GB466954A - Improvements in or relating to thermionic valve circuits - Google Patents

Abstract

466,954. Valve amplifying circuits. BULL, C. S. Nov. 6, 1935, No. 30730. [Class 40 (v)] In a hexode or like valve, gain control is effected by applying the signal in opposite phase to the two control grids, and varying the difference between the bias potentials on the two grids, either manually or by the incoming signals. The Figure shows a hexode valve in which the control grids 4, 6 are coupled by condensers to the input transformer 11, and the cathode 2 is connected to the centre tap which is earthed. Negative potential from terminal 19 normally maintains the outer grid biassed to cut-off, the inner grid being at a cathode potential. For the purpose of deriving voltages for automatic gain control, the anode is coupled to the diodes 24, 28, the voltages developed across the diode load resistances 25, 29, being applied to the control grids in opposite senses. These voltages are preferably proportional to the grid bases of the two control grids. It is shown that under these conditions the valve characteristic is approximately parabolic, which is desirable for the prevention of distortion. In a modification, Fig. 3 (not shown), the outer grid is biassed to cut-off by a cathode resistance and the inner screen current, and the potential of the inner grid is controlled by a single diode. When a signal is received negative bias is imposed on the inner grid, causing the total cathode emission to diminish and therefore the negative bias on the other grid to be diminished.