The reason it happens is because when your amp is turned on and running, there is a constant supply of voltage running through your amp.
However, the pop that you hear from the standby switch on your tube guitar amplifier is actually normal.
Making sure there is no DC voltage anywhere on the audio signal cables will reduce the chance of clicks or pops if the audio cable is disconnected and re-connected with the amplifier up. This amp must have a problem This is a common complaint we hear. The switch shorts out this resistor for a short time after power-up. Something in the region of 10K to 22k would be normal. This resistor will have a value that is high enough that it does not effect the audio signal but does carry the hopefully very low capacitor leakage current. I would expect the pre-amp to not only have a series capacitor on the output but also have a resistor to ground at the output after the capacitor to maintain a 0V DC condition on the cable that goes to the main amplifier. This is why the short needs to remain for a short time after the pre-amp power is applied. The short provides a path for this charging current without introducing a voltage at the main amplifier input. When the pre-amp is first switched on, this capacitor needs to charge up to its normal working voltage. The reason for shorting out is that the pre-amplifier will have a series capacitor at the output. Should not matter if you short out the pre-amp output.
