The quasioptical gyrotron (QOG) is under development at the Naval Research Laboratory as a high power source of millimeter-wave radiation. An experiment has been designed to convert the QOG into a quasioptical gyroklystron by the addition of a prebunching resonator driven by an external source. One goal of the experiment is to use the prebunched electron beam to selectively drive the longitudinal mode in the output resonator that has optimum frequency detuning, which should improve the efficiency. The output power of the gyroklystron is constrained by oscillation of the prebunching resonator, which is formed by a pair of spherical mirrors. A centered hole in one of the mirrors is used to couple power into the resonator, the size of which is optimized in cold tests. The experiment will investigate longitudinal mode selection, efficiency enhancement, and phase-locking in an open resonator.