GAIN AND BALANCE CONTROLS 



the usual value and making Rj^ and R^ approximately equal. A higher 

 value of HT positive is generally required. 



Feedback gain control, to fill in between steps, is applied to d.c. amplifiers 

 in exactly the same way as to a.c. coupled. 



HT+ 



HT* 



HT* 



HT+ 



HT- 

 Figure 12.31 Figure 12.32 



Gain balance 



We have seen earlier thattit is particularly important to arrange that the 

 gain of the two halves o^Xht first amphfying stage be made equal, and this is 

 achieved by a differential gain control, i.e. one which increases the gain of one 

 half and reduces it in the other. 



This may conveniently be achieved by differential variation of the anode 

 loads {Figure 12.33). If the valves can be selected for ^,„'s within, say, 5 per 



>/?/ 



Input 



Figure 12.33 



Figure 12.34 



cent of a norm, then in the worst possible case the amplifier will have to 

 compensate for a 10 per cent unbalance in g^ by providing up to 10 per cent 

 unbalance in anode load. This it can do if i?j is chosen 10 per cent of Rj^, or a 

 bit more for safety. 



189 



