PUBLIC ADDRESS SYSTEMS 



131 



The output transformer at the right of the figure is designed to 

 match accurately the impedance of the tubes to that of the number of 

 receiver-projector units which has been found to give the greatest 

 flexibility under the varying conditions of commercial operation. 

 This amplifier, speaking in telephone terms, is worked at a 

 gain of 23 miles, a power amplification ratio of about 200, the 



Incut Transformer 



MEDIUM POWER AMPLIFIER 



Output Transformer 



Filament 

 Current 

 Transformer 



A.C. 750 Volts from 

 Supply Rectifier 



Fig. 9. 



maximum output being about 40 watts. The plate circuits of the 

 tubes are supplied at a d-c. potential of 750 volts. As has been 

 pointed out previously, this amplifier gives a practically uniform 

 gain for all the important frequencies in the voice range. This high- 

 power amplifier is shown mounted on the supporting rack, in Fig. 13. 

 The apparatus on the rear of the panel is protected with a sheet metal 

 cover and integral with this cover is a disconnecting switch, which, 

 when the cover is removed, cuts off the high potential from all the 

 exposed parts on the set. 



For indoor installations where the audience is small the power 

 output given by the high-power amplifier is not required and a medium- 



