78 BELL SYSTEM TECHNICAL JOURNAL 



filament was hot. This loss increased with frequency in the manner 

 characteristic of that of the capacity-resistance combination between 

 cathode and grid which was described above. Present indications are 

 that, at least in part, the loss may be ascribed to the resistance existing 

 between the cathode and the region of potential minimum. 



Of the three current paths through the tube, one more still remains 

 to be considered. This is the grid-plate path. The relations involved 

 here are more readily seen by considering first a low-frequency ex- 

 ample. Here the electron stream passes through the spaces between 

 grid wires, afterward diverging as the plate is approached. Electro- 

 static force from the grid acts not only on the plate but also on the 

 electrons in the space between. It is evident, then, that the path 

 which, when the cathode was cold, constituted a pure capacity changes 

 into an effective capacity different from the original in combination 

 with a resistive component. The losses would be expected to increase 

 with frequency just as they did in the grid-cathode type. The change 

 in grid-plate impedance is particularly noticeable when it is attempted 

 to adjust balanced or neutralized amplifier circuits with the filament 

 cold, in which case the balance is disturbed when the cathode is heated. 



As yet, no accurate expression for this grid-plate impedance has 

 been obtained, either at the low frequencies where transit times are 

 negligible or at the higher frequencies now particularly under investi- 

 gation. The reason for this lies in the repelling force on the electron 

 stream of the negative grid so that the assumption of current flow in 

 straight parallel lines is not valid in so far as current from the grid to 

 the plate is involved. 



It has been shown that both the cathode-grid path and the grid- 

 plate path contain resistive components with corresponding losses 

 which increase with increase of frequency. This loss may be cited as 

 a reason why triodes with negative grids cease to oscillate at the higher 

 frequencies. If it were not for these losses, external circuits could be 

 attached to the tube having such phase relations as to satisfy oscilla- 

 tion conditions, so that the negative grid triode could be utilized in 

 the range which is now covered by the triode with positive grid. 



V. Triodes with Positive Grid and Slightly Positive Plate 



When the grid of a three-element tube is operated at a high positive 

 potential with respect both to cathode and plate, electrons are at- 

 tracted toward the grid, and the majority of them are captured on their 

 first transit. Those which pass through the mesh and journey toward 

 the plate will be captured by the plate if its potential is sufficiently 

 positive with respect to the cathode. 



