[en] Conventional millimeter amplifiers (coupled-cavity TWTs or gyro-klystrons) are limited in power by the maximum current that can be accommodated in a single beam. Cathode current density, beam optics, and the magnetic field necessary to confine the beam, combine to limit beam current and add cost and bulk to the device. If the microwave source is designed as a pulsed klystron operating at a high voltage, larger lateral as well as axial dimensions can be employed. Beam optics become easier and permanent magnet periodic (PPM) focusing is possible. A high efficiency also results, because of the low perveance. A number of klystrons can then be fabricated on single substrate, using a deep-etch lithography technique. They can be water-cooled individually, and operated in parallel. Several such modules can be stacked to form a klystron brick, requiring a relatively low voltage for the peak and average power produced. The brick can be provided with a single output, or with individual, spatially-combined radiators. The design of a 4 x 10 x 1.5-inch module producing 500 kW peak, 500 W average at 91 GHz, and operating at 120kV, 10 A, is described