The overall effect of the original developments was to produce a highly effective power amplifier tube, whose anode characteristic is very much like that of a pentode, but which has greater efficiency as a result of reduced screen current. A further bonus was that third harmonic distortion was much reduced relative to a comparable pentode (Terman pp 198–9). Beam tetrode audio power output valves were introduced in 1937, and quickly replaced conventional pentodes in this application. Later developments produced beam power tubes which were capable of high-power output at frequencies extending into the UHF region.