Patent GB-362419-A (GB)
Title: Improvements in detector-relay systems
IPC: H03F1/50
Filed: 1930-05-30
Abstract: 362,419. Valve amplifying circuits. WHITE, S. Y., 1931, Broadway, New York, U.S.A. May 30, 1930, No. 16714. Convention date, June 25, 1929. Void [Published under Sect. 91 of the Acts]. [Class 40 (v).] Relates to direct-coupled aperiodic valve circuits for wireless reception, or for gramophone reproduction. The initial grid-bias on each valve is graded from stage to stage to give the proper operating potentials ; constant-coupling is ensured over a wide frequency range; automatic gain control is provided; motor-boating is prevented ; reverse reaction is utilized to ensure stability; and provision is made to compensate for the increasing resistance of coupling-elements by " ageing " and for the substitution of one type of valve for another with different characteristics. The absence of reactive coupling-elements eliminates distortion due to phase-displacement, or to the production of harmonics. As shown in Fig. 1, a triode valve VT1, normally designed to operate with a plate voltage of 180, is supplied with 10 volts from a battery B1 or less, producing an output current of a few microamps, so that the effective valve impedance is several megohms. A decrease of 0À3 volts in the grid bias reduces the plate current from 800 to 50 microamps, as shown by the curve marked 4 in Fig. 2, the switch SW1 being closed to short-circuit resistance RS and the switch SW2 being open. The curves 6, 7 of Fig. 2 show the normal plate-current grid-volts characteristic for the valves VT1, VT2 when operating with normal plate voltages of 180 and 100 respectively. The steepness of the curves 1 .. 5, Fig. 2, is found to be practically independent of the plate voltage for values between zero and 22 volts. By closing the switch SW2, Fig. 1, so as to bring a resistance R2 of from 1 to 50 megohms across the grid circuit, the curves 1 .. 5 remain practically the same, but the stability of the system is increased. The setting of the initial grid-bias is, however, critical. The resistance RS is merely inserted to measure the grid-circuit impedance. When receiving a modulated carrier wave, the plate current from VT1 increases whilst that from VT2 decreases, according to the strength of the carrier. To prevent the output from VT2 falling to the lower part of the curve, the grid-bias on the valve VT1 is automatically regulated by one of the methods hereafter explained. Rectification is progressive, i.e. distributed over both valve stages. In Fig. 4, aperiodic back-coupling between the two valves is introduced by means of a resistance RG, so as to permit of stabilized high-frequency amplification. Above a certain value of RG the system acquires a " trigger " action and can be used as a trip relay, without self-oscillation. A resistance RG, Fig. 6, common to the plate circuit of VT2 and the input circuit of VT1 can be used to replace the grid-bias battery GB1. Here the effect of the varying reactance of the loud-speaker LS with frequency lessens the amount of reaction applied through the resistance RG, and so tends to ensure a straightline frequency response. In Fig. 8. the filament of an auxiliary valve VT4, operating as a diode, is energized by the output current from' the power valve VT3, and functions to regulate the effective grid-bias on the input valve VT1 in order to ensure automatic gaincontrol. Or a resistance in series with a choke in the output circuit of the valve VT3 may serve the same purpose. In an alternative arrangement, a triode valve is used in place of the valve VT4 to byepass excess current from the plate of the valve VT2, thereby maintaining the output from VT3 at constant strength. Or the grid of the valve VT4 may be connected directly to the input circuit of the set. As shown in Fig. 11, the automatic gain-control valve VT4, with its filament energized by the output current from VT3, is also made to form one of the chain of amplifiers. The resistance of the filament of valve VT4 also serves as a feed-back coupling. Mains-driven sets. As shown in Fig. 13, grid-bias for the valve VT3 is derived from the points c, d through the plate-filament path of the valve VT2<SP>11> and similarly from the points b, c through the valve VT1<SP>11> or the grid of VT2<SP>11>. Automatic gain-control is secured through the auxiliary valve VT4, the filament of which is energized in part by the normal current flowing through the mains potentiometer resistance PR, and in part by the output from the power valve VT3. A gramophone pick-up PU may be switched into the input, as shown. Hum disturbances in the first two valve stages are balanced out against a similar but out-of-phase component in the power stage VT3, the phase of the latter being finally adjusted by means of a potentiometer FP shunted across the filament. The aperiodic nature of the inter-valve couplings also tends per se to the reduction of hum. The condenser C3 reduces the signal-current components flowing in the section c, d of the resistance PR, and thus limits signal feed-back. Also the direction of flow of signal components in section e, d is opposed to that in section b, c this again helping to reduce signal feed-back. The auxiliary valve VT4 may be arranged in the chain of amplifiers, as before. Gain regulation may also be effected by using a second resistance in parallel with PR in place of the valve VT4. In a modified arrangement, the valve filaments are heated directly by raw alternating current, each filament having a shunt potentiometer for reducing hum. The section d, e of the resistance PR, Fig. 13, may be omitted, so as to reduce the load on the mains, thereby improving the smoothing action of the supply filter. Stress is laid on the advantage of automatically regulating the grid-bias applied to the rectifier valve as the strength of the incoming carrier varies. In Fig. 19 a resistance which varies in value with the current flowing through it, such as a tungsten-filament lamp LP, is inserted in the resistance PR to increase the effective grid-bias control on the valve VT1 as the incoming signal varies. Signal feed-back is substantially reduced because both resistances PR and PR1 contain inphase signal components from the valves VT and VT2, which tend to balance each other on the grid of VT1 if the sections a1, b1 and a, b are properly proportioned. The regulating-lamp LP may be replaced by one or more triode valves inserted in the chain of amplification. For a three-stage set, the necessary phase-correction is secured by inserting the regulating lamp LP in the upper resistance PR1. The field-windings of a moving-coil speaker may be inserted in series with the resistance PR, and energized by the D.C. plate current from the power valve. A screened grid valve may be used as such or provided with a switch to connect together either the plate and screen or screen and grid, so as to function as a triode, either for amplifying high or low frequency currents, or as a rectifier. For instance, in one arrangement, two screened-grid stages are followed by a triode stage for gramophone reproduction. The high degree of amplification so secured may be used in combination with a photo-electric input. A push-pull arrangement of this sort is described, automatic gain-control being effected in both stages. A reversing-switch may be inserted across one-half of the output transformer in order to test the " balance " of each push-pull channel, either of which can be used alone without introducing distortion. An arrangement is also described in which the grid of the power valve is positively biased to the extent of passing 5 milliamps grid current, a choke or resistance being connected across the grid and filament as desired by means of a change-over switch. In Fig. 25 the platefilament path of a screened-grid valve V1, connected to function as a triode, is energized by the voltage across the plate and grid of the power stage VT2. With the switch SW12 on the right-hand contact, the amplification gain is increased. In an alternative form, in which a gramophone pickup can be combined with either two or three stages of amplification, the space-current path of a preceding amplifier is traversed by the grid current from the power valve. To secure automatic gaincontrol over a wide variation of input frequency, the arrangement of Fig. 27 is used. The rectified current increases the potential drop across resistance R11. But as the plate current drops simultaneously, the voltage also falls across the points b, k, which determines the initial biasing- voltage on the grid. This gives a double or differential gain-control on the grid of the valve V1. The provision of resistances R2, R2<SP>1> in shunt with the plate and grid of the valve VT2, produces the same effect as connecting resistance R2 to a potential-developing point more positive than the point c. The grid tapping-point on the resistance PR is selected so as to apply an A.C. component which neutralizes any residual hum, produced for instance by changing one of the amplifying valves for another of different characteristics. Fig. 30 shows a chain of four high-frequency valves V4<SP>11> .. V4<SP>11> followed by detector-amplifiers VT1<SP>1> VT2<SP>1> and a parallel output stage VT3, VT3<SP>1>. All the operating voltages are derived from the mains, together with the energizing-field for the loud-speaker LS. The lamp LP co-operates with the resistance PR to ensure automatic gain-control. A gramophone pick-up PU may be switched into the grid circuit of the valve VT1<SP>1>. The aerial resistance AR gives an aperiodic input. The tuning-condensers TC .. TC3 are ganged to a common control. The condenser BC3 in combination with the transformer P1 ensures a constant-frequency coupling between the valves V1<SP>11> and V2<SP>11>, a similar provision being made between the valves V3<SP>11> and V4<SP>11>. Automatic gain-control is effected by the connection of the cathode of the first valve V1<SP>11> to the point d throu