I have a 1969 Silverface Princeton Reverb. It sat unused for about a year. I recently plugged the amp into the outlet, without a signal going into the amp, and you can hear a faint sound of the tremolo. If I adjust the intensity higher I can clearly hear it through the amp with no guitar plugged in. If I adjust the volume it follows. The reverb has no effect if adjusted. With the volume set to 0, I cannot hear the tremolo, but there is slight hum. It is not a 60 hertz cycle hum. This was not the case a year ago. By the way, the bias cap and the cap can were replaced about 5 years ago. Anyone has an idea what is going on with the amp and the tremolo?