RADIO BROADCASTING TRANSMITTERS 



129 



in Fig. 4. As will be seen, it consists of seven panel units with a 

 screen enclosure in the rear. The first unit on the left is the oscillator- 

 modulator. This is essentially a low-power transmitter capable of an 

 output of 50 watts and 100 per cent modulation. It is followed by 

 three push-pull stages amplifying modulated radio-frequency power. 

 The first power-amplifier stage, which employs two 250-watt tubes, 

 occupies the second unit. The tubes for the second power stage, 

 which are water cooled, and the associated tuned output circuit are 

 contained in the third and fourth units, respectively. The final power 



Fig. 4 — Western Electric 7-A (50-kw) radio transmitter. 

 Oscillator-amplifier assembly. 



stage, incorporating six water-cooled tubes each capable of a peak 

 output of approximately 40 kw., occupies the fifth unit. The last 

 two panels constitute the front of an electrically screened enclosure 

 housing the output circuits for this latter stage. All of the panels 

 are aluminum covered with several coats of black lacquer grained by 

 rubbing with abrasive paper. A full complement of meters is pro- 

 vided, the cases of which are either grounded or mounted behind 

 glass for the protection of the operating personnel. In designing the 

 equipment, special consideration has been given to safety. Access 

 to the apparatus in the rear of the panels can be had only through 

 the door on the left which is held closed by a bolt operated by the 

 hand wheel shown. The rotation of this wheel opens the transmitter 

 control circuits putting the equipment out of operation. It then 

 grounds the high-voltage supply busses and finally withdraws the 

 bolt. As an additional precaution a manually operated disconnect 

 switch for the main power supply is provided just inside the gate 

 which can be opened on entering. Access to some of the tubes is 

 had by opening the glass windows in the various panels, but these are 



