The scaling of operating parameters and the tendencies in the efficiency of the step-tunable gyrotron operating in a set of modes in a cavity of a given length are considered. Results of calculations are reported for design of a ~1 kW, step-tunable gyrotron oscillator operating from 150 GHz to 600 GHz in steps of about 15 GHz. Detailed results are presented for the efficiency, power losses in the cavity, and the starting conditions. Designs of the electron gun electrodes and of an output mode transducer are also presented. Full modulation of the radiation via the modulating anode of a magnetron injection gun is considered. The gyrotron would be of use for plasma diagnostics and spectroscopy, including electron paramagnetic resonance spectroscopy (EPR).