The purpose of this work is to determine the efficiency of low-voltage multibeam klystrons in the Ka-frequency range, using an electron-optical klystron-analog system operating in the Кu-range. Methods. The natural frequencies and main dimensions of the resonators were found by solving the Maxwell equations by the finite difference method in the time domain (FDTD) with a rectangular spatio-temporal partition grid. The parameters characterizing the interaction (characteristic impedance, coupling factor, relative electronic conductivity, Q-factor) were calculated by numerical integration and differentiation methods of the obtained distributions of the electromagnetic field of the resonator. Evaluation of the effectiveness of various designs of multibeam klystrons was carried out using the well-known one-dimensional programs «AJDISK» and «DISKLY». Results. Two designs of multibeam klystrons were considered: with six and nine resonators. It is shown that the use of the electron-optical and magnetic systems of the low-voltage multibeam klystron of the Ka-band allows us to create an efficient amplifier operating in the Ku-band with an output power from 0.5 kW (version with 6 resonators) to 1 kW (version with 9 resonators). Conclusion. The proposed designs of low-voltage multibeam klystrons make it possible to obtain efficient amplifiers in the Ka-frequency range with an output power of up to 1 kW. At the same time, the use of an electron-optical system of a klystron-analogue operating in the Ku-band ensures a reduction in production costs.