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The general characteristics of the ideal output tube for broadcast receivers are discussed briefly with respect to specific electrical and acoustical requirements. Considerations of practical power-tube design indicate that the tube most nearly approaching the ideal characteristics is one having an accelerating grid (screen) and a control grid which does not require power. The limitations of conventional output tetrodes and pentodes with respect to the ideal are treated and are illustrated by means of oscillograms and models showing field-potential distributions. It follows that homogeneous potential fields and directed electron beams having high electron density can be utilized to minimize these limitations. These design features indicate the feasibility of a tube suitable for operation as a class A amplifier having substantially second-harmonic distortion only and capable of high power output, high efficiency, and high power sensitivity. The theoretically proper geometric structure for beam power tubes is developed. The theory is substantiated by performance data obtained from actual tubes.