Be it further enacted, That until all bonds authorized by a vote of the electorate under this Act shall have been retired, the council each year shall levy, as may be necessary, a total tax on all property subject to taxation by the city and calculated to produce revenues sufficient to include the amount required to pay the principal and interest due in that year, if any; provided, that the council may appropriate other municipal revenues for such purpose, in which case a corresponding reduction may be made in the property tax levy. Such a total tax levy shall include revenue requirements for general municipal purposes. [As replaced by Priv. Acts 1963, ch. 74, § 9]