SAN ANTONIO – Clear Channel Communications Inc. on Monday delayed a vote on a proposed $19.35 billion buyout of the radio and billboard company and said it was talking to bidders about a revised offer.

The company had already received enough proxies to defeat the proposed buyout, which would require a two-thirds vote for approval. The shareholder meeting scheduled for Tuesday was postponed until May 22.

Clear Channel said its board was talking with private equity firms Bain Capital Partners LLC and Thomas H. Lee Partners LP about increasing the price to $39.20 per share from $39 and letting shareholders pick between getting paid in cash or stock of the new company with current shareholders limited to a combined stake of 30 percent.

The Clear Channel board turned down a similar proposal last week, saying the changes would delay the vote by up to 90 days with no certainty it would be approved.

Since last week, Clear Channel said, some of its largest shareholders have asked the board and the company’s financial adviser to delay the vote and reconsider the buyers’ sweetened offer.

Institutional Investors Services and Glass Lewis & Co. advised shareholders to vote against the sale, saying the price was too low. Two big shareholders – Fidelity Management & Research Co. and Highfields Capital Management, which together own 15 percent of the company’s shares – indicated they would oppose the buyout.

ISS warned, however, that rejecting the buyout could be risky.

“Turning down the certain $39 offer requires faith in the ability of a standalone [Clear Channel] to generate shareholder value,” ISS said. “If shareholders do vote down this transaction, the verdict on that decision will not be rendered until sometime in the next 12 to 18 months.”

Clear Channel is the nation’s largest radio broadcaster with about 1,100 stations. Traditional AM and FM radio stations have been hurt by competition from satellite radio and MP3 players.

About 40 percent of Clear Channel revenue comes from its 90 percent share of the world’s largest billboard company. Revenue from billboards has been growing as digital displays replace some old-fashioned signs, and motorists can’t ignore billboards as easily as they can change radio stations.