HARD VALVES 



rather gently. When the anode potential is at B the retarding field is much 

 less and the electrons hit the anode hard, knocking out 'secondary electrons'. 

 These travel back to the screen because it is more positive than the anode, 

 so that the net anode current is less than expected — hence the kink. At C 

 the secondary emission is even stronger, but because the anode is now more 

 positive than the screen the secondary electrons fall back on the anode. 

 There is a device called the dynatron which makes use of tetrode kink: the 

 author has never found it being used in biological work so it will not be 

 described. On the whole tetrode kink is a bad thing, and to get rid of it 

 further elaboration is needed. The valves which overcome it are the pentode 

 and the beam tetrode. 



PENTODE 



In the pentode yet another grid is included, between screen grid and anode. 

 This is maintained at a fixed negative potential with respect to the screen, 



Figure 8.22 



Figure 8.23 



usually by connecting it to the cathode (Figure 8.22). It is called the suppressor 

 because its function is to suppress the flow of secondary electrons from anode 

 to screen by screening the anode from the screen. There is then no tendency 



146 



