Patent GB-513162-A (GB)
Title: Arrangements for switching over from normal to spare amplifiers in electrical signalling systems
IPC: H03F1/54
Filed: 1938-03-04
Abstract: 513,162. Repeating systems. STANDARD TELEPHONES & CABLES, Ltd., HODGSON, K. G., and ROCHE, A. H. March 4, 1938, No. 6844. [Class 40 (iv)] In a repeater in which a spare amplifier A2 is adapted to be switched in automatically upon failure of the normal amplifier A1, the two amplifiers are connected in parallel during the period of switching over so as to reduce the interruption of the transmission to a minimum. Current for the amplifiers is preferably obtained over the line. Heating current is fed in parallel to the amplifiers (which employ indirectly heated valves) through relays FN, FS. The H.T. supply is taken via a smoothing circuit L1, K1 to the amplifier A1 through relays A,B, C and to the amplifier A2 through a high resistance R3 and relay S. Relays K, L are normally operative when the covers of the amplifiers are in position. If the heating circuit of the amplifier A1 fails, FN falls back followed by J, which gives an alarm. Relays E, F operate and switch in the amplifier A2, after which MCO pulls up and locks.- The H.T. supply is disconnected from the amplifier A1 and the resistance R3 is cut out to render the amplifier A2 fully operative. Relay Q disconnects the heating current from the amplifier A1 and puts in a dummy load resistance R1, while relays G, H switch out the amplifier A1. The two amplifiers are thus connected in parallel for a short. period, during which the amplifier A1 remains operative owing to the thermal inertia of the valves. The amplifiers should have high input and output impedances in order to avoid undue reduction of gain. If the H.T. supply fails, one or more of relays A, B, C falls back and brings about switching over in a similar manner, except that MCO operates simultaneously with E, F in order to render the amplifier A2 fully operative as rapidly as possible. To switch back to the normal amplifier A1, a key PB2 is actuated to operate relay R which reconnects the heating current and H.T. supply to the amplifier A1 and starts a motor M. After a delay to allow the amplifier to reach its operative condition, the latter closes contact m to connect up relay D, which disconnects G, H to switch in the amplifier A1 and unlocks MCO. This relay falls back slowly disconnecting E, F to switch out the amplifier A2 and restoring the supply circuits to normal. Relay D falls back when contact m opens, and stops the motor M. The change over to the spare amplifier may be effected manually by a key PB1, which simulates a failure of the heating circuit. If the heating circuit of the spare amplifier fails, the relapse of FS closes an alarm circuit and connects in a dummy load resistance R2. Relay S operates when the full H.T. supply is connected to the spare amplifier, and its relapse when that amplifier is switched in controls an alarm circuit. The heating supply circuit includes a sluggish relay N, which, when the current is restored after a failure, allows R to operate and switch in the normal amplifier in the manner described above. The spare amplifier may be fed normally with reduced heating current through a resistance which is cut out by the operation of Q. Fig. 2 shows a modification in which the outputs are connected in series instead of in parallel, the inoperative amplifier being short-circuited on the output side, and the amplifier inputs being terminated in resistances R5, R6 when not in use.