The use of a cluster of cavities in frequency multiplying gyro‐amplifiers is considered. An analytical theory has been developed to maximize the second harmonic current and optimize the drift section length for the case of a single low‐Q input cavity, operating at the fundamental cyclotron harmonic, and bunching clustered cavities operating at the second‐harmonic. MAGY simulations have been conducted to benchmark the theory and further study the detailed characteristics of cluster‐cavity gyro‐amplifiers. The theory and MAGY code simulation agree. In the small signal regime, the bandwidth of a cluster‐cavity device (with a pair of cavities in the cluster) is twice that of a single cavity device, while both have the same peak bunching. With a gyro‐TWT output section, a peak power of 247kW, efficiency of 24.2&percent; and bandwidth of 1.08&percent; have been simulated using a cluster of cavities as a buncher. Also, the power‐bandwidth product is 105kW×MHz, which is double that of the single cavity buncher case. © 2003 American Institute of Physics