Patent GB-309902-A (GB)
Title: Improved means for operating wireless sets from supply mains or machine generators
IPC: H04B15/00
Filed: 1928-08-16
Abstract: 309,902. Telefonaktiebolaget L. M. Ericsson. April 17, 1928, [Convention date]. Thermionic amplifiers.-Multi-valve amplifiers for wireless reception &c., supplied with current from a potentiometer connected across a rippling source, have their electrodes so connected to the potentiometer that the ripple produced in the several valves is balanced out in the output circuit of the last valve. Fig. 4 shows a twovalve resistance-coupled set with the loudspeaker S connected directly in the plate supply circuit of the second valve. The grids, filaments, and plates of the valves are connected to such points on the potentiometer as will give the required direct current voltages to the electrodes. If the loud-speaker is to be unaffected by ripple, it is necessary that ripple should be applied to the grid G2 in opposite phase to that applied to the plate P2 from the point B2. The amount of grid ripple required depends on the amplification factor, and is represented by the fictitious point G<1>2 on the potentiometer. In the preceding valve, the ripple present in its plate voltage (which becomes the grid voltage for the second valve) may be represented by the fictitious point P<1>1. If the tappings C1, O1, B1 for the first valve are moved bodily along the potentiometer until the points G<1>2, P<1>1 coincide, the required balance is set up, and no ripple flows in the speaker circuit. In the finished apparatus, the tappings may be fixed in position, but it is desirable that one tapping, preferably the grid tapping for the first valve, should remain adjustable to compensate for differences in valves used with the set. Fig. 6 shows a threevalve receiving-set with resistance couplings r1, r2 between the valves. The arrangement is such that practically the whole of the D.C. voltage available is applied to the plate P<3> of the last valve. Phase-correcting impedances may if necessary be inserted across portions of the potentiometer. In Fig. 8, the potentiometer is divided into sections, and filament current is derived from tappings on the potentiometer or by inserting the filaments in series. The filaments should have considerable heat capacity to prevent ripple from affecting their electron emission. Reaction is provided by a coil L<11> connected in series with a condenser K<1> as a shunt to the coupling resistance r1. The last valve has a choke-capacity output circuit, and the speaker S is connected by condensers K4, K5 between the plate and the filament. The high inductance df the choke D substantially prevents ripple voltage from reaching the plate P3, and the various potentiometer tappings have therefore to be set so that no ripple voltage reaches the grid G3, or the plate P2. When a choke coil is used as an inter-valve coupling, ripple can be eliminated in the same manner as for resistance coupling provided that the impedance of the choke at ripple frequencies is much larger than the internal resistance of the valve. Fig. 10 shows a circuit with transformer coupling between the valves, and with the speaker S is in the plate supply circuit of the last valve. The ripple received by the grid G, consists partly of a direct pick up from the point C, and partly of ripple transferred from the preceding valve through the transformer T. The sign of the latter may if necessary be reversed by changing the connexions of the primary coil. By leaving the tapping C<1> adjustable, the total ripple reaching the grid G2 can be adjusted to neutralize in the loud-speaker S the ripple directly picked up at the plate tapping B2. As further precautions against hum effects a resistance Ro may be connected across the transformer secondary coil, and a condenser may be connected across the potentiometer from its negative end to an adjustable point near the positive end.