POWER PACKS 



A and O. The output voltage plus the reference voltage* appears between 

 O and C, and a definite fraction between O and B is applied to the other 

 input of the diiferential amplifier. Any error in the output voltage produces 

 a signal at D which is fed forward to the absorber valve grid. The sense of 

 the connections is such that a tendency for the negative output voltage to 

 increase, increases the absorber valve current and increases the voltage 

 drop produced across the 5k resistor. 



The upper part of the circuit, concerned with the positive supply, uses 

 conventional series stabilization, but notice how the input to the amplifier 

 valve is derived by comparing the positive output with the negative output 

 rather than with a second reference tube, thus achieving a useful saving in 

 components. 



Wide-range HT supplies 



When developing a new piece of apparatus it is extremely useful to have 

 a power supply yielding HT voltages continuously variable over a wide 

 range, say to 350 V. Control by variable series resistor is not satisfactory 

 because it gives very poor regulation. Potentiometric control is not very 

 practicable unless only a very light load is to be drawn, since for the output 



o + 



Unstabilized 

 supply in 



+ o 



Variable forward - 



stabilized supply out 



— o 1 * * o — 



Figure 37.11 



impedance to be small at all settings of the output control a value of poten- 

 tiometer is required so low that it bleeds a great deal of current. What we 

 can do, however, is to supply the power via a cathode follower whose grid 

 potential is variable. Since the grid current is negligible the grid may be 

 potentiometrically fed from a component of high resistance, and therefore, 

 since the potentiometer current is low, it may easily be cold-cathode- 

 stabilized. The scheme is shown in outline in Figure 37.11. Notice that 

 the output is described as 'forward-stabilized'. It is protected from mains 

 voltage fluctuations but not from load current fluctuations, since the output 

 impedance is not very low, being \jg^ for the cathode follower, a few 

 hundred ohms. Nevertheless such a device is extremely useful, and a 

 version used by the author appears in Figure 37.12. 



It is possible to combine the functions of control and rectification in a 

 single valve. When this is done it is called a cathode-follower-rectifier. 

 A power supply along these lines has been described by Walker^ > 400 V, 

 > 100 mA) and another by Perry^ > 350 V, 40 mA). 



* The addition of the reference voltage is incidental to the operation. It allows a larger 

 fraction of the output to be used for control purposes. 



592 



