This circuit has been designed primarily for use with valve amplifiers. When the amplifier is switched on, filament voltage is supplied first and the anode potential a few minutes later. When there has been no input signal for a while, the anode voltage is switched off again automatically.
When the mains is switched on, a set pulse is supplied to bistable (flip-flop) IC3b via R16-c6. The Q output (pin 13) then goes high and the bistable resets itself via R5-C15, whereupon relay Re1 is energized via T3 and the valves are provided with heater voltage. After a delay, dependent on time constant R13-C4. the potential at the clock input of IC3a reaches a level that causes the bistable to toggle so that its Q output (pin 1) goes high. Relay Re2 is then energized and switches the high voltage to the values. During the operation of the amplifier, the inverting input of IC1 and the non-inverting input of IC2, both of switch circuits are connected as comparator, are provided with a voltage of above 6 V by potential divider R1-R2-R3-R4. The audio signal from the preamplifier or output amplifier (one channel suffices) is fed to both ICs. The earth of this signal is connected to the potential divider, which means that the supply and earth lines of the amplifier and the delay circuit must be well isolated from 1mV or greater. When the signal level is about 60 mV or grater. the output of either IC1 or IC2 will go high, depending on the polarity of the signal. Transistor T1 is then switched on via R6 or R7,which result in C3 is charged slowly via R18. When there has no signal input for a few minutes, the voltage across C3 rise to a level where IC3b gets a clock pulse. This result in IC3a being reset, whereupon T2 is switched off. so that relay Re2 is deenergized and the high voltage is removed from the values. Transistor T1 is provided with base current via R11 and D3. so that the clock input of IC3b remains low. This bistable resets itself almost immediately. however, via R15-C5. The interval between clock pulse and reset is so short that Re1 remains energized: heater voltage to the valves is, therefore, maintained.
There are two keys for user operation: S1 resets IC3a whereupon the high voltage is reapplied to the valves; S2. when pressed. causes a clock pulse to be generated that switches the amplifier to standby.