400 BELL SYSTEM TECHNICA L JOURNA L 



of the simple diode complicates the noise problem, the factor T remains of 

 great importance. It is therefore desirable to control the tube processing 

 in manufacture so as to insure adequate available emission in every tube. 

 An ''activity test" is made on completed tubes for this purpose. This test 

 consists of reducing the heater voltage by an arbitrary amount (usually 10%) 

 and observing the change in one of the tube characteristics which is sensitive 

 to changes in available emission. The characteristic used for tubes like 

 the 717A and 6AK5 is the transconductance. If the transconductance de- 

 creases by more than about 20% for a 10% reduction in heater voltage, 

 with the other operating voltages held constant, insufficient available emis- 

 sion is indicated and T is higher than for more "active" tubes. 



In the case of a pentode with negligible impedance in each of its leads, as 

 shown in Fig. 10(b), the fluctuation in the cathode current is the same as 

 that given in equation (18), but the noise component in the plate lead is 

 larger, Thompson, North, and Harris'' showed that it can be written as 



4 = 26/,A/[^^^^^±-^] (19) 



where Ip is the d-c plate current and hi is the d-c screen current. It was 

 mentioned above that T can be made as low as 0.20 by providing adequate 

 available emission. From the design standpoint, equation (19) also shows 

 that the screen current should be as small as possible. For normal operat- 

 ing conditions in a pentode, the screen current is influenced by the screening 

 fraction (fraction of area blocked off by grid wires) of the screen grid and 

 by the amount of space current turned back to the screen in the screen-plate 

 region. The way to get a minimum screening fraction and still obtain the 

 desired function of the screen grid of reducing the plate-grid capacitance is 

 to use wire of as small diameter as possible. The presence of a suppressor 

 grid, at cathode potential, placed between the screen and the plate to pre- 

 vent interchange of secondary electrons, causes a certain proportion of the 

 space current which would otherwise go to the plate to be turned back to the 

 screen. Here again, this effect is minimized by using as fine wire as possible 

 for the suppressor grid. It was pointed out in an earlier section that fine 

 wires are desired for the control grid. The ideal for each of the three grids in 

 an IF pentode would be a conducting plane which offers no resistance to 

 the passage of electrons other than the influence of its potential. 



When impedances are connected in the various leads of a pentode the 

 noise components discussed above will, in general, be different due to the 

 influence of fluctuation voltages developed between the elements of the tube. 

 In particular it is found experimentally in the \'HF range of frequencies 

 that when an impedance Z is introduced between the grid and cathode, as 

 shown in Fig. 10(c), the noise component in the plate lead rises more than 



