76 BELL SYSTEM TECHNICAL JOURNAL 



Of course in actual practice, it is necessary to provide a d-c path for 

 the space current of the tube, which can be accomplished by shunting 

 the condenser, C5, with a high impedance choke. 



It is often the case that a stopping condenser is desirable in the X4 

 position, instead of a direct connection between grid and tuned circuit. 

 This stopping condenser and the accompanying leak are advantageous 

 inasmuch as it has been found by experience that an oscillator oper- 

 ating with a leak and condenser combination is inherently much more 

 stable as regards change of frequency with change of battery voltage 

 than an oscillator with a d-c low resistance path from grid to filament, 

 even when a battery is employed to impose a negative bias on the grid. 

 The explanation for this improved stability lies in the fact that the grid 

 leak tends to keep the grid resistance, Vg, constant. It frequently hap- 

 pens, when the leak and condenser combination is used, that difficulty 

 is experienced in avoiding "blocking" when a large enough condenser 

 to have negligible reactance is employed. In such cases the required 

 value of C5 may be chosen in the manner indicated in connection with 

 Fig. 4, which allows for a finite reactance between grid and tuned 

 circuit, or else, as another alternative, the plate may be directly con- 

 nected to the tuned circuit so that X^ is zero, and stabilization may be 

 accomplished by choosing the value of C4 in accordance with the re- 

 quirements then imposed by (8), which refer to Fig. 3 and necessitate 

 the value of capacity shown in the figure. Another possible stabilizing 

 arrangement is shown in Fig. 4, where either capacity or inductance 

 may be used on the plate side, depending on the value of capacity at 

 d on the grid side. Yet another possible modification of Fig. 4 would 

 be to use an inductance on the grid side. This would require a very 

 small capacity on the plate side, and probably is less convenient than 

 the arrangement indicated in the figure. 



In all three of the cases considered thus far, the equations show that 

 the value of the stabilizing capacity or inductance depends upon the 

 values of L], L2, M, and C3 so that if the frequency of the oscillator 

 were varied intentionally, by changing L], for instance, then a dift'erent 

 value of stabilizing capacity or inductance would be required to secure 

 independence of frequency and battery voltage at the new frequency. 

 If, however, the circuit were so constructed that M were zero, and Li 

 and L2 were made so that they remained always equal to each other, 

 then the value of the stabilizing element would depend upon C3 only, 

 and the frequency could be changed by varying Lx and L^ simultane- 

 ously without destroying the stabilizing adjustment. 



