GAIN AND BALANCE CONTROLS 



feedback R^ is split into two parallel parts and the cathodes are joined by a 

 variable resistance (Figure 12.29). This usually involves the use of a negative 

 HT supply, as in d.c. amplifiers. When Ry is reduced to zero the stage has a 



HT+ 



HT+ 



HT+ 



HT* 



Figure 12.28 



HT + 



HT+ 



^AA/^A^ 



AAAAA^ 



• HT- 



HT* 

 Figure 12.29 



/2Ry >Ri( 



long tail, R^, as before ; and the anti-phase gain — if both outputs be used — is 

 gm^L- As Rj' is increased, cathode follower-like action becomes possible and 

 the anti-phase gain falls. It is clear from the symmetry of the system that the 

 mid-point of Ry must remain at a fixed potential and therefore as far as 

 potential changes are concerned the equivalent circuit of each half is as that 

 shown in Figure 12.30. 

 We have 



total gain = j-^-^ 



187 



