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BELL SYSTEM TECHNICAL JOURNAL 



springs shown. The heating element consists of a winding of re- 

 sistance wire inserted in the block concentric with the thermostat. 

 The assembly is mounted in a thermally insulated box, shown on its 

 side in the photograph. Two of these units are provided, one located 

 on each side of the oscillator-modulator unit directly below the win- 

 dow. A detachable handle for adjusting the contacts of the thermo- 

 stat and a suitable thermometer extend through the box to the front 

 of the panel. The brass mounting block is provided with a groove 

 to receive the bulb of the thermometer. The thermostat does not 

 operate directly in the heater circuit but controls the grid bias of a 



Fig. 9 — Simplified circuit schematic of 7-A (50-kw.) radio transmitter. 



vacuum tube in the plate circuit of which a suitable relay is placed. 

 The quartz plates are ground to oscillate at the assigned frequency 

 at approximately 50 deg. C, and the final adjustment is made by 

 varying the operating temperature. The temperature coefficient of 

 the plates varies from 30 to 100 parts in a million per deg. C. The 

 degree of constancy attained necessarily depends on the diligence of 

 the operating personnel. With proper maintenance the maximum 

 deviation has been held to ± 30 cycles for long periods of time. 



A simplified circuit schematic is shown in Fig. 9. Features of the 

 electrical design are the modulation system, the push-pull amplifier 

 stages with cross neutralization, the capacity coupling arrangement 

 used to facilitate control of parasitic oscillations, and the provisions 

 for the suppression of harmonics. The modulating amplifier is a 

 50-watt tube operating at 750 volts. The audio power stage employs 

 a 250-watt tube at 1 ,500 volts. In this manner, ample audio-frequency 

 voltage and power are provided to effect complete modulation without 



