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BELL SYSTEM TECHNICAL JOURNAL 



grid circuit to prevent the operation of the transformer from being 

 affected by variations in the grid circuit capacities. 



More than one coupling circuit has been used to obtain the necessary 

 selectivity. Several of these have been connected together with 

 either series or shunt condensers to obtain the equivalent of a multi- 

 section band-pass filter. Networks of this type in which the elements 

 have been chosen in accordance with the previously mentioned 

 equations have been employed as band filters because of their simplicity 

 and compactness, and the relatively low cost of the inductance ele- 

 ments. They have been used to connect two equal or unequal 

 impedances as well as to operate from an impedance directly into the 

 grid circuit of a vacuum tube. 



One of these networks was used between the plate circuit of a 

 shielded-grid vacuum tube and the grid circuit of a second shielded- 

 grid vacuum tube at 84 kilocycles. The gain characteristic of a stage 

 of this type is shown in Fig. 4. The capacitance Cj represents the 



80 84 



FREQUENCY 



88 92 



KILOCYCLES 



Fig. 



4 — Transmission characteristic of a transformer coupling circuit operating 

 between shielded-grid tubes. 



capacitance between the plate and shield of the first tube plus the 

 winding capacitance and capacitance C2 represents the winding capaci- 

 tance and effective input capacitance of the second tube. It will be 

 of interest to know that the voltage amplification obtained over the 

 transmitted band was approximately equal to the amplification of the 

 first shielded-grid tube. Consequently, the transformer network 



