The design and experimental results of a 140-GHz gyro-amplifier that uses a dielectric-loaded, severless confocal waveguide are presented. The gyro-traveling wave amplifier uses the HE ₀₆ mode of a confocal geometry with power coupled in and out of the structure with Vlasov-type, quasi-optical couplers. Dielectric loading attached to the side of the confocal structure suppresses unwanted modes allowing zero-drive stable operation at 48 kV and 3 A of beam current. The confocal gyro-amplifier demonstrated a peak circuit gain of 35 dB, a bandwidth of 1.2 GHz, and a peak output power of 550 W at 140.0 GHz.