GB514762A - Improvements in or relating to thermionic amplifiers - Google Patents

Abstract

514,762. Electron multipliers and circuits therefor. FERRANTI, Ltd., and TOMLIN, G. M. June 3, 1938, No. 16630. [Classes 39 (i) and 40 (v)] A thermionic amplifier of the type employing a thermionic valve having a cathode, a control electrode, an accelerator electrode, an anode and an additional electrode at which secondaryelectrons are produced when it is bombarded by primary electrons of sufficient velocity has potentials applied to the electrodes of the valve such that useful primary electrons impact on the additional electrode and is provided with means for varying the mutual conductance of the valve without varying the interrelationship between the potentials. This means may comprise a variable resistance in series with the high tension supply or a potentiometer connected across it. In the example shown, resistances 8, 11 and 12 of 5,000, 60,000 and 40,000 ohms and 17, 18, 19 of 1,300, 20,000 and 120,000 ohms are connected in series between the slide and earth and form two paths across the high tension supply P.N. The junction of 12 and 11 is connected to this screen-grid 4, and the junction of 11 and 8 to one end of the secondary winding 7 of an intermediate frequency transformer, the other end of which is connected to the control grid 3. The junction of 17 and 18 is connected to the cathode 2 and of 18 and 19 to the additional electrode 6. These paths form potential dividers across the source and since the total resistance of 8, 11 and 12 is less than that of 17, 18 and 19 the potential developed across 17 (1,300 ohms) is less than that across 8 (5,000 ohms) while the valve warms up and the control grid 3 is more positive than the cathode 2. When the valve becomes conducting it forms a path in parallel with the resistances 18 and 19 and the potential developed across 17 therefore increases and the control grid gradually becomes less positive to the cathode. The mutual conductance of the valve may be varied by means of the variable resistance 14 and the resistances of the sections of the two potential dividers are so chosen that when steady conditions are reached the relationship between the electrode potentials is such that secondary electrons are produced at the auxiliary electrode.