A transformerless amplifer is described using four tubes connected in a bridge circuit and so driven that each tube delivers equal power to a common load. The ac equivalent circuit of the amplifier may be represented by a generator of voltage 2µe(symbol) in series with the plate resistance rp of one tube. The amplifier is characterized by wide frequency response, low distortion, and good transient response. Negative feedback is easily applied, and there is no direct current flowing through the load. Two complete audio amplifiers are described using this output circuit. One of these delivers 18 w to a load of 200 ohms, with a total intrmodulation distortion of only 0.7%. It weighs only 4 lb, 3 oz, including the power supply and preamplifier. This amplifier circuit is also suitable for use in aircraft equipment and other applications where compactness and light weight are major considerations.
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