I have Park G25R amp from 90's, designed by Marshall. This amp is humming on clean and drive channel and hum is getting louder if I turn volume up. There is no guitar sound to hear. I hear guitar sound only for about half second, when I turn the amp off. Refers it to something? So far I have checked the pots. They are noisy but working. Also I have checked the bridge rectifier, which should be also ok. Second thing is I think a design flaw, cause its always been so. When I switch from drive channel to clean, the amp is making really horrible noise, when reverb is turned on. Without reverb it is making pop. Is it possible to add a resistor, which will ground the voltage from switching?