AMPLIFIERS 283 



provided for simultaneous reproduction at both Philadelphia and 

 Washington. There were three complete and essentially equivalent 

 channels of equipment actually in use and a fourth complete channel 

 held in reserve as a spare. 



Several stages of so-called voltage amplification were required pre- 

 liminary to the final or power stage. There is, of course, no essential 

 difference between a voltage amplifier and a power amplifier, the term 

 "voltage amplifier" being applied to those preliminary stages of an 

 amplification system the function of which is so to amplify the output 

 of the pick-up device as to supply adequate driving voltage to the 

 grids of the power stage. Theoretically, inasmuch as no energy is 

 absorbed in the ideal grid circuit, this voltage increase might be 

 supplied entirely by a high ratio input transformer. However, there 

 are practical difficulties to the design of such a single stage amplifier 

 and therefore multistage vacuum tube amplification is employed. 



As a matter of convenience the voltage amplification for this system 

 was obtained through the use of several separate amplifier units in 

 tandem. This arrangement not only enabled the ready replacement 

 of any unit of the system in case of failure, but it also facilitated the 

 insertion of a pad, control potentiometer, or other network at any 

 desired point. Several of these devices were required, and of course 

 each introduced a loss. Thus the gross amplification of the system 

 used for reproduction at Philadelphia was approximately 160 db and 

 for Washington 240 db, although the actual difference in level between 

 microphone output and loud speaker input was but from 80 to 90 db. 



The general scheme of the amplification system is shown in Fig. 1. 

 yli is a single-stage, single-tube Western Electric No. 80A amplifier 

 ^lightly modified to meet the particular conditions of use; it has a 

 gain of 30 db, and employs a Western Electric No. 262A vacuum tube. 

 This tube has an equipotential cathode, the heater being operated on 

 10-volt 60-cycle alternating current and the anode being supplied from 

 rectified alternating current. ^2 is a 2-stage amplifier having a single 

 Western Electric No. 262A vacuum tube in the first stage and push- 

 pull Western Electric No. 272A tubes in the second stage. It has a 

 gain of 50 db. The cathodes of the tubes are energized with low- 

 voltage 60-cycle alternating current and the anodes with rectified 

 alternating current. A^, the final or power amplifier, is a single stage 

 amplifier employing two Western Electric No. 242A vacuum tubes in 

 parallel on each side of a push-pull circuit, thus having four tubes per 

 amplifier. Two of the Az amplifiers were used in parallel on each 

 channel, and were capable of supplying 60 watts each, or a total of 

 120 watts, to the loud speakers. These are r.m.s. values. The instan- 

 taneous peaks of power of course could equal twice this value, or 720 



