Patent GB-663583-A (GB)
Title: Improvements in or relating to negative feedback amplifiers
Inventor: JACOBSEN BENT BULOW
IPC: H03F1/54
Filed: 1949-04-19
Abstract: 663,583. Thermionic valve amplifiers. STANDARD TELEPHONES & CABLES, Ltd. April 19, 1949, No. 10335/49. Class 40 (vi). When the heating filament of a valve in a multistage negative feed-back amplifier burns out, the faulty valve is automatically byepassed and phase inversion introduced in the feed-back loop so that the feed-back remains negative. In one embodiment, Fig. 1, the heaters of the valves 1, 2, 3 of a negative feed-back amplifier are shunted by thermistors 24, 25, 26 in series with the windings of relays 27, 28, 29, respectively. The resistance of the relays and the thermistors are proportioned so that the terminal voltages of the filaments remain constant if one filament burns out, as described in Specification 524,627, [Group XXXV]. The burning out of the filament of a valve causes the associated relay to operate, and connect the input of the valve to the primary winding of a phase-inverting transformer, 30, 31 or 32. The secondaries of transformers 30, 31, associated with valves 1, 2 are connected to the inputs of the following valves 2, 3, while the secondary of the transformer 32 associated with the final stage 3 is connected to the output transformer through a blocking condenser 33. The relays may be replaced by indirectly-heated thermistors with their heaters connected in series with the directly-heated thermistors across the heaters of the valves, Fig. 2 (not shown). In another modification, Fig. 3 (not shown), the secondaries of the phase-inverting transformers are connected in the anode circuits of the valves in place of the coupling networks, and the coupling networks are connected across the primary windings. In this embodiment the capacity between the relay contacts provides part or all of the neutralizing capacity for the grid-anode capacity of the valve. In another modification, Fig. 4, relays 35, 36 with two pairs of normally open contacts are connected in series with thermistors 25, 26. Operation of relay 35 or 36 energizes relay 39 or 41, respectively and also a relay or thermistor heater in the feed-back network. On failure of valve 2, the left-hand contacts of relay 39 open to disconnect the coupling condenser 16 from the anode of valve 2 and the right-hand contacts close to connect the input to valve 3. If the output valve 3 fails, the left-hand contacts of relay 41 open to disconnect the coupling network 15 from the anode of valve 2 and the righthand contacts close to connect the output of valve 2 to the output transformer. The normally open contacts of the relays may be replaced by indirectly-heated thermistors, while the normally closed contacts may be replaced by a directly-heated thermistor, Fig. 5 (not shown). The necessary phase inversion in the feed-back path may be produced by the operation of a relay with change-over contacts to remove a short-circuit from the secondary of a transformer and apply the feed-back voltage to the primary, Fig. 6 (not shown). In another arrangement, Fig. 7, the heating coil of a thermistor 56 is connected in a bridge circuit with resistors 57, 58, 59. The heating coil of a second thermistor 55 is connected across the bridge. The thermistor 56 normally has a low resistance and effectively short-circuits the secondary winding 49 of a transformer, while the thermistor 55 has a high resistance owing to the balance of the bridge. On the burning-out of a valve filament, connector 37 is earthed, and the heater of thermistor 56 is short-circuited, the bridge is unbalanced, and thermistor 55 is heated to connect the feed-back to the primary winding 48 of the transformer.