The Causes for the Increase of the Admittances of Modern High-Frequency Amplifier Tubes on Short Waves

By recent measurements of input loss, output loss, and feed-back capacitance of modern high-frequency amplifier tubes (pentodes) up to 300 megacycles, a considerable increase of these values in the short-wave range has become manifest. Contrary to the opinion, expressed in several recent publications, for many types of valves the main cause of this increase must not be sought in electron-transit-time effects, but in the action of capacitances, mutual inductances, and self-inductances of the tube electrodes and of their leads within and without the tubes. A general theory of the effect of these quantities on input admittance, output admittance, feedback admittance, and mutual admittance is put forward for tetrodes, pentodes, hexodes, etc., used as high-frequency amplifiers. By three series of measurements it is shown, that about one to two thirds of the input damping of modern European high-frequency valves of normal dimensions on short waves must be ascribed to inductive effects and not to transit times. Several measurements on transit-time effects are described, showing that the transit time between the input grid and the screen grid may not be neglected as compared with the transit time between the cathode and the input grid. Theoretical formulas for inductive effects are well checked and those for transit-time effects are not so well checked by measurements. Causes for the latter deviations are given. Output admittance and feed-back admittance are almost wholly due to inductive effects in the short-wave region, as shown by several measurements described.