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A 137-GHz gyrotron with a high Q (~6000) cavity has been built and tested in short-pulse operation. Output power of 0.1-20 kW and efficiency of up to 26 percent were observed at 65-kV, 0.03-1.4-A operation in the TE031 mode. Frequency pulling of 2-kHz/V was measured. This small frequency pulling value indicates that a long pulse or CW gyrotron with high stability (<1-MHz bandwidth) is possible. Operation at currents of 3-5 A, which is very far above threshold, resulted in the excitation of many higher axial modes.