544 BELL SYSTEM TECHNICAL JOURNAL 



at midband, gave a gain-band product of 1290 mc. The bandwidth of 203 

 mc was considerably greater than the average for these tubes. Similar 

 results on 35 experimental tubes yielded the following averages: Low-level 

 gain 10 db; Bandwidth 103 mc; Gain-band product 916 mc. The 416-A 

 tubes produced by Western Electric Company exhibit comparable aver- 

 ages with much less spread; for example, a recent sample of 138 tubes had 

 average values and standard deviations as follows: 



Table I 

 Gain and Bandwidth or 138 W. E. Co. 416-A Triodes 



Low-level gain .... 



Bandwidth 



Gain-band product . 



St'd. Dev. 



1.1 db 



9 mc 



350 mc 



It is indeed gratifying to realize that such a remarkable tube can be 

 produced with such uniformity. 



o+B 



o-B 



Fig. 19. — Stabilizer circuit. 



In operating these tubes, it has been found that small variations in 

 gain due to power line fluctuations and due to other disturbing influences 

 can be minimized by using a stabilizing bias network which provides a 

 large amount of negative feedback for the dc. path. This circuit is similar 

 to one proposed by Mr. S. E. Miller of the Bell Telephone Laboratories 

 for use in coxial repeaters which also use high transconductance tubes. 

 In this circuit, shown schematically in Fig. 19, a few volts negative are 

 applied to the cathode through a suitable dropping resistor. In the absence 

 of plate voltage, the grid draws current, being positive with respect to 

 the cathode. When plate voltage is applied, the drop in the cathode re- 

 sistor tends to bring the cathode nearer ground potential until a stable 

 voltage is reached. The resistor is set to a value which allows the desired 

 cathode current to flow and subsequent variations in gm or plate voltage 

 then have little effect on the total cathode current. 



Maintaining the cathode current constant does have an appreciable 

 effect on the gain of the tube when operating at high output levels. This 

 is characterized by a decrease in gain as the driving power is increased. 



