FIRST BOSTON <FBC> STARTS ALLEGHENY <AG> BID First Boston Inc said it has started its previously-announced tender offer for all common shares, 2.19 dlr cumulative preference shares and 11.25 dlr convertible preferred shares of Allegheny International Inc at 24.60 dlrs, 20.00 dlrs and 87.50 dlrs respectively. In a newspaper advertisement, the company said the offer and withdrawal rioghts will expire April Nine unless extended. The offer is conditioned on receipt of at least a majority of Allegheny voting power on a fully diluted basis and on receipt of at least two thirds each of the preference and preferred shares. A merger approved by the Allegheny board in which remaining common, preference and preferred shares would be acquired at the tender prices is to follow the offer. Receipt of the minimum amounts under the offer would give First Boston sufficient voting power to assure approval of the merger without the affirmative vote of any other shareholder, the company said.