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Signal and noise in microwave tetrodes are discussed with particular emphasis on their behavior as space-charge conditions are varied in the grid-screen, or drift, region. The analysis assumes that the electron-stream velocity is single-valued. For particular conditions the noise figure may be substantially improved by increasing the space-charge density in the grid-screen region until an entering electron encounters a field of a certain magnitude. The noise reduction is largely due to the cancellation in the output of the noise produced by the random cathode emission. The method of noise reduction described is applicable only when the transit angles of both input and drift regions are fairly long. In a forthcoming paper, H.V. Neher describes experimental results which broadly agree with the theory.