Patent GB-429429-A (GB)
Title: Improvements in radio receiving systems and valves used therein
IPC: H03G3/26
Filed: 1933-11-22
Abstract: 429,429. Valve circuits for wireless reception. ROBINSON, E. Y., Garvary, Dryden Road, Bush Hill Park, Enfield, Middlesex, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. Nov. 22, 1933, Nos. 31159 and 33213. [Class 40 (v)] In order to permit accurate aural tuning, a receiver in addition to comprising means for rendering the output independent of signal strength also comprises means for decreasing the sensitivity on departure of the tuning of the receiver from resonance with the received signal. The selectivity of the circuits producing the ordinary A.V.C. potentials is arranged to be less than that of those used for producing the aural tuning control. The circuits preferably embody means for inter-channel noise suppression. Fig. 1 shows two high-frequency amplifiers I, II the grid bias and hence the amplification of which is controlled inversely as the strength of the signal output from the first, by the rectified output from a diode 3 incorporated in the valve II and fed from the secondary winding 2 of the intervalve coupling as described in Specification 429,428. The output of the valve II is transferred by a transformer 4, 5 to the diodes 6, 7 incorporated in a low-frequency amplifying, valve III for full-wave rectification. The D.C. and a variable proportion of the signal potentials across the load resistance 9 of the rectifiers 6, 7 are fed to the input grid of the valve III through resistance 13 and condenser 14 respectively. The valve is of the type described in Specification 408,256, the second and fourth grids being connected to the high tension source, the third to a tapping on the potential divider 12 across the high tension source, and the fifth to the cathode. As the potential of the first grid becomes more negative the current in the cathode resistance 10 falls making the third grid less negative and hence diverting current from the second grid to the anode load 8 which during the reception of weak signals was not energized. Thus interchannel noise suppression is obtained and also a rise in amplification of the valve III as the tuning of the receiver approaches resonance, enabling aural tuning to be employed. The diode 3 has a delay voltage applied thereto from the cathode resistance of valve II. In a modification Fig. 3, (not shown), the valve II is omitted, and the extra selectivity for the aural tuning control obtained by coupling together two intervalve transformers. The first low-frequency amplifier contains two diodes which are used for the two independent volume controls, and is of the pentode type. The manual volume control varies simultaneously the D.C. and A.C. potentials applied to this valve but in a fixed proportion. In a further embodiment, Fig. 4, the diode 33 produces the normal volume control potentials which are applied to the preceding H.F. amplifier I, and to the L.F. amplifying pentode IIIb, through the load resistance 9 of the diode 36 which produces signals for low frequency amplification and aural tuning control voltages which increase the gain of the valve IIIb as the tuning approaches resonance. The diode 33 is fed from the primary 18 of the H.F. transformer 18, 22 through condenser 25, and the diode 36 from the secondary 22. In another modification, Fig. 5, (not shown), a valve of the type shown in Fig. 1, III, controlled for aural tuning, has its first grid controlled by two diodes as in Fig. 4 and its third grid is a muting electrode negatively biassed in the absence of signals but made positive during reception by a third diode rectifier. The other modifications consist, in the main, of means for rendering different the selectivity of the circuits feeding the demodulator and volume control diodes which may all be separate. Specification 407,317 also is referred to.