HARD VALVES 



The presence of the transformer modifies shghtly the procedure for graphi- 

 cal design, for we have now two kinds of load. (1) The 'static load' or 'd.c. 

 load', which is the load relevant to the determination of the mean working 

 point as defined by the steady — or zero frequency — bias, anode voltage and 

 current. The static load is the resistance of the transformer primary winding, 

 which is usually negligible. (2) The 'dynamic load' or 'a.c. load' which is 

 the load as seen by the load when handling signals, and which is given by 

 Rj^. The diagram for graphical design is drawn in the following manner. 



Choose a mean working point. Drop a vertical straight line through it 

 to the voltage axis. This is the d.c. load line and the HT required may be 



read off. Draw the dynamic load through the working point, having the 

 appropriate slope {Figure 8.16). 



Choice of transformer ratio — For a given input signal the maximum power 

 is transferred to the load when the optimum transfer conditions are met. In 

 practice these conditions are deliberately not met; let us see why this is. 



If the load resistance reflected into the valve circuit is other than r^ the 

 system is mismatched, and more voltage swing will be necessary at the valve 

 grid to develop the same power in the load. This is usually a minor problem 

 and can be dealt with by a bit more amplification at an earlier stage. A 

 much more serious difficulty is to get a load condition such that the greatest 

 voltage swing may be appUed to the valve grid without taking the working 

 point outside the working region. The 'optimum load' is the load into which 

 the valve will deliver the maximum signal power without distortion. We 

 saw that with the direct coupled power amplifier low values of load were 

 unfavourable from the point of view of possible input swing. For a triode 

 it can be shown that the optimum load is twice r^, for the loss of output 

 power due to the mismatch is overshadowed by the possibility of increasing 

 the input. We shall not prove it rigorously, but the reader may care to 

 experiment with some load fines on an actual published anode characteristic. 

 The output power and efficiency of a power amplifier may be read off" as 

 follows. In Figure 8.17, if the grid swings between Vg-^ and Vg^, the anode 

 current swings between £■ and Fand the anode voltage between G and H. Now 

 these are peak-to-peak values. For a sine wave input the R.M.S. voltage 



142 



