PUBLIC ADDRESS SYSTEMS 



137 



volts, 60 cycles, and delivering 751) volts direct current for the plate 

 circuits is employed. A potentiometer arrangement provides a 

 direct-current supply at 350 volts for the speech-input amplifier 

 tubes. Full wave rectification is obtained and a filter consisting of 

 a large series reactance coil and bridged condensers is used to render 

 the direct-current output suitable for this use. Included in the 



Fig. 14b. 



power equipment is a step-down transformer for supplying the fila- 

 ments of the power amplifier. For the larger installations, employing 

 the rectifier, the total power drawn from the commercial supply is 

 1500 watts. 



For installations of a size not requiring the use of the high-power 

 amplifier, a compact motor generator set is provided consisting of a 

 350-volt d-c. generator driven by a suitable motor, the total power 

 drawn from the supply mains being about 500 watts. A low-voltage 

 generator for supplying direct current at 12 volts for the operation 

 of the amplifier tubes is incorporated in this motor generator set. 

 A filter is necessary and a reactance coil and a 12-volt storage battery 

 is floated across its output. This supplies the transmitters and the 

 tube filaments. The necessary indicating meters are provided on a 



