VACUUM riBES FOR RADAR I.F. A .\f J'/JFIERS M)-> 



When the grid is made more positive the plate current rise is reduced because 

 Mi2 increases. The grid wires should be as small as possible so as to block 

 oflf no more of the area of the structure than is necessary. .Vn ideal grid 

 would be an intinitesimally thin conducting plane which offered no resistance 

 to the passage of electrons through it e.xcept that due to its electrostatic 

 potential (sometimes called a ''physicist's grid"). In the 6AK5 tube the 

 grid-cathode clearance is 0.089 mm (O.OOvSS inch), the control-grid pitch is 

 0.0127 mm (0.0050 inch), and the wire size is 0.0010 inch. E.xperiments 

 have shown that substantially higher transconductance could have been 

 realized, with the same spacing, if smaller wires and smaller pitch had been 

 used, but the mechanical difficulties would have been much greater. 



The way to achieve a high band merit from the tube design standpoint is 

 thus to use as close grid-cathode clearance as practicable, to operate the 

 tube at as high a current density as the emission capabilities of the cathode 

 will permit, and to keep the stray capacitances as low as possible. It was 

 noted above from (11) that Bo is independent of A. However, if it were 

 possible to maintain the same grid-cathode clearance with a large tube as it 

 is with a small one, the larger tube would have the advantage that the stray 

 capacitances in the tube would be a smaller fraction of the total capacitance 

 so that the band merit for the tube would be higher. It would also be closer 

 to what can be realized when the tube is used in an actual circuit because 

 the capacitances added by the socket, the wiring and the circuit elements 

 would be less important. However, the practical mechanical limitations 

 controlling the minimum grid-cathode clearance have been such that the 

 band merit is roughly independent of the tube size over a moderate range of 

 sizes of high transconductance receiving tubes. 



3. Input Conductance 



Two factors tend to make the input conductance of tubes higher at high 

 frequencies than at low frequencies. One is the effect of lead inductances 

 and the other is the effect of transit time. If the loading produced by these 

 effects is no more than that required to get the desired bandwidth, it may be 

 no particular disadvantage for stages other than the first one in the amplifier. 

 As will be seen later, however, this effect in the input tube increases the 

 noise figure. The practical result of a consideration of these effects is that 

 the leads are made as short as possible and that small tubes are used in order 

 to use close grid-cathode clearances when the frequency at which the tubes 

 are to be used is above about 10 mc. The expression deri\'ed by North^ for 



^ "Analysis ot the Effects of Space Charge on Grid Impedance," D. O. North, I. R. E. 

 Proceedings, Vol. 24, No. 1, January, 1936. 



