THEORY OF MULTI-ELECTRODE VACUUM TUBES 59 



with variations in plate voltage. The plate-current family of charac- 

 teristics should show only small variations with plate voltage (so long 

 as Ep is higher than £«)> depending on the magnitude of the change in 

 the ratio of I„ to h- 



Families of such characteristics are shown in Fig. 4. They are seen 

 to correspond very closely indeed with similar characteristics for a 

 triode. The It curves vary so little with plate voltage that the 

 families of characteristics taken at £,, — 135 volts and £,, = 180 volts, 

 coincide within the breadth of the curves. The plate current curves 

 show a small variation with E,,, as was discussed previously. 



In Fig. 5 the transconductance and amplification factor are shown as 

 functions of the plate voltage for four different values of grid bias and 

 with the screen potential maintained constant at 75 volts. At the 

 normal operating point P, the transconductance is 1375 micromhos 

 and the amplification factor is 550. The shape of these curves is 

 typical of that for screen-grid tubes and pentodes. Throughout the 

 normal operating range, the transconductance curves have about the 

 same degree of flatness as the plate-current curves. This is to be 

 expected from consideration of the plate-current curves of Fig. 4. 

 Since these curves change only slightly with variations in plate 

 potential, their slopes or transconductance values also change but 

 slightly. The amplification-factor curves are very similar in form to 

 the plate-resistance curves of Fig. 2. This follows at once from 

 equation 17, for since 5,„ remains nearly constant with variations in Ep, 

 IJL must vary in the same manner as R,,. 



In Fig. 6 the transconductance, plate resistance, and amplification 

 factor are shown as functions of grid voltage for three different values 

 of the screen voltage and with the plate voltage maintained constant at 

 180 volts. These curves are also typical of those found for several 

 multi-electrode tubes. The transconductance curves agree in form 

 with those for a triode, as would be expected from the plate-current 

 curves of Fig. 4. The plate-resistance curves are similar in form to 

 those for a triode, but rise more rapidly with increasing negative grid 

 bias. The amplification-factor curves, however, are entirely different 

 in form from those for a triode, since they rise with increasing negative 

 grid bias, whereas in triodes the amplification factor decreases with 

 increasing negative grid bias. The reason for this difference is that the 

 plate resistance in many multi-electrode tubes, over the normal range 

 of operation, increases more rapidly with decreasing plate current than 

 in triodes. At sufficiently large negative values of grid bias and very 

 low plate currents, the amplification-factor curves frequently reach 

 maxima and then fall rapidly as they do in triodes. 



