252 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



CONTROL GRID 



grid, do introduce distortion in the tube when used as an amplifier 

 under the conditions assumed. To prevent this phenomenon, which 



occurs only under abnormal 

 conditions, a fifth electrode 

 known as a suppressor grid is 

 placed between the screen grid 

 and the plate. This suppressor 

 grid (fig. 8) is connected to the 

 cathode or to ground potential. 

 Thus a zero potential is estab- 

 lished in the space between 

 the screen grid and the plate. 

 Under these conditions the 

 screen grid has no effect on the 

 electrons splashed out and all 

 return to the plate so that no 

 distortion can be introduced 

 due to this cause. The elec- 

 tron tube containing the five 

 electrodes is called a pentode. 



20 VOLTS 



H|l|l|lH 



Figure 7. — Secondary emission in a tetrode. 



It will produce large power outputs with relatively small signal 

 voltages applied to the grid. 



CONTROL GRID 



SUPPRESSOR 

 GRID 



RECTIFIERS 



Recently the term mutator 

 has been suggested to cover 

 those forms of electronic devices 

 which are designed for rectify- 

 ing alternating current for power 

 purposes. The term mutator 

 thus includes grid-controlled 

 hot-cathode rectifiers, mer- 

 cury-arc rectifiers, and igniter- 

 type mercury-arc rectifiers or 

 ignitrons. 



If the vacuum-type triode 

 previously discussed be changed 

 by the addition of gas under 

 low pressure and if the grid be 

 changed so that it completely 

 surrounds the cathode, then a tube possessing some different and 

 desirable properties is obtained. If this tube be connected in the 

 circuit of figure 9, practically no current will flow from the cathode 

 to the anode when the grid is free or not connected to any source of 



HI— II 



Figure 8.- 



-Prevention of secondary emission by flftii 

 electrode in a pentode. 



