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BELL SYSTEM TECHNICAL JOURNAL 



the residual value of the grid-plate capacitance. This reaction differs 

 from that predicted from the low-frequency capacitance measured on 

 a cold tube because of the inductance of the screen-grid lead and be- 

 cause of the electron space charge. The reaction can be measured by' 

 observing the variation in the input impedance resulting from a varia- 

 tion in the tuning and loading of the output circuit. Experimentally 

 determined values are given in Fig. 10. 



100 

 80 



60 

 40 



20 



10 



40 



60 80 100 200 400 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 8 — The input resistance as a function of frequency. 



The double pentode tube has been found useful as a high quality 

 class A amplifier, a class B amplifier, a frequency multiplier, and as a 

 modulator at frequencies of 300 megacycles per second and below. Its 

 performance in these various modes of operation is quite comparable to 

 the performance of conventional pentodes of similar ratings at much 

 lower frequencies. Stable operation with some gain has been obtained 

 at frequencies as high as 500 megacycles. Because of the increased im- 



