I don't know if it only works on just my amp or everybody else's too.
Any way what i do is connect the input to headphones socket, and turn the gain and all volumes up, except the master volume. At low master volumes, the amp screams and ear pircing whistel, if you turn up the master volume , the sound gets louder and lower, It's sounds a bit like an oscilliscope sound generator type thingy..
i've not played about wich this much, out of fear of blowing up my amp, but i can manage to get it to "play"
a few notes.
Please try this, and get back to me... tell me if it works, or if this is a well known old trick...
I love effects, but i haven;t get an fx box of my own though...
oh yeah i forgot, my amp is a transistor amp, if that makes any difference