For insurance granted pursuant to this subchapter
the Secretary shall fix and collect a premium charge in an amount not exceeding one-half of 1 per centum of the outstanding investment
for the operating year for which such premium charge is payable without taking into account the excess earnings, if any, applied, in addition to the minimum annual amortization charge
, to amortization of the outstanding investment. Such premium charge shall be payable annually in advance by the investor, either in cash or in debentures issued by the Secretary under this subchapter at par plus accrued interest: Provided, That, if in any operating year the gross income shall be less than the operating expenses, the premium charge payable during such operating year shall be waived, but only to the extent of the amount of the difference between such expenses and such income and subject to subsequent payment out of any excess earnings as hereinafter provided.