GB507271A - Power transfer circuit - Google Patents

Abstract

507,271. Valve circuits. STANDARD TELEPHONES & CABLES, Ltd. Aug. 19, 1938, No. 24481. Convention date, Sept. 2, 1937. [Class 40 (v)] A spare amplifier is brought into operation automatically when the output from a working amplifier becomes sub-normal, the amplifiers being changed over by removing a blocking bias from one and creating a blocking bias on the other. An alarm circuit may be completed whenthe spare amplifier is in use. Under normal conditions amplifier 5 is in use, the grid of the valve being connected through R5 to a point at earth potential on potentiometer 8 which comprises resistances R1, R2, R3 whilst the grid of the spare amplifier 5<1> is maintained at a potential of - 24 volts over resistances R4 and R6. A pair of gas discharge valves 12 connected in parallel (one valve only would suffice) have their plates connected between R1 and R2 on potentiometer 8 so that the normal voltage relative to the cathodes is 114 volts, whilst the potential of their grids is determined by rectifier bridge 13 fed from the output 3 of amplifier 5. If the output falls below a predetermined amount, the negative bias on one or both tubes 12 drops below the firing point of the tubes so that current flows in the tubes. This produces a voltage drop of about 24 volts across R4 so that the blocking bias on the spare amplifier is removed and this amplifier becomes active. At the same time the voltage at the junction of R2 and R3 is materially lowered so that a blocking bias is created on the normal amplifier 5. A relay A may be provided as shown to close an alarm circuit when the spare amplifier is in use. The return to normal output on the circuit 3 does not affect the discharge across tubes 12. The apparatus may be restored to normal when required by momentary operation of key 17. Key 18 enables the spare amplifier to be put into use when tests are to be made on the normal amplifier 5.