WIDE-BAND OPEN-WIRE PROGRAM SYSTEM 361 



so as to have very high inductances the desired phase distortion re- 

 quirements were met while at the same time the necessary flatness of 

 gain characteristic was obtained at the low frequencies. 



Figure 10 shows the transmission circuit of the Una amplifier and 

 monitoring amplifier. This device has a 600-ohm input and output 

 impedance and consists of two stages of push-pull amplification. The 

 potentiometer is a balanced slide wire having a continuous gain adjust- 

 ment over a range of 6 db. A balanced input transformer serves to 

 connect the potentiometer to the grids of the two push-pull vacuum 

 tubes which function as the first stage of this amplifier. The first 

 stage is connected to the second or power stage by means of resistance 

 coupling which gives better results both as to phase distortion and 

 low-frequency gain characteristics than if transformer or retard coil 

 coupling were used. Resistances are provided in the grid circuits of 

 the second stage so that the high-frequency characteristic may be 

 adjusted as required. The power tubes are connected to an output 

 transformer which has the unique feature of providing a monitoring 

 outlet which is not materially affected by voltages produced at or 

 beyond the line terminals. The transformer, as may be observed, 

 consists of three balanced windings arranged as in the form of the 

 well-known hybrid coil used in two-wire telephone repeaters, with the 

 exception that the two low impedance windings are of unequal ratio,* 

 the line windings having many more turns than the monitoring wind- 

 ings. The ratio of the windings is such that the voltage at the monitor- 

 ing terminals when said terminals are closed through 600 ohms is 30 

 db below the voltage at the line terminals. Resistances are inserted in 

 series with the monitoring winding so that an impedance of 600 ohms 

 will be presented at the monitoring terminals. 



The average gain of the amplifier with the potentiometer set at its 

 maximum position is 33 db. Of 100 amplifiers measured, the gain 

 at 35 cycles averaged .10 db less than the gain at 1,000 cycles while 

 from 100 to 8,000 cycles the gain was constant within .05 db. The 

 delay at 50 cycles is approximately .6 millisecond greater than it is 

 at 1,000 cycles. From 150 to 8,000 cycles the delay is substantially 

 constant and is only a small fraction of a millisecond. The amplifier is 

 capable of handling an output power 9 db above reference volume 

 without noticeable distortion. 



At several points along a program circuit taps or branches are pro- 

 vided so as to connect various broadcasting stations to the program 

 circuit and also to connect to other program circuits which form part 

 of a broadcasting network. Points where such connections or branches 

 are made are commonly called bridging stations. At some points as 



