Experiments have been performed at NRL to determine the saturation behaviour and maximum conversion efficiency of emission gated electron beams in slow wave circuits. Simulations suggest that single-pass conversion efficiency exceeding 50% to the fundamental harmonic may be achieved with moderately tight bunches in a radically tapered helix having a velocity ratio of 3:1. The NRL Twystrode Experiment is a flexible instrumented amplifier yielding information on a wide parameter space, allowing thorough verification of analytical and simulation models. The electron beam (from a gridded thermionic cathode) can be modulated in any degree from DC up to average-to-peak beam current ratios of less than 0.15. The beam voltage is variable from below to well above synchronism. The Twystrode Experiment operates at UHF (487 MHz), a low frequency for which rugged, longlife gridded thermionic cathodes are available. The experimental results, and especially the experimentally verified simulation models, forecast the X-band performance obtainable with future Field Emission Array cathodes. Such FEA twystrodes are a prime candidate for an improved vacuum power booster in the next-generation Microwave Power Module. The experimental results will be compared to both particle simulations and analytic calculations.