THEORY OF MULTI-ELECTRODE VACUUM TUBES 



83 



It is of interest to compare the characteristics of this co-planar-grid 

 tube with those of a three-electrode tube designed also to operate at a 

 plate potential of 130 volts and at a plate current of 35 milliamperes. 

 The plate current-grid voltage characteristics of such a triode are 

 shown in Fig. 21. The dynamic characteristic is for a load resistance 

 of 1140 ohms which matches the plate resistance of the tube at the 

 operating point P. At zero grid bias on the dynamic characteristic, 

 the plate potential is 92 volts, a drop of only 38 volts from the operating 



2.0 



,- 1.5 



1.0 



50 



b- 30 



20 



10 



2 -.^,,,^ 



3 ' —^ 



28 30 32 34 36 38 

 HARMONIC OUTPUT IN DB BELOW P 



40 



44 46 



Fig. 22 — Curves showing the output power and plate efficiency of a triode and 

 a co-planar-grid tube with varying effective values of second and third harmonics 

 expressed in db below the fundamental. 



potential of 130 volts. In the co-planar-grid tube the dynamic 

 characteristic cuts the axis of zero grid bias at a plate potential of 27 

 volts, a drop of 103 volts from the operating plate potential. The 

 larger plate-voltage variation in the co-planar-grid tube results in 

 higher efficiency than in the triode. The reasons for this difference are 

 identical with those previously discussed in the case of the power 

 pentode. 



In Fig. 22, curves are shown comparing the power output and plate- 



