SOFT VALVES 



current shall build up quickly to its full value, and to facilitate this it is 

 necessary that there be an adequate supply of electrons near the cathode. 

 To achieve this the tube contains a subsidiary cathode and anode between 



Trig, in 



^^ 



-o+/ 



1 



Figure 7.14 



which a 'keep alive' discharge is permanently maintained. The hght output 

 from the keep-alive discharge ensures a sufficiency of photoelectrons in the 

 neighbourhood of the main cathode. 



PRIMED STABILIZER 



This is a voltage stabilizer tube to which a 'primer' electrode has been added. 

 The primer is returned to a positive supply potential via a high resistance 

 {Figure 7.15). The advantage of the primed stabihzer is a reduced striking 



Figure 7.15 



voltage for a given running voltage, making the ancillary circuitry easier 

 to design. 



THYRATRON 



If a wire grid is interposed between the cathode and anode of a soft hot- 

 cathode diode, and the connection to it is brought out, the performance is 

 modified in the following manner. 



Suppose the grid be made a few volts negative with respect to the cathode, 

 the electrical field produced tends to reinforce the space charge, and assists 

 it in returning electrons to the cathode and in retarding those which escape 

 from it. The average electron velocity is therefore lower and an anode 

 potential which normally would produce ionization now fails to do so. To 

 produce ionization the anode potential has to be made more positive by an 

 amount proportional to that by which the grid has been made negative. The 

 ratio between negative grid 'bias' and critical anode potential to cause 

 ionization is a constant and is called the 'control ratio' of the thyratron. 



When once ionization does occur current flows through the valve, 



128 



