THERMIONIC VACUUM TUBES 



59 



It has been the practice in high frequency amplification to use 

 tubes with ju's between 6 and 10, interstage transformers being selected 

 to step up the voltage as much as is possible consistent with the 

 desired flatness of the amplifier characteristic. In general, the larger 

 the ratio of the transformer, the more pronounced is the peak of the 

 characteristic. Other things being equal, the most suitable tubes are 

 those with the smallest internal electrostatic capacities. The largest 

 of these capacities, in general, is that between grid and plate and 

 tubes have been produced in which this does not exceed 5 n /j. f. and 

 in which the internal plate resistance is about 20,000 ohms. 



In amplifying the higher frequencies the feed-back which occurs 

 through the tube may require attention. In section 13, it was pointed 

 out that an inductive output for a tube gives rise to a negative resist- 

 ance characteristic in the input which means that feed-back is occur- 

 ing. To eliminate the possibility of singing and also to eliminate 

 unequal amplification of different frequencies which feed-back intro- 

 duces, various means of neutralizing it have been proposed. 29 One 



Fig. 30 



such means is illustrated in Fig. 30 which is drawn to show radio re- 

 ception with a loop antenna. Note that the grid of the first tube is 

 joined to one end of the loop and the plate is joined to the other end 

 through the balancing condenser C b , the filament being joined to the 

 midpoint of the loop. When C b is chosen equal to C 3 the capacity 

 between grid and plate, it is evident that the feed-back occurring 

 through the tube is just balanced by that occurring through C b . By 

 adjusting the condenser C b so as to permit of feed-back, very large 

 amplification may be obtained at a single frequency but at the ex- 

 pense of flatness of characteristic. 



29 See Patent No. 1,183,875 issued to R. V. L. Hartley, and Patent No. 1,334,118 

 issued to C. \Y. Rice. 



