Convicted Watergate conspirator John W. Dean III said last night that he knows of nothing in previous administrations that compares with Watergate in scope, and called upon former President Richard M. Nixon to "come forward and tell what really happened."

He said that until Nixon does so, and permits public access to the tapes in his possession, "the history of the Nixon years will be a history of Watergate."

Dean spoke and answered questions before a sometimes raucous crowd in MIT's Kresge Auditorium.

James R. Miller, the committee's lecture subdirector, said yesterday that $500 of Dean's fee was contributed by the MIT Undergraduate Association, but said it would violate Dean's contract to disclose the total amount of the fee.

Dean said in his speech that he is "troubled by commercializing on Watergate," but that he is "in serious debt."

He said he thinks "given the personalities that were involved, a Watergate was inevitable."

"Had the president not wanted this type of activity, it never would have happened," he said.

Dean described his four-month prison term as "depressing," and said that he will work to improve the system of justice in America.

"I am personally offended by a case I know of where a young person convicted of possession of one ounce of marijuana got ten years while I got four month," he said, provoking prolonged applause.

No Chance

Asked whether he thought the other convicted Watergate defendants would ever go to prison, Dean said, "They don't have much of a chance on appeal."

"But these are powerful men with a lot of powerful friends," he added. "Mitchell is a friend of Rockefeller. It's not inconceivable that if Ford decides not to run again, or runs and loses, he might pardon Mitchell and the rest."

Asked whether the Nixon administration had instructed the Justice Department not to permit the case of the killing of four students at Kent State to come before a grand jury, Dean said the possibility was "very potential."