The phase-modulation sidebands obtained from a klystron amplifier with beam-voltage modulation may be used to give mixing action. The output resonator is used to select a phase-modulation side-frequency component to give power output at a sum or difference frequency. The method of obtaining phase modulation in a beam-type amplifier is described. The construction and operating details are given for a klystron amplifier which has a mixer output of 1 watt over a 20-Mc band for a 6,000-Mc television relay system.