104 BELL SYSTEM TECHNICAL JOURNAL 



is much larger than for the conv^entional tube. Radiating fins are 

 employed to compensate for the decrease in heat radiating ability of 

 the plate which would otherwise occur because of its short length. 

 In Fig. 5 is shown a photograph of this tube. , It will be noted that 

 the tube electrodes are supported directly from their leads. The com- 

 plete absence of auxiliary supporting members either of metal or of 

 insulating material and the large size of leads reduce radiation, eddy 

 current, and conduction current losses. That portion of the inter- 

 electrode capacitances due to the supporting structure is also made 

 small by this method of support. 



The inter-electrode capacitances are given below, together with the 

 corresponding values for the type 242.4 tube, which has the same plate 

 dissipation rating but is designed for use at lower frequencies: 



High Frequency Tube 242.-1 



Plate to grid 3 ^^f 13.0 utii 



Grid to filament 2 ju/^f 6.5 n^i 



Plate to filament 1 txixi 4.0 mm^ 



The decrease in capacitance by a factor of approximately 4 makes 

 possible a much greater improvement in performance in the 60- to 150- 

 megacycle-per-second frequency range than the corresponding degrada- 

 tion in performance due to the lower mutual conductance and the in- 

 creased electron transit time resulting from the increased spacing. 

 The material increase in plate impedance makes it necessary to employ 

 an anode potential approximately twice as great with the high- 

 frequency tube. 



Output and efficiency curves are shown in Fig. 6. (For the sake 

 of uniformity, curves taken from published papers have, in most 

 cases, been redrawn.) The particular shape of the output curve is 

 due to the manner in which the applied anode potential was reduced 

 with increasing frequency to minimize the danger of tube failure from 

 the increased energy losses which occur at high frequencies. The 

 limiting frequency as set by the inter-electrode capacitances and lead 

 inductances is given by the authors as 230 megacycles. An extension 

 of the efficiency curve to higher values indicates that the tube will 

 probably fail to oscillate before this limit is reached. From this it 

 can be inferred that the decrease in efficiency in the range from 150 

 to 200 megacycles is due largely to the effect of the relatively large 

 transit time, since the authors' method of arriving at the output by 

 taking the difference between the measured input and the measured 

 plate losses includes circuit and lead losses as a part of the output. 

 A comparison of the data of Fig. 1 and Fig. 6 shows that at 100 

 megacycles the output of the type 242^ tube is 2 watts and the corre- 



