Summary form only given. Experimental studies of a three-cavity gyroklystron amplifier employing stagger-tuning have been completed. Each cavity operates in the TE/sub 011/ cylindrical mode. Drive power is directed into a passive TE/sub 411/ coaxial resonator which surrounds the TE/sub 011/ input cavity. Power is coupled to the circular-electric mode inside the inner cavity by four axial slots placed every 90 degrees in azimuth. The cold-test resonant frequency is 34.89 GHz and the Q is 188 (primarily diffractive due to the coupling slots). The intermediate (buncher) cavity employs an annular ring of lossy ceramic against the upstream endwall to provide the desired cold-test Q of 194 and a resonant frequency of 34.62 GHz. The downstream end of the output cavity employs an outward radial step and a nonlinear uptaper to achieve a cold-test Q of 175 and a resonant frequency of 34.83 GHz. The drift tubes between cavities are loaded with lossy ceramic rings to suppress instabilities.