479,599. Valve circuits for wireless reception. BLUMLEIN, A. D., and CONNELL, W. H. July 8, 1936, No. 18892. [Class 40 (v)] The selectivity of a receiver is controlled by applying volume control bias potential to an amplifier or frequency changer having a negative feed-back path associated therewith. The feed-back is such that frequencies adjacent to the signal frequency are attenuated relatively to the signals bv an amount depending on the gain in the amplifier. Fig. 1 shows an intermediate frequency amplifier 6 with the grid bias controlled from an automatic volume control line 5 and with a series circuit 7a, 7b giving feed-back. Anode current is byepassed by a flatly tuned choke 8. Alternatively, Fig. 2 (not shown), the negative feed-back impedance may be in series with the anode circuit decoupling condenser so that the effective slope of the valve is only that of the anode, instead of, as shown in Fig. 1, the combined slope of screen and anode. The negative feed-back impedance may comprise two parallel tuned circuits in series, one resonant above and the other below the signal frequency or series and parallel tuned circuits connected in parallel. Instability may be avoided by extending the winding of the parallel tuned circuit to form a neutralizing winding connected to the input grid, Fig. 4 (not shown). A less efficient circuit may be used to provide feed-back by using a tightly coupled and flatly tuned choke tapped at 13, 14, 15 to form a bridge in which the circuit 7 is balanced against a resistance 12. A step-up in the feed-back voltage may be obtained by using a remote tapping 16 for the input circuit and neutralizing potential may be derived from a tapping 17. With a bridge circuit, the impedance of the circuit 7 may be less at the carrier frequency than the resistance 12 to introduce a small positive feed-back at this frequency. A switch may be provided to remove the volume control bias from the valve provided with negative feed-back to obtain high selectivity with large input voltages, which switch may also increase the bias on a further amplifying valve to compensate volume output. To ensure that selectivity varies to the required extent with signal strength, the control voltage may bias valves both with and without feed-back and having different bias resistances and in the one case carrying supplementary current so that different effective bias is applied to the sets of valves. Fig. 8 shows a combined automatic and manual selectivity control for an amplifier 16 without and an amplifier 6 with negative feed-back. Automatic volume control bias is applied to both the grids and equal bias resistances 21, 22 are employed. A potentiometer 24 is, however, connected between the cathodes with the slider connected to high tension positive through a resistance 23 so that selectivity may be adjusted and the gain maintained substantially constant. To preserve a better balance of sound output, a bass cut may be effected with the cutting of the higher modulation frequencies due to the negative feed-back. Fig. 9 shows an arrangement in which automatic volume control potentials are applied to a low-frequency amplifier 35 from the resistance 26 in the diode load circuit. Delayed automatic volume control is produced by the voltage drop across resistance 27a and by potentiometer 29, 30 connected to the high-tension supply. The cathode load 27a, 28 of the valve 35 is made large compared with the inverse of the mutual conductance so that with a comparatively small condenser 36 negative feed-back of low frequencies occurs. High - frequency cut - off may be produced by an inductance in series with condenser 36. Specifications 290,202, [Class 40 (v)], and 440,337 are referred to.