130 BELL SYSTEM TECHNICAL JOURNAL 



to the input of the power amplifier, when the program is to be trans- 

 mitted to a local audience; or to be connected, through a transformer 

 of proper impedance, to the long distance lines when the program 

 is to be transmitted to a distant audience, or to a radio-broadcasting 

 station. The filaments of the tubes are supplied from a 12-volt 

 storage battery, while the plate circuits obtain direct current at 350 

 volts from the power supply equipment mentioned later. Arrange- 

 ments are also provided for using 130 volts instead of 350 volts under 

 certain conditions. The proper grid potentials are obtained by 

 utilizing the drop over a resistance in the filament circuits of the first 

 two tubes, and for the third tube small dry cells furnish the grid 

 potential. The maximum gain with this amplifier is 85 miles, which 

 expressed as a power ratio is 1.2 x 10 8 . Under this condition the out- 

 put is approximately 3/l0 of a watt. The front and rear views of 

 this amplifier, mounted on the supporting rack, as shown in Fig. 13, 

 where the gain regulating potentiometer, the rheostats for controlling 

 the filament, and transmitter currents, the three tube mountings 

 with protective gratings and the jacks which permit the connection 

 of instruments for determining the current flow in the filament, plate 

 and transmitter circuits, will be noted. Great care was taken in the 

 design of this amplifier to obtain as nearly as possible equal amplifica- 

 tion of all the important frequencies in the voice range. The trans- 

 formers, and the retardation coils in the plate circuits were chosen 

 with this consideration in mind. 



Power Amplifiers 



For practically all of the larger installations the maximum power 

 possible with the system is required and the output from the trans- 

 mitter-amplifier is taken directly to the high power amplifier. Re- 

 ferring to Fig. 9 it will be seen that this is a four-tube amplifier so 

 connected that but one stage of amplification is obtained. Usually 

 alternating current at 12 to 14 volts is used for heating the filaments 

 of these tubes, the latter being connected in what we know as a push- 

 pull arrangement. It will be seen that each side of the push-pull 

 arrangement consists of two power tubes in multiple. It is inter- 

 esting to note that this push-pull arrangement of the tubes will deliver 

 somewhat more power for equal quality than the same number of 

 tubes connected in the ordinary multiple arrangement, since the 

 tubes may be worked beyond the straight part of their characteristic. 

 The grid potential is chosen to permit the largest variation of cur- 

 rent without distortion and is obtained from a group of small flash- 

 light batteries. 



