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This paper describes the observation of pulse shortening in an 11.4-GHz magnicon amplifier due to competition with a parasitic gyrotron mode. This effect was observed following the installation of a new electron beam collector. The parasitic mode occurred only at high operating powers, when the beam transverse momentum was high, and the growth of the parasitic mode caused the power in the output pulse to drop substantially. We analyze the competition between the gyrotron and magnicon modes in the output cavity using a time-dependent multimode gyrotron simulation code that has been specially modified to model competition between synchronous magnicon and nonsynchronous gyrotron interactions, and compare the results to experimental observations. We also analyze the effect due to the change in the collector.