Abstract: In a high efficiency amplifier system a Traveling-Wave Tube (TWT) power amplifier is driven to saturated output by a second TWT (driver TWT) which is preferably but not necessarily operated at saturation. Broad band operation, over two octaves, is achieved by harmonic conditioning. In addition to the usual phase shifter harmonic conditioner, an interstage equalizer and an input equalizer are added. The attenuation vs. frequency characteristic of each of these two equalizers is properly proportioned to cause the driver TWT to develop a specific desired ratio between harmonic amplitude and fundamental amplitude output at each frequency. When a specific driver and power amplifier TWT were operated in a laboratory test and the two equalizers were adjusted for optimum fundamental output power, the interstage equalizer had maximum attenuation at the second harmonic frequency (relative to the low frequency end of the operating band) and the input equalizer had maximum attenuation at the third harmonic frequency.