DIRECT-COUPLED PRE-AMPLIFIERS 



In the case of pentode cathode-followers the bias is determined by the 

 screen potential, and it is this which must be varied to find the zero grid 

 current condition. 



The range of anode voltages which it is necessary to provide for triode 

 cathode followers, and of screen voltages for pentode versions, lies between 



+ 40 



*25 



HT + 



HT- 

 Fi^ure 39.10 



+25 and +40 V for the types of valve tested— Mullard EF37A and ME1400. 

 The author has not tried this technique with pentodes, as a variable supply 

 carried on the cathode is not easy to arrange. With triodes, however, it is 

 not too difficult; the first method which occurs to one is Figure 39.10, but 



-« 9V grid- bias 

 [ battery: taps every 

 -!- 1V2V 



in 



t 



{ HT battery: 

 I taps every 

 i 9V 



JL 



Figure 39.12 



this is not satisfactory. For correct operation, the resistance seen looking 

 back into the network supplying HT to a cathode follower ought to be low ; 

 to achieve this the resistor values in the divider chain have to be so small 

 that a current is wasted down them of a magnitude which is intolerable in 

 battery-operated gear. A promising alternative is to feed the cathode fol- 

 lower from another cathode follower {Figure 39.11). In this case the output 

 impedance of the variable HT supply to the lower valve is Ijg^ for the 

 upper, and the potential-divider chain can have as high a value as is wished. 

 However, in view of the extra valve involved, the method finally adopted is 

 shown in Figure 39.12. The required HT is selected to the nearest 9 V by 



625 



