570 BELL SYSTEM TECHNICAL JOURNAL 



quarter-wave Lecher system formed by removing one set of leads. 

 Since only half of the total charging current to the inter-electrode 

 capacitances flows through each set of leads, the losses due to the lead 

 resistances are also reduced. In the tubes under discussion the electron 

 transit time limitation has been met by the use of extremely small 

 inter-electrode spacings so that full advantage may be taken of the 

 increased frequency range. 



For the purpose of confirming the above conclusion, efficiency curves 

 have been obtained on the large size tube, as shown in Fig. 1, when 

 operated both single- and double-ended. The results are shown in 

 Fig. 4. It will be observed that the efficiencies for double-ended 

 operation are always higher than for the single-ended case over the 

 range covered by the experimental data. In fact, usable outputs are 

 obtained at frequencies well beyond the point where the single-ended 

 tube fails to operate. The ratio of the cut-off frequencies for the two 

 tubes happens to be 1.23 for the particular conditions under which 

 these data were obtained. 



Output and efficiency curves for the large size tube are shown in 

 Fig. 5. The values of 60 watts at 300 megacycles and 40 watts at 400 

 megacycles compare quite favorably with outputs reported from 

 radiation-cooled magnetrons. When the problems of modulation and 

 the complications of the magnetron's magnetic field are considered, the 

 advantages of the negative-grid triode become more apparent. The 

 improvement in power output made possible by this departure in design 

 is illustrated by the comparison plot shown in Fig. 6. 



The double-lead arrangement is also responsible for an increase in the 

 upper frequency limit at which stable operation as an amplifier may 

 be secured. 



The primary cause for instability of the triode amplifier is the inter- 

 action between the input and output circuits which results from the 

 admittance coupling between these circuits provided by the grid-plate 

 capacitance. A second source of coupling is that caused by common 

 impedances in the two circuits in the nature of the self and mutual 

 inductance of the tube leads. At moderately high frequencies this 

 latter coupling is usually of negligible importance. Stable operation is 

 thus possible when suitable means are provided to compensate or 

 "neutralize" the admittance coupling. At ultra-high frequencies 

 lead-impedance coupling can no longer be neglected. It may, of course, 

 be minimized by the use of short leads. The ultimate solution is to 

 provide independent leads for the input, output and admittance neu- 

 tralizing circuits. The double-lead tube is an attempt to fulfill these 

 conditions. It will be observed that the only common impedance 



