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The Naval Research Laboratory (NRL) is investigating the gyroklystron amplifier (GKA) concept. The purpose of this program is to assess GKA characteristics in order to determine if such a device is capable of achieving the high efficiency, gain, and output power predicted by numerous studies. The first in a series of tubes to examine GKA behavior has been fabricated and is currently under testing. This tube utilizes three rectangular cavities operating in the TE101 mode. Design goals included 100-kW output power, 30-percent efficiency, 40-dB gain, and operation at a center frequency of 4.5 GHz. Test results have shown reasonable agreement with theoretical predictions. In particular, efficiencies as high as 33 percent have been observed with a saturated output power of 52 kW. Bandwidths of 0.4 percent have been measured. In addition, the device can be operated under certain conditions as a phase-locked oscillator. These results suggest that the desirable characterstics which have been predicted for the GKA can be realized with a microwave tube.