Ampeg V-4B 60cyc hum issue

About a month ago I bought a demo Model V-4B from Sweetwater. A few days in to playing at my house I noticed my amp puts out an audible 60 cycle hum for 12 seconds every 2-1/2 minutes. It only does it when a bass is plugged in, standby is off and the bass's volume and tone are fully on. It does it in both grounded and ungrounded outlets and in the grounded outlet with a ground loop adaptor. It does it both when playing and just and when the bass is at rest in the stand. It doesn't happen everytime I've played and it didn't happen at my neighbors house. I've been contracting the Sweetwater service dept and they've had me film the amp behaving this way and had me try it at another power source and came to conclusion that maybe I have a "dirty" neutral in my house. Does this sound about right for this amp? The fact that when it does act up it's every 2-1/2 minutes for 12 seconds makes me wonder if I'm picking up interference from another electronic device or from the amp itself? The basses I've experienced this hum with are both passive, Fender American Standard P-bass and a Gibson Thunderbird.

 

Good point. I've got a lot of chargers plugged in in my house.

 

No problem dont. But I'm now bettin it is interference. I really find out when I take it the band practice space and cook on it a little.