THEORY OF MULTI- ELECTRODE VACUUM TUBES 77 



in the transconductance curves. The rapid falling off in transcon- 

 ductance on either side of the maxima, with change in grid voltage, also 

 indicates that a large amount of distortion would result if the tubes 

 were operated with large swings in grid potential. 



Space-charge-grid pentodes are capable, then, of yielding very high 

 amplification because of their extraordinarily high transconductance; 

 but, practically, they are limited to use as voltage amplifiers for fairly 

 small inputs, since operation over the wide range necessary for large 

 power output results in prohibitive distortion. These statements 

 apply also to space-charge-grid tetrodes. These tubes have the 

 advantage over ordinary triodes, however, of yielding high values of 

 transconductance at comparatively low voltages applied to the plate. 



Co-Planar-Grid Tubes * 



Thus far only multi-electrode tubes have been considered in which 

 the electrodes are arranged in concentric order one about another. In 

 the case of the power pentode it has been shown that this arrangement 

 results in characteristics yielding comparatively high output power at 

 high amplification and efficiency but with relatively large percentages 

 of harmonics. It has also been pointed out that the high plate- 

 circuit efficiency is largely due to the presence of a positive grid held at 

 a fixed potential, which permits larger plate voltage swings than would 

 otherwise be possible. 



It is evident also from the underlying physical principles that in such 

 a structure, with three concentric grids between the plate and cathode, 

 even though the lateral spacings are fairly wide, enough shielding is 

 interposed between the plate and cathode to make the plate resistance 

 high. The question then arises: Is there any possible way of obtaining 

 the advantages of a positive grid in tubes designed to operate at 

 comparatively low plate voltages without the accompanying high plate 

 resistance of the pentode structure? 



This objective is accomplished reasonably well by a four-electrode 

 structure in which the positive grid is placed in the same plane as the 

 control grid. The arrangement of the electrodes in such a structure, 

 which will be referred to as a co-planar-grid tube, is shown schemati- 

 cally in Fig. 17. The lateral wires of the two grids are asymmetrically 

 arranged, the positive and negative lateral wires being arranged in 

 pairs close together. The object of this arrangement may be described 

 roughly as follows: By such an arrangement, when the control grid is 

 very negative, the field about each negative wire very largely neutral- 



* This section, "Co-Planar Grid Tubes," is an addition to the material publishetl 

 in Electrical Engineering. 



