THERMIONIC VACUUM TUBES 



61 



ages. It consists of a Wheatstone bridge in which two similar tubes 

 form one pair of arms. The output circuit is the branch in which 

 the galvanometer is ordinarily placed. When a voltage is applied 

 to the two tubes in such manner that the potential of one grid is 

 raised by the same amount as the grid of the other is lowered, the 

 bridge becomes unbalanced and current flows through the output 



Fig. 33 



branch. For small applied voltages the amplification of the circuit 

 is very nearly distortionless. The circuit has the obvious disadvantage 

 of requiring a close balance between the tubes and is therefore liable 

 to require careful adjustment during use. 



The push-pull circuit possesses one marked advantage over the 

 resistance-coupled d.c. amplifier described in Sec. 18 for in it current 

 flows through the output branch only when voltage is applied to the 

 input. For the same reason it is also useful for amplifying low fre- 

 quency alternating voltages. 



V. Amplifier Power Supply 



The proper power supply for amplifiers is an item of prime im- 

 portance. 



25. Plate Voltage Supply. The principal requirement placed on 

 plate voltage is that it be steady. For this reason storage batteries 

 are usually best, but good dry cells are more often used and when 

 fresh, prove very satisfactory. The principal trouble encountered 

 in the use of dry cells arises from an attempt to use partially rundown 

 cells. A dry battery should be tested periodically for voltage, the 

 reading being taken while the battery is delivering a current at least 

 as large as that drawn by the amplifier. Whether dry cells or storage 

 cells are used for plate voltage, in general not more than four stages 



