458 



BELL SYSTEM TECHNICAL JOURNAL 



and cushioned in sponge rubber. The tubes are electrically connected 

 in cascade by means of resistance-capacity coupling, so that the 

 whole amplifier system is stable over long periods of time and is also 

 uniformly efficient over the required frequency band. Grid bias for 

 the small tubes is supplied by the potential drop across a resistance 

 in the filament circuit; the power requirements for the low level 

 stages of the amplifier are filled by 6-volt filament batteries and 135- 

 volt plate batteries, all located externally where they can be checked 

 and replaced conveniently. 



The amplifier system is divided into units of convenient size as 

 shown in Fig. 8. Associated with each of the three banks of photo- 



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Fig. 8 — Schematic diagram of the complete television channel and the relative 

 voltage levels of the signal along the channel. 



electric cells is a two-stage unit known as the photoelectric cell amp- 

 lifier; the combined output of these three units is carried to a four- 

 stage unit known as the intermediate amplifier whose output is of 

 sufficiently high level to be carried outside the copper cell cabinet to 

 the three-stage transmitting power amplifier on the relay rack. A 

 four-stage unit known as the receiving power amplifier is also on the 

 rack, and serves to amplify the signal from the other station to a 

 level which will yield an image of satisfactory contrast when it is 

 translated into a light variation by means of the neon lamp. The 

 final stage of this amplifier consists of two special 250-watt tubes in 



