The Senate Select Committee on Ethics said yesterday it had obtained "substantial credible evidence" of financial improprieties by Sen. Herman E. Talmadge (D-Ga.) and voted to move its nearly 6-month-long probe of the 22-year Senate veteran into a more serious trial phase.

The Senate panel voted 4 to 1 to hold formal hearings on allegations concerning Talmadge's claims for official reimbursements of Senate expenses, gifts to the senator and "certain of his campaign expenses and campaign contributions."

The committee chairman, Sen. Adlai E. Stevenson (D-Ill.), said after a 2 1/2-hour meeting yesterday of the panel that he had informed Talmadge of the charges and would release a list of them today.

Those voting for the expanded inquiry were Stevenson; the vice chairman, Sen. Harrison Schmitt (R-N.M.); Sen. Charles McC. Mathias Jr. (R-Md.), and Sen. Robert Morgan (D-N.C.). Stevenson said another member, Sen. John G. Tower (R-Tez), would have voted with the majority had he been present.

Only Sen. Abraham A. Ribicoff (D-Conn.) voted to end the probe. After the committee session, Ribicoff left alone showing obvious anger.

There is currently a federal grand jury also investigating Talmadge's finances and Ribicoff said after the vote he objected to the Senate runing a parallel probe. Ribicoff, who has not attended the committee's meetings for months, said he fears the Senate investigation will affect the outcome of the grand jury deliberations.

Talmadge, who was in Georgia yesterday, issued a statement through his office saying that he welcomed the investigation "so that the facts and the truth can be known."

Talmadge also said that he hoped the committee's investigation would clear him "of any intentional wrong-doing."

The committee's investigation of Talmadge has been under way since June 8 after he revealed he had accepted cash gifts from political supporters and friends in Georgia and had not reported them on his Senate disclosure forms.

Since then he had been under a steady barrage of news reports which have disclosed allegations of other unreported gifts, illegal Senate reimbursements, insider land deals and the existence of a secret bank account here through which $39,000 in unreported campaign contributions and illegal Senate reimbursements were funneled.

The Ethics Committee vote yesterday opens the way toward full Senate action on the Talmadge case. Several committee members compared its effect to that of an indictment by a grand jury within the Senate. If the ethics panel determines that Talmadge has been guilty of wrongdoing it could recommend penalties ranging from a simple reprimand to expulsion from the Senate.

Th last formal hearing by the Senate on misconduct by a senator was in 1967 when the Senate censured then-Sen. Thomas J. Dodd (D.-Conn.) for financial wrongdoing.

Carl Eardley, special counsel to the Ethics Committee for the Talmadge case, said yesterday that he did not expect the hearing on the charges to be convened before the Senate meets again Jan. 15.

Under the committee's rules, Talmadge can ask for the hearing to be either open or closed but a final decision is up to the committee members. Both Stevenson and Schmitt told reporters yesterday that they were in favor of opening the hearing to the public.

The committee originally had planned to wrap up its investigation of Talmadge last fall, but the investigation was prolonged by the disclosure of the secret bank account by Talmadge's former chief aide, Daniel Minchew.

The delay in the investigation could cause procedural problems for the committee, Stevenson has said. Under a Democratic Party agreement, the three members of the panel who are Democrats have agreed to rotate off the committee when the Senate reconvenes. The Republicans have no such agreement.

Stevenson has attempted to set up a separate new committee made up of the present committee members to handle the Talmadge matter after the ethics panel's membership is changed. The move would avoid the necessity for a full review of the case by new committee members. Stevenson said yesterday that the plan was still under consideration and that he had spoken about it with Senate Majority Leader Robert C. Byrd (D-W. Va.)

When the hearing is held Minchew will be a major witness against Talmadge, Eardley said. Minchew has turned over two boxed of documents and testified before Ethics Committee investigators and the committee itself seven times since the investigation began.

He has alleged that he set up a secret account at Talmadge's direction in the Tiggs National Bank here in 1973. During 1973 and 1974 Minchew deposited $13,000 in illegal Senate reimbursements in the account and another $26,000 in campaign contributions siphoned off the senator's legitimate campaign account.

Earlier this month in a pseech in Georgia Talmadge called the account "simply a case of embezzlement by a former aide." The Georgia senator has denied any knowledge of the account or the whereabouts of the funds that went into it.

The Ethics Committee is also investigating $37,125 in improper Senate reimbursements which Talmadge obtained between 1972 and 1977. Talmadge paid back the money Aug. 18 after an audit of his staff books and said the funds were requested due to a "staff oversight."

The federal grand jury is also looking into the secret bank account. But Eardley indicated to reporters that the grand jury's focus does not appear to be on Talmadge but rather on his former chief aide, Minchew. Both Eardley and the panel members declined yesterday to speculate on the outcome of the grand jury's investigation.

Eardley did not cancel out the possibility of a negotiated end to the Ethics Committee's inquiry on Talmadge. He said there was nothing in Senate rules to prohibit Talmadge from "admitting that he committed certain violations of Senate rules and that he is willing to take the consequences."