640 BELL SYSTEM TECHNICAL JOURNAL 



circuit designed to afford a high degree of stabiUty. This was con- 

 nected to the input of the modulating ampHfier through two radio- 

 frequency stages, also employing 50-watt tubes. These two stages 

 precluded the possibility of the oscillator frequency being appreciably 

 altered by effects due to modulation. The modulating amplifier 

 employed two 250-watt tubes in parallel and operated on the Heising 

 system. In the standard equipment, the audio stages involve one 

 50-watt tube and two 250-watt tubes in parallel. To meet the more 

 rigorous requirements of television with an ample factor of safety, 

 this portion of the transmitter was removed from service and a 

 specially constructed three-stage amplifier was substituted. As shown 

 in the drawing, the latter consisted of two 50-watt resistance-coupled 

 stages and a final power stage based on a 5-kilowatt water-cooled 

 tube which raised the signal currents to a power level of approximately 

 one half kilowatt. 



In order that it might be possible to check the performance of the 

 radio transmitter under all operating conditions, a suitable monitoring 

 rectifier was constructed and coupled to the output circuit of the radio- 

 frequency power amplifier. A circuit was run back to suitable switches 

 on the television control panel so that either the output of the photo- 

 electric cell amplifiers or the rectified output of the radio transmitter 

 could be impressed on the pilot lamp of the television transmitter. 

 By comparing the two images, it thus became a relatively simple 

 matter to detect any serious maladjustment in the radio apparatus. 



The problem of providing a suitable transmitter for the speech 

 channel was rendered quite simple by the fact that at the time there 

 was in process of development at Whippany a 50-kilowatt equipment 

 intended for broadcasting applications. The detailed description of 

 this transmitter is beyond the scope of the present paper. It may be 

 said, however, that it consists of a piezo-electrically controlled master 

 oscillator employing a 50-watt tube directly followed by a 50-watt 

 modulating amplifier. Modulation is by the Heising system, em- 

 ploying one 50-watt and one 250-watt tube in the audio stages. The 

 output of the modulating amplifier is amplified by three push-pull, 

 neutralized, radio-frequency stages the last of which employs six 

 water-cooled tubes at approximately 17,000 volts. This set is capable 

 of delivering 50 kilowatts (unmodulated carrier) to the antenna and 

 during modulation instantaneous peaks approaching 200 kilowatts are 

 attained. 



The radio receiver employed at Whippany for the reception of the 

 synchronizing signals at 185 kilocycles presents no features of unusual 

 interest. A double-tuned input circuit was used followed by three 



