Charter Co. today became the second publisher in a month to put a major newspaper on the auction block when it announced that it wanted to sell the Philadelphia Bulletin, that city's second biggest newspaper.

Last month the Chicago-based Tribune Co. said it hoped to sell the New York Daily News, a money-losing tabloid that nonetheless is the nation's biggest circulation general newspaper.

Until last year, when it was passed by the morning Inquirer, the afternoon Bulletin had the largest daily circulation among Philadelphia's newspapers. The Philadelphia Daily News is third. Last month, the Philadelphia Journal, a tabloid that specialized in sports and local news, folded after unions refused to give it any concessions.

The Bulletin provided printing facilities for the Journal and sources estimate the afternoon paper lost $2 million in profits with the demise of the Journal.

In making the announcement today, Charter cited the same reasons as the Tribune in deciding to dispose of the newspaper property. Both said they were unwilling to continue to provide the capital needed to keep the newspapers going.

The Bulletin lost nearly $11 million in the first six months of 1981 and sources said the paper, despite union concessions it won last August, has been losing money at an annual rate of nearly $30 million in the past several months. Charter--based in Jacksonville, Fla.--bought the Bulletin a year ago from the McLean family, which had owned it since 1895.

Charter said it is having discussions with a prospective buyer for the Bulletin that it did not identify. The paper's executive editor, Craig Ammerman, told the staff this afternoon that negotiations are under way with a group that has substantial financial backing and one that the staff would not feel ashamed to work for.

Last August, Charter said it would attempt to return the Bulletin to the preeminence it once held in Philadelphia.

But in a statement today, Charter said the "performance of the newspaper has fallen significantly short of established financial targets despite stepped-up cost-reduction measures and marketing plans put in place since August."

About the same time the Bulletin received concessions from its unions, the New York News folded the Daily News Tonight, its year-old attempt to compete with the New York Post venture in the afternoon field, and announced a series of cost-saving measures as well. Nevertheless, News executives predicted the tabloid would lose $11 million in 1981. Sources said the losses will be higher.

News circulation, more than 2.2 million a day 10 years ago, has declined to about 1.5 million, in large part because its middle-class readership has moved out of the city to the suburbs.

Neither Charter nor the Tribune Co. have set deadlines by which purchasers must be found, but industry sources in both cities doubt either paper will survive under its current owner beyond six weeks.

Several companies have talked to the Tribune Co., including Warner Communications and Gulf & Western. Spokesmen for both conglomerates have said that it is unlikely they would purchase the Daily News. There have been reports that the Gannett newspaper chain and the Times Mirror Co. are interested in the News as well.

However, most industry insiders doubt either publishing company would buy the Daily News, which needs at least a $50 million injection of cash to upgrade its outmoded printing facilities and has union contracts that would make many potential buyers back off.

The Tribune Co. is the publisher of Chicago's biggest newspaper, the Chicago Tribune, and owns broadcast properties, other smaller newspapers, paper mills and last summer bought the hapless Chicago Cubs baseball team.