408,256. Valve amplifying circuits. ROBINSON, E. Y., Garvary, Dryden Road, Bush Hill Park, Enfield, Middlesex, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. July 4, 1932, Nos. 18859/32 and 8217/33. [Classes 39 (i) and 40 (v).] The gain of a valve amplifier is adjusted by varying the negative bias on a control grid interposed between two positively-charged electrodes. These may comprise two grid electrodes, or a grid and anode. The negative bias may be adjusted by hand or automatically by a rectified carrier wave. The arrangement minimizes cross modulation and distortion. Fig. 3 shows a tuned amplifier with input circuit 9 energizing the inner grid IG, and output circuit 11 connected to the anode A. The screen-grid SG is connected to the positive side of the anode supply source, and the gain control grid GG situated between screen-grid and anode, is connected to a potentiometer tapping 12 across a source 13. Amplification is reduced as the grid GG is made more negative, its effect being to reduce anode current and increase screen current. In Fig. 8, the valve has two connected screen-grids SG1, SG2, one on either side of the gain control grid GG, and a further outermost grid L connected to the cathode. Fixed bias for the input grid IG and variable bias for the gain control grid are taken from resistances 10<a>, 12 in the cathode lead. The grid L may be omitted, Fig. 9, and the screen grids and gain control grid may be connected to suitable tappings on a potentiometer across the HT supply. For superheterodyne reception, a local oscillation may be applied at 28 to the gain control grid GG ; or the local oscillation may be applied to the innermost grid, and the incoming oscillation to the gain control grid. By back-coupling the circuit of one or both of the screen grids to the tuned circuit 9 of the inner grid, the valve may be used to generate the local oscillation (Fig. 10, not shown). Fig. 11 shows such a circuit with both screen grids SG1, SG2 connected to HT+ through a back-coupling coil 9<b >coupled to the coil 9. The anode circuit 11 is tuned to the beat frequency and is connected through the coil 9<b> to HT +. The back-coupling coil 9<b> may be connected in the circuit of an additional grid next to the input grid IG and may be fed from HT + through a voltage-dropping resistance (Fig. 12, not shown). This additional grid may consist of one or more rods parallel to the cathode ; or it may be replaced by a separate anode co-operating with lateral extensions of the cathode and innermost grid (Fig. 13, not shown). The grids IG and GG may be of non-uniform pitch. In the tuned amplifier shown in Fig. 14, the bias on the gain control grid GG is adjusted by the tapping 10<b> on a resistance in the cathode lead. The whole of this bias, or a fraction thereof determined by the potential divider 29 is applied to the input grid IG. Fig. 17 shows a three-valve radio receiver, employing as the first stage a frequency-changing valve V1 of the kind described with reference to Fig. 13, and as second stage an amplifier V2 as shown in Fig. 8. The third valve V3 is a pentode output valve combined with two diode rectifiers operating respectively as signal rectifier, and as rectifier for gain control. The signal output from the rectifier anode A2 appears across resistance 37 and is applied through a potentiometer 38 to the input grid of the pentode. The automatic gain control voltage derived from the anode A3 appears across the resistance 41 and is applied through smoothing elements 42, 43 to the gain control grids of the first and second stages V1, V2, whilst a fraction determined by a tapping 44 is applied to the innermost grid of the second valve V2. In a modification shown in Fig. 18, the third stage V3 comprises a triode amplifier combined with a double diode rectifier, and is followed by a fourth stage V4 comprising a triode output valve. The double diode acts as a full-wave rectifier, giving audio and D.C. voltages across the resistance 37. The audio-frequency voltage passes from a tapping on the resistance 37 through a condenser 50 to the triode grid. The D.C. voltage is applied through resistance 49 to the grid of the triode, and amplified D.C. voltage appears across a resistance 51 in the cathode lead. This voltage is applied through a resistance 42 to the gain control grids of the first two stages V1, V2, and a fraction is also applied to the innermost grid of the second stage V2 by the potential-divider 44. The unilateral conductivity at the grid GG of the second valve V2 prevents any substantial positive voltage from developing at the left-hand end of the resistance 42. The innermost grid of this valve may thus be kept at negative potential. In a modification (Fig. 9, not shown), a valve incorporates a double diode rectifier within its envelope, and the D.C. voltage resulting from rectification of the incoming wave is applied to a gain control grid interposed between two screen-grids in the valve as in Fig. 8. The audio-frequency voltage from the rectifier is applied through a condenser to the innermost grid, and the amplified audio voltage on the anode is passed on to a succeeding stage through a condenser. The Specification refers to features of valve construction, which form the subject of Specification 408,328. Disclaiming reference is made to Specifications 266,325, 271,850, and 312,698, [all in Class 40 (v)].