A secret cache of $100 bills allegedly kept in an old overcoat in Sen. Herman E. Talmadge's apartment has been traced to a Texas bank, sources said yesterday.

According to knowledgeable sources, 77 $100 bills which the Georgia Democrat's former wife Betty claimed were part of the cache were numbered in sequence, which allowed investigators for the Senate Select Committee on Ethics to trace them.

The Texas origin apparently means that the bills were not from a secret account at Riggs National Bank here which has been linked to Talmadge by his former senior aide, Daniel Minchew.

But the bills raise another question - whether Talmadge had additional sources of income. In April , Betty Talmadge told Senate committee investigators that when they separated in 1974 she took the overcoate money, which was in an envelope bearing the name "Harry P. Anestos," and kept it at her home in Georgia.

A handwritin expert who testified to the committee said Anestos was not the person who wrote the anem on the envelope. The handwriting specialist said Minchew did not write the name, but he did not testify about whether Talmadge or his wife might have.

According to knowledgable sources, Talmadge's former wife did not come forward with information about the hidden money cache until committee investigators learned of it through another source. But she has cooperated with the investigators since they questioned her in April about the money.

In his opening statement four weeks ago Talmadge told the Senate ethics hearing that his attorneys would prove that the money given to the Senate investigators could not have been in his overcoate in 1974 and that it did not come from the secret Riggs account.

A spokesman for the senator declined to comment on the overcoat issue. Talmadge's attorneys have not yet dealt with that part of the case, although it is expected to be covered when Betty Talmadge testifies early month.

Talmadge is accused of five charges of financial misconduct including filling thousands of dollars in false Senate expense vouchers for non-existent office expenses and converting additional thousands of dollars in campaign contributions to his personal use. Last year, after news accounts raised questions about Talmadge's finances, the senator ordered an independent audit of his office accounts and eventually repaid $37,125 in Senate expense overpayments.

But the Georgia senator has repeatedly denied any blame for another $13,000 in false expense vouchers and $26,000 in mostly unreported campaign contributions that were funneled by Minchew into the secret account in the Riggs bank.

In testimony so far, a federal auditor has said he traced more than $16,000 from the secret bank account to Minchew and said another $5,000 went back into a legitimate Talmadge campaign account. But the auditor said he could not account for about $18,000 that was also withdrawn from the secret account.

Minchew says that he gave the untraced money to Talmadge, to the senator's late son Bobby and to his ex-wife Betty in envelopes stuffed with $100 bills.

Talmadge, a 23-year-Senate veteran, declared last year, and again last week, in an interview with a Georgia radio station, that he relied for his personal day-to-day spending money on gifts of $5 and $10 pressed on him by supporters during his travels in Georgia.

But the senator's aides - most of whom have remained fiercely loyal - have testified to the committee in public and in private that they never saw cash passed to Talmadge with the exception of traditional contributions handed over during campaign swings.

When the hearing resumes June 4, attorneys for the ethic committee are likely to suggest that the senator got his walking-around money from the secret cache in the overcoat.

The $100 bills turned over to the committee by Betty Talmadge could therefore end up as the single most important piece of evidence remaining to be presented to the hearing, if it ties Talmadge to improper sources of money.

So far the only other evidence introduced against Talmadge has been the word of Minchew, who is a self-confessed liar and thief. Documents supporting Minchew's word, which he supplied to the ethics committee, have been questioned by other witnesses.