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Two new single-velocity electron stream amplifiers, which use neither slow-wave guiding nor resonant circuitry, are described. The major part of the amplification is obtained in a drifttube region where either the tube diameter or the stream diameter varies periodically. The period length is a quarter-plasma waveength; the gain mechanism is one of reinforcing the simple harmonic motion of the electrons in the stream. A general theory is presented from which a gain formula is obtained for rippled wall, rippled stream, and velocity jump amplification. Design curves are shown. An experimental rippled wall amplifier operating in the range 1 to 4 kmc is discussed.