surface reason for his visit. He is running his "Merry TubaChristmas" concert in Washington (as well as New York, Chicago, Los Angeles and Dallas, all in a single week) as a part of his lifelong campaign to convince music lovers that the tuba is a noble, lyric instrument.

At concert time last night, from a stage set amid the brightly lit state Christmas trees, the majestic sound of massed tubas floated softly, golden-rich over an audience that was sometimes hushed, sometimes singing along, shivering slightly but warm in its applause.

The band was a bit smaller than anticipated, but still impressive. "There were just under 200 when we stopped counting," Phillips said, "and a few more came after that, so I think we hit 200."

On stage, playing for a crowd that outnumbered them by abit more than 2 to 1, the improvised band had a high proportion of players from local high schools, some with school emblems on their instruments, others had decorated their tubas with tinsel of Christmas garlands, and there were two antique instruments supplied by Mark Elrod of the Heritage Americana band that operates out of Catholic University.

Elrod played an enormous Civil War instrument that went straight back over his shoulder without the usual curves, and a friend played an ophicleide with a rather awkward-looking set of keys rather than valves.

"I came up from Jacksonville to play in this," said real-estate agent Gene Butts. "I heard about it and just wanted to play in a tuba band. It's not every day you get a chance to play with 200 other tubas."

Two band players form Loudoun County High School thought the concert was "a lot of fun" and confirmed their band's opinion that "you have to be crazy to play the tuba."

"I started the tuba as a joke in the fifth grade, but now I love it," said Fred Hocker. "They asked us what instrument we wanted to play and everybody was saying violin and things like that, but my friends told me I had a big enough mouth to play the tuba."

"Tuba players all have speech deflex," added schoolmate Stuart Lay. "It's one thing that brings us all together."

The audience seemed to consist largely of hard-core tuba fans and proud parents; and comments were generally enthusiasic.

Earlier, in a lunchtime coversation, Phillips and Fennell presented the tuba's claims antiphonally.

Fennell: "It's an instrument of the present."

Phillips: "And the future."

Fennell: "And the future."

One reason for this emphasis on the tuba's modernity is that its past history has been relatively brief and not terribly distinguished. It is one of the newest orchestral instruments, having reached that status around 1835, shortly after its invention. And like that other newcomer, the saxophone, its work record is heavily tilted toward such sniff-provoking activities as Dixieland jazz, circus music and the Germanic brass band.

Asked to imitate the sound of the tuba, 99 people out of 100 will automatically shout, "oom-pah-pah," a sound that makes Phillips wince as he hastily points out that the tuba is responsible only for the "oom"; French horns traditionally are in charge on the "pah-pah."

While the instrument is fairly new, it has the oldest name in the orchestra. You may search Latin poetry in vain for any word that looks like "violin," "oboe" or "clarinet," but "tuba" is there, unchanged, in two of its greatest lines.

You may have wondered why Mozart, when he came to one of those lines, "Tuba mirum spargens sonum," in his "Requiem," used a trombone rather than a tuba. The reason is simply that the tuba did not yet exist; the closest equivalents were the serpent (which looked like a serpent) and the ophicleide, which "looked something like a baritone sax, according to Phillips. "Neither of these instruments had the range of the tuba, its expressiveness or its reliability."

The oldest poet of the tuba was Quintus Ennius (239-169 B.C.) and his single line about it contains a piece of onomatopoeia that is still current today: "At tuba terribili sonitu taratantara dixit." ("But the tuba, with a terrible sound, said taratantara.")

From the sound Ennius evokes, one must assume that he was writing about some other instrument-probably one like a bugle-rather than the one which was patented by A. G. Guichard of Paris 2,000 years later. The Latin language lacked the word "oom-pah-pah," but if he had heard the sound Ennius would surely have invented the right word.

By the time tubas got invented, brass instruments were generally being made of brass, but Phillips notes that, "If it has a cupped mouthpiece an instrument is brass even if it's made of wood-that's how they are classified. Actually, the total tuba is perhaps 95 percent flesh and blood and 5 percent metal. The important part is all behind the mouthpiece-the musical intellect and artistry that the metal part amplifies."

This flesh-and-metal instrument has a potential range that extends an octave below the bottom of a piano keyboard up to an octave above middle C, Phillips notes, and it can play as lyrically, as gracefully, as nimbly as a flute or violin. Among the stereotypes he is trying to combat are the idea that a big instrument like the tuba is s slow instrument and that it must be played by tall, chunky people like himself.

"Some of the best tuba players coming up now are tiny little wisps of humanity," says Phillips, and Fennell adds that, "If they want to, they can blow you right out of the room."

But volume, contrary to popular belief, is not the tuba's forte. "It's not really as loud as most brass instruments, and certainly not as penetrating," says Phillips. "The problem may be that it looks loud; the biggest obstacle to the tuba's acceptance is visual. Just as the association of size with slowness is a psychological thing."

Phillips' own association with the tuba began when he was in the seventh grade, when he was studying violin.

?That was during World War II," he recalls. "And the tubist in the high-school band joined the army during the school year-and they asked me if I wanted to take his place. So I learned to play the tuba part in 'Pomp and Circumstance' before I learned to play a scale."

His high-school bandmaster got him a summer job with a small circus-then, a few years later, he received "an offer from Ringling Brothers that ended my academic career at the University of Missouri." During his three years with Ringling Brothers, he got to know many veterans of the old Sousa band, and I became "steeped in the pride and tradition of that great band."

"The circus was a whole world," he recalls. "At that time, there were 1,500 people traveling with Ringling Brothers, which was about three times the population of my home town in Missouri."

While he was playing with the circus, the Julliard School of Music called-offering a scholarship.

"Once I got to New York," he recalls, "it was anything and everything. I played with the Voice of Firestone and the Hit Parade, the Symphony of the Air, the opera and ballet companies and the Sauter-Finnegan jazz orchestra, and I was a founding member of the New York Brass Quintet. There were four or five free-lance tubas around New York at that time, but I don't think any of them were busier than I."

Phillips sometimes seems a bit quixotic when he starts talking about the future of the tuba, but he is convinced that he has an instrument whose time has come, and he can cite figures to support the idea. "In 1960, there was only one American university with a full-time tuba teacher. Last year, there were 78. This is partly a result of the brass quintet movement; brass teachers have learned that when a tuba is added to the faculty, forming a quintet, members of the brass department can become not just teachers but artists in residence. Economically, in terms of employment for tuba players, this growth is the equivalent of the creation of 77 symphony orchestras-$1.5 million in annual revenue generated for tuba players."

Through a nonprofit foundation he has established and through a 2,000-member international organization, the Tubists Universal Brotherhood Asscociation, Phillips is seeking to open new horizons for the tuba. One concrete result of his efforts has been the commissioning of hundreds of new compositions featuring the tuba-a vast escalation of a repertoire that used to stop short after one concerto by Vaughan Williams and one sonata by Hindemith.

"I will not be satisfied," Phillips sums up, "until the major concert managers include solo tuba players among their clients, along with all the sopranos and pianists. I am making all this effort for young tuba players, so that they will know that they have a great tradition and a good future.

"I'm not just blowing my own horn."