Power amplifier tube protector circuit

The circuit shown in the figure is a power amplifier tube protector circuit.

Working principle: In the figure, K is the power switch, T1 controls a small power transformer, and T2 is the power transformer for the power amplifier. IC1 (S1-S4) is a four-way analog switch CD4066. When the power is turned on, K1 is closed, and a 12V DC voltage is obtained on C1. At this time, S3 is turned on because the control end is at a high level, and the S4 control end is set to a low level, and S4 is disconnected. Since the voltage on C2 cannot change suddenly, when the power is just turned on, S1 is turned on because the control end is at a high level, the light-emitting diode in the photocoupler IC2 lights up, the photosensitive tube is turned on, and the audio signal output by the power amplifier volume potentiometer is short-circuited to the ground. After S1 is turned on, C3 is charged, and after a few tenths of a second, S2 is turned on, followed by VT, and relay J is attracted to connect the power supply of the power amplifier part. VD7 feeds back the high voltage after S2 is turned on to the control end, making S2 self-locking. After that, C2 is charged, and the potential of the S1 control end gradually decreases. Since the COMS analog switch has the characteristic that the on-resistance changes gradually with the potential of the control terminal, as C2 charges, the light-emitting diode in IC2 gradually dims, and the resistance presented by the photosensitive tube gradually increases. If the volume potentiometer of the power amplifier is at a large opening, the volume gradually increases, preventing the power-on shock. Finally, IC2 is cut off, and RP can freely control the volume. When shutting down, K is disconnected, and the charge on C1 is quickly discharged through R1 and the light-emitting diode LED. Since C5 and C6 have large capacities and the voltage still exists, VD5 and VD6 are cut off, and IC1 and IC2 do not stop working immediately, and J is still attracted. At this time, S3 is disconnected because the control terminal becomes low level, and C4 starts to charge. As the potential of the S4 control terminal gradually rises, its on-resistance gradually decreases, and the resistance presented by the photosensitive tube in IC2 gradually decreases, and the volume of the power amplifier is gradually turned down. Later, due to the discharge of C5, J is released, and the power amplifier is powered off.

Component selection: LED is Φ5mm red light-emitting diode. IC2 is 4N25 photocoupler. Other components are as shown in the figure.