HARD VALVES 



concomitant space charge is sufficient to turn back secondarily emitted elec- 

 trons. The anode characteristics of beam, tetrodes and power pentodes are 

 similar and design procedure is the same for each. 



Figure 8.25 



Warning — The anode circuit of power pentode or beam tetrode valves 

 must never be broken whilst the screen potential is applied. If this is done 

 all the cathode current will go to the screen, which is of insufficient thermal 

 capacity to withstand the resulting temperature rise. It is extremely hkely 

 that the screen will melt and the valve be destroyed. 



Equivalent circuits of valve amplifying stages 



The voltage gain expression for a simple triode amplifier i,iRl(R + '*«)' 

 suggests the equivalent circuit for the stage introduced in Figure 8.13, namely, 

 a pdVg volt generator in series with a resistance /'„, feeding the load Rj^. 

 Similarly, Figure 8.26a shows an appropriate equivalent circuit for a pentode 



9m^K 



m 'g 



'a 



R. 



g bVg 



(a) 



Figure 8.26 



or beam tetrode stage. Since r„ is usually so much greater than 7?^, its con- 

 tribution to the two resistances in parallel is generally negligible and the 

 valve may be regarded practically as a generator of constant current g^dVy 

 feeding the load Rj^ (Figure 8.26b). 



148 



