REPEATER DESIGN — NEWFOUNDLAND-NOVA SCOTIA LINKS 255 



open-circuited, it causes a permanent direct short-circuit across the 

 heater chain. The Hne voltage will be temporarily increased by about 95 

 volts while the fuse operates (1 min) and will then drop to 12 volts below 

 normal. 



Amplifier. 



The amplifier circuit is shown in Fig. 3. It consists of two 3-stage 

 amplifiers connected in parallel between common input and output trans- 

 formers with a single feedback network. This circuit arrangement al- 

 lows one amplifier path to fail without appreciably affecting the gain 

 of the complete amplifier (less than 0.1 db for all faults except those on 

 the grid of VI and the anode of V3, but the overload point is reduced by 

 about 5 db and distortion at a given output level is increased (about 

 12 db for second harmonics). Care has been taken to ensure that the 

 open- or short-circuiting of a component in one amplifier path will not 

 affect the performance, life or stability of the remaining path, and this 

 involves the duplication of certain components. 



Mixed feedback is employed to produce the required output imped- 

 ance ; the current feedback is obtained from the resistor feeding the high- 

 voltage suppl}^ to the output transformer, and the voltage feedback is 

 developed across a two-turn winding on the output transformer, which 

 also serves as a screen. The output of the feedback network is fed in series 

 with the input signal to the grid of VI. The gain response of the amplifier 

 is chiefly controlled by the series-arm components in the feedback net- 

 work, which resonate at 600 kc. 



The input transformer is built out as a filter and steps up in imped- 

 ance from 55 ohms to 17,000 ohms. Protective impedances minimize the 

 effect of a short-circuit on the grid of one of the first-stage tubes. The 

 anode load of the first stage resonates at 600 kc, and is roughly the in- 

 verse of the feedback network so as to give constant feedback loop gain 

 over the working frequency band. The output tube has about 5.5 db of 

 feedback from its cathode resistor, and the pair of output tubes feed the 

 output transformer, which steps do^vn from 5,000 to 55 ohms. 



Specially designed long-life tubes are used.'^ The first two stages are 

 operated at about 40 volts on the screens and anodes; each anode cur- 

 rent is 3 ma, giving a mutual conductance of 5.1 ma/v. The output stage 

 is operated at 60 volts on the screen, -|-15 volts on the suppressor grid 

 to sharpen the knee of the Va/h characteristic, and nearlj^ the full high- 

 voltage suppl}^ of 90 volts on the anode ; the anode current is 6 ma, giving 

 a mutual conductance of 6.6 ma/v. The tube dynamic impedance is ap- 

 proximately 300,000 ohms. To obtain an anode current nearest to the 



