The operational theory of vacuum-tube power amplifiers with complex load impedances is being studied. The theoretical analysis proposed earlier Reports 1, 2, and 3 was checked experimentally with amplifiers operating in the steady state at low power levels. A description is given of tests made with the amplifier of a type XQHB sonar system. Agreement between experiment and the theory was fairly good. The difficulty of measurement with pulsed operation and the fact that the theoretical assumptions are only partially satisfied by the amplifier were some of the reasons for the disagreement. The appendixes consider the envelope of response of a pulsed amplifier and correction of power output.