Carl (Cobby) Lynch, the convicted leader of the "fat lady" drug ring and the man said to be responsible for shuttling hundreds of thousands of Preludin diet pills into the illegal D.C. drug market, was sentenced to 20 years in prison yesterday.

The sentencings of Lynch and other alleged drug ring members by U.S. District Judge Louis F. Oberdorfer ended a massive investigation into the ring, which recruited fat women to go to various east coast cities to wheedle Preludin prescriptions from doctors. Twenty persons have been convicted and sentenced in the case.

In its final presentation of materials to the judge, government prosecutors portrayed Lynch as a skilled but illiterate businessman in his chosen field of drug-peddling who took up the slack in the illegal Preludin market here in 1974.

Even the judge commented on Lynch's abilities at the sentencing, recommending that the Hillcrest Heights man educate himself in prison and direct his talents for the good of society when he is released. Oberdorfer said the Lynch case appeared to represent an infrequent instance of the government's capture of a person at the head of a criminal enterprise, and said he was "impressed by the extent to which what you did caused trouble."

Preludin, which can be legitimately used as an appetite suppressant, is used by D.C. heroin addicts to boost the "high" of heroin. It has been unusually popular in the city for that purpose since the early 1970s, according to drug investigators.

A crackdown here on certain doctors and pharmacies in 1974 limited the ease with which Preludin could be obtained, investigators said. Lynch already was a Preludin dealer here, they said, and when his sources here dried up, he expanded his operation and, in effect, opened branch offices.

Assistant U.S. Attorneys Peter O. Mueller and Charles J. Harkins Jr. of the major crimes division described the operation in this way:

At first, the "fat ladies" would go to various east coast cities from Washington under the direction of a group leader or manager who would supervise them during their trips. Later, however, to cut transportation and hotel costs, Lynch rented apartments and maintained a "fleet of company cars" in various cities.

The drugs would be brought back into Washington, the prosecutors continued, and be distributed largely in the 4th and M streets NW and 11th and O streets NW areas.

Lynch proposed in his business, the government asserted, paying as much as $18,000 cash at one point for property he purpased. At another time during his 6 1/2 year scheme, Lynch - also known as "The Pink Man," "Big Man," "Fat Man" and "Boss," - paid $3,000 to customize a $14,000 Cadillac he bought so it would be the same color pink as Preludin pills, the government said.

The eight final persons convicted in the scheme were sentenced yesterday in the presence of several deputy U.S. marshals and under tight security. Women and children in the spectator section occasionally would burst into sobs as the sentences were pronounced.

Among those sentences yesterday was Lynch's wife, Myrtle (Sister) Lynch, who is custodian at the J. Edgar Hoover FBI building here. The government called her "an active participant in one of this city's largest and best-run illegal drug systems," and she received a prison term of 1 to 3 years.

Others sentenced were Milton Battle, 1 to 3 years; John W. (Sweetmeat) Brinner, 1 to 3 years; Tyrone J. Johnson, up to 6 years under the Youth Corrections Act; Theresa Lucas, 1 to 3 years; Thomas McCloud, 1 to 3 years, and Emma Jean Ward, 2 to 8 years.

Three doctors, two from New York and one from Philadelphia, also are serving jail terms in connection with the scheme.