Jay Ammett, the former president of Warner Communications Inc., testified today that he is the "only person in the entire world" who claims that Warner Chairman Steven Ross directed an allegedly illegal cash fund established at the communications conglomerate in the early 1970s.

But Ammett, the key government witness -- who himself awaits sentencing after pleading guilty in a related case -- seemingly did little to support the government's allegations during two days of testimony.

In his opening remarks last Wednesday, Assistant U.S. Attorney Nick Akerman had implicated Ross in the creation of the cash fund. Ammett has been testifying at the fraud trial of Solomon Weiss in U.S. District Court here.

Weiss, formerly assistant treasurer at Warner, was indicted last year for allegedly accepting $170,000 in cash bribes. The money was paid into the cash fund, prosecutors contend.

The government says that these bribes were paid to induce Warner to purchase 40,000 shares of stock in Westchester Premier Theatre in Tarrytown, N.Y. Several top mafia figures have been tied by federal investigators to the once-famous 3,500-seat theater, which went bankrupt in 1978.

In his testimony, Ammett said he discussed the stock purchase with Ross in 1973, but he said Ross declined to get the company involved. According to Ammett, Ross suggested that perhaps Leonard Horwitz, a Westchester Premier official, "ought to meet with Sol Weiss and maybe he could be helpful in the Warner cash fund." Ammett said that he then arranged for Weiss to meet with Horwitz. He said he told Weiss about his conversation with Ross.

Government prosecutors tried to get Ammett to obtain evidence against Ross, according to Ammett. On Feb. 9, l981, Ammett telephoned Ross to warn him that the federal prosecutors "wanted me to be wired" to record a conversation with Ross.

Later, on the same day, Ammett testified he made another telephone call to Ross, which was taped by federal authorities. According to a transcript of that call introduced in the trial, Ross said to Ammett, "Anything that pertains directly to the trial . . . don't talk to me."