BEAM TETRODE 



for electrons knocked out of the anode to do anything but fall back on to 

 it again. 



With the pentode the anode characteristic is as in Figure 8.20 and voltage 

 amplifiers may be designed graphically in the same manner as for triodes 

 (Figure 8.23). Since at the bottom of the diagram the curves tend to close 

 up, the bottom edge of the working region is bounded by an arbitrary level 

 at which the cramping becomes intolerable. The left-hand edge is bounded 

 by a voltage just above the knee voltage. The remainder of the working 

 region is bounded by the hyperbola of maximum anode dissipation, and by 

 the maximum permissible anode voltage and current, as in the triode. The 

 amplification may be taken as g^R and is typically 150 times. Pentodes 

 may also be used for power amplification, and will deliver more power for a 

 given mean anode voltage and current than will a triode, as explained in 

 Figure 8.24. Due to the cramping of the curves at the bottom, pentodes, 



Triode power 

 rectangle 



Triode 

 characteristic 



Pentode 

 characteristic 



Figure 8.24 



whether high-level voltage or power-amplifying, tend to cause more dis- 

 tortion than triodes. The optimum load for power pentodes is only about 

 1/5 to 1/10 of r^; but as ;•„ is so high, of the order of hundreds of thousands 

 of ohms, the optimum load usually works out higher than for a triode. 



BEAM TETRODE 



A beam tetrode is a carefully made — and therefore expensive — valve suitable 

 only for power ampHfication {Figure 8.25). The screen grid is wound in the 

 shadow of the control grid (as illuminated by the cathode) as a result of 

 which there is a clear run for the 'beaming' of electrons on to the anode, 

 and incidentally the screen current is extremely small. 'Beam forming plates' 

 at cathode potential are included which constrain the beam to strike the 

 anode at right angles. Tetrode kink is prevented because: (1) secondary 

 electrons released by primary incident electrons which strike normally are 

 of minimal energy ; and (2) because the valve is a power valve and because 

 the primary electrons are beamed the electron density is so great that the 



147 



