The invention is a high-power transconductance circuit (HVTC) comprising three direct-coupled stages which can be substituted for a final-stage power-amplifying vacuum tube in an audio amplifier. The HVTC consists of an input stage, a driver stage, an output stage, and a power conditioner. The input to the HVTC is a composite signal consisting of an AC component and a DC component. The input stage conditions the input composite signal for input to the driver stage. The driver stage transforms the input composite signal into the driving signal for the output stage. The output stage utilizes one or more power transistors to drive a load. The power conditioner supplies regulated power to the HVTC. The input composite signal is direct-coupled through the input stage and the driver stage to the output stage.