GB399068A - Improvements in or relating to thermionic amplifying arrangements suitable for use in radio receivers - Google Patents

Abstract

399,068. Valve circuits for wireless reception. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.-(Assignees of Beers, G. L. ; 119, Summerfield Avenue, Collingswood, New Jersey, U.S.A.) Jan. 26, 1933, No. 2571. Convention date; Jan. 26, 1932. [Class 40 (v).] In a receiver provided with automatic volume control, the damping of the tuned circuits is also automatically varied with changes in signal strength, so as to combine good reproduction of the local station with high selectivity for distant working. A manually-operated control may also be provided. As shown in Fig. 1, the amplified signal is fed back from the input circuit of the second detector 9 of a superheterodyne circuit through a branch-line and condenser 53 to the grid of an auxiliary volume-control valve 41. An increase in signal strength lessens the initial negative bias on the grid of that valve and reduces the " gain " of the I.F. stages 1, 3, 5, 7 by increasing the potential drop across the output resistance 55 which supplies grid bias to the I.F. valves from a common potentiometer line 31. Two other auxiliary valves 59, 61 serve to vary the selectivity response. The space-current path of the valve 59 is in shunt with the tuned output circuit 23 of the I.F. valve 3. A resistance 71 in the input circuit of the valve 59 is connected in the output of the first I.F. stage 1, and during reception of weak signals applies a strong negative grid bias, so that the platefilament impedance of the valve 59 is large. An increase of signal strength, on the other hand, lowers the valve impedance so that it damps the coupling circuit 23 and widens the frequency-response curve, at the same time reducing selectivity. A second control valve 61 similarly shunts the primary winding of the transformer 27 coupling the I.F. valves 5, 7. The grid of the valve 61 is biased from the plate circuit of the A.V.C. valve 41, so that as signal strength increases the coupling between the valves 5, 7 is progressively damped. The resistance 71 of the valve 59 may be replaced bv a manual control. In an alternative arrangement, Fig. 2, control valves 89, 91, 93 are shunted across the output circuits of the I.F. valves 76, 77, 79 and operate as before to reduce the "gain" or energy-transfer between these stages, and at the same time to broaden the frequency response. Amplified signals are fed back through a condenser 105 to the grid of a "master" control valve 95, the corresponding change in the potential drop across the output resistance 99 being transferred through a phase-reversing valve. 107 to render the grids of valves 89, 91, 93 progressively more positive with increasing signal-strength.