Defense Secretary Harold Brown acknowledged yesterday that the United States would have "some uncertainty" about Soviet compliance with some sections of SALT II, including the provision banning more than one "new type" of missile during the life of the treaty.

Nevertheless, Brown said, American intelligence can catch the Russians if they ever try to cheat in a way that would significantly alter the strategic balance.

Brown acknowledged that the ban on more than one "new type" missile would not prevent the Soviets - or the United States - from replacing all their existing missiles with new ones. He insisted, however, that these new missiles would have to be such close copies of the existing versions that they would not provide any significant military advantage.

Specifically, Brown said, the new versions could not significantly exceed their predecessors in throw-weight - the amount of payload a rocket can deliver - or in the number of nuclear warheads carried by each rocket. These, Brown said, were the militarily significant measures.

At yesterday's Senate Foreign Relations Committee hearing on verification of the new treaty, Brown was asked about the prospects for new Soviet missiles by Chairman Frank Church (D-Idaho), who referred the secretary to a report in yesterday's New York Times. The article said "some senators" felt the treaty contained a loophole after hearing secret testimony from CIA Director Stansfield Turner that the Soviets could replace all their missiles with new models.

Air Force Gen; David C. Jones, chairman of the Joint Chiefs of Staff, testified to the same effect in an open session of the committee last week.Jones said that if the Soviets chose to deploy new versions, they would get no appreciable military advantage from doing so.

Brown repeated this point yesterday. He asked committee members to imagine what would happen if he went to the Armed Services Committee to request billions of dollars for "entirely new missiles" that have the same payloads, the same number of warheads, the same yield and the same principal dimensions as their predecessors.

"I would be accused, and quite rightly, of wasting the taxpayers' money," Brown said, adding that the Soviets would be doing just that.

Brown's remarks illustrate the extraordinary complexity of the "new-type" provision. Though he said the permissible new versions of old rockets would have to have the same yield (explosive power) and same number of warheads, this is not precisely true. In fact the treaty permits increases in yield provided they do not cause a change of more than 5 percent in the weight of a rocket's payload. The treaty also permits reductions, but not increases, in the total number of warheads a rocket carries.

The American objective for the new-type provision was to limit Soviet options for modernizing their missile force. According to American intelligence estimates, the Soviets have at least four new missiles now in development.

Some critics of the treaty charge that the Soviets will be able to deploy all of these rockets, notwithstanding the new-type provision of SALT II.

Administration officials acknowledge that this may happen. But they say that the new-type provision - which limits variations in the size, weight and throw-weight to plus or minus 5 percent, compared to existing missiles, and bans any increase in the number of warheads - will mean these new missiles won't pose any added threat to the United States.

Officials acknowledge privately that some spokesmen for SALT II have been too loose with their language in describing the new-type provision, leaving the impression that the Soviets will be frozen in place by the treaty except for the one new-type missile the treaty permits. In fact they won't be frozen, just severely limited, these officials say.

Gen. Jones testified that the new-type definition allows the Soviets to replace a Chevrolet with another Chevrolet, but not with "a Cadillac or a Volkswagen."

At yesterday's hearings senatorial frustrations spilled out in public for the first time during these deliberations.

John Glenn (D-Ohio) criticized Chairman Church for rushing through the verification issue, saying the committee was not prepared for yesterday's hearing. Glenn also charged that the committee hadn't adequately probed U.S. ability to monitor Soviet compliance with the treaty.

Church defended the committee's work, and said Glenn could have more meetings on verification if he wanted them.

Joseph R. Biden Jr. (D-Del.) criticized the committee's rule limiting senators to 10 minutes of questioning at a time. This made follow-up questions impossible, Biden said.

Glenn said he thought the United States would be unable to know if the Soviets exceeded the 5 percent limit on improvements in missile characteristics using existing means of gathering such intelligence. Unless satisfied on this point, he said, he would vote against the treaty.

Brown replied that there might be some uncertainty about some precise figures, but he assured Glenn the Soviets could not deploy a strategically significant new weapon that violated the treaty without being detected.

Without SALT II, Brown said, the Soviets could hide many aspects of their strategic weapons program (the treaty bans deliberate concealment).