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Transient plasma-physics/electromagnetic codes were used to analyze microwave tube circuits excited by an unmodulated electron beam. The objective was to assess the value of Mission Research Corporation's codes "SOS" and "MAGIC" (for 3- and 2-dimensional analysis, respectively) for analysis of oscillation in microwave tubes. For the 2-D case the Q of a cavity due to beam loading was obtained and corroborated by other methods. For the 3-D cases the computed resonance frequencies in a resonated section of coupled-cavity TWT were corroborated by cold-test measurement. Despite the coarse frequency resolution inherent in the relatively short-duration transient analysis performed for a higher-order antisymmetric mode, it was possible to isolate several of the resonances sufficiently to determine the negative Q due to beam loading. Thus the capability of these programs to perform quantitative stability analysis on microwave tube circuits plus beam was demonstrated.