1964 Mercury M-12a noise

[journeyman]

The early Ampegs did not use a grounding plug, so with nothing plugged in, they have that ground hum. Does it dissappear when you plug your guitar in? My M-12 is the same. I recently got a 1958 Jet and had the amp tech install modern Switchcraft input jacks and there is no ground hum when I unplug the guitar. Let us know, because if it does this with the guitar plugged in, there are other issues that can be addressed.