DESIGN FACTORS OF THE 1553 TRIODE 



511 



still be for xi — > but will not increase so strongly as x as before but 



much more slowly, about as x~ . The grid dimensions should consequently 

 be made as small as possible while still maintaining a transmission fraction 

 at no less than 0.5 and at the same time not allowing mean deviations in 

 pitch more than about 15%. 



In the 1553 our best grid techniques today have led to a stretched grid 

 (which does not move appreciably during temperature cycling) having a 

 transmission factor of approximately 0.7, a pitch distance of .001'' and a 

 mean deviation in pitch of less than 15%. For such a grid further de- 

 creases in input spacing without refining the grid will not pay oflE very 

 rapidly, since we are on the maximum slope portion of the function F4. 



0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 



RATIO OF INPUT SPACING TO GRID PITCH, X,/p 



Fig. 9. — Dependence of gain-band product on grid pitch. 



Limitations on Anode-Grid Spacing, xt 



In considering the choice of output spacing we must attain a balance 

 among the following considerations: 



a. The optimum transit angle 62 = 2.9 radians requires a spacing which 

 varies with plate voltage and with frequency. For 250 volts and 

 4000 Mc/s, this optimum is .022". 



b. The anode heat dissipation must be closely watched because the glass 

 seal in this type of tube is very close to the anode. For the 1553, 

 a maximum of 50 watts per square centimeter of anode active sur- 

 face is safe. With a maximum cathode current density of 180 

 ma/cm^, set by life considerations, heat dissipation limits the plate 

 voltage to 275 volts unless the current is lowered. 



c. If the anode is moved too far out, keeping its voltage constant, then 

 in order to draw the desired current the grid must go positive, 

 perhaps drawing excessive grid current. The grid shielding factor ju 

 cannot be reduced without harming the transadmittance and feed- 



