On/off delay for valve amplifiers

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) IC_3b via R_16-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 IC_3a 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 IC_3b gets a clock pulse. This result in IC_3a 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 IC_3b 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 IC_3a 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.