Prudential will invest $600 million in Bache.

The insurer’s contribution to its Prudential-Bache subsidiary will convert an existing loan to preferred stock bearing a floating dividend rate equal to the interest rate that the unit had been paying--in effect, shifting the entry from one part of the balance sheet to another and allowing the brokerage firm to report higher earnings more in line with its competitors, a spokesman said. The unit reported a 1984 net loss of $113.1 million but had net income of $20.4 million for the first half of 1985.