I realize volumes have been written and careers spent on susceptibility and emissions but ask your indulgence as I demonstrate my inexperience. My interest has recently turned to tube amplifiers with a seemingly fully functional kx 100b as my first victim. The only issue I know of is a slight 60 hz hum at the speakers (both channels) that gets louder and softer with the volume control. I am not calibrated to what's normal but don't think it excessive and figure I would chase it in the future with recap and the usual stuff (although I have scoped the power supply and it looks good to me....besides I thought power supply noise would show up at 120 hz...but I digress). My intention was to study tube theory in parallel with taking measurements on the amp to gain some practical experience. I realize it is seldom a good idea to just poke around on stuff, especially stuff that can kill you so I am careful and methodical. I did not get too far. I started at the phono preamp and put a scope probe one resistor away from the gate of 12AX7 (R9 for those that want to follow along). Looked normal to me i.e. looked like the input which I had at about 10mvac from signal generator. Tone volume responded accompained with the aforementioned underlying 60hz. This is where things went off the rails. Not wanting to continually change the scope setting yet wanting to know dc as well as ac I connected a Fluke 77 to the same place as the scope probe. When I did so the 60hz got a lot worse (both thru the speakers and on scope). So I go into disconnect and turn stuff off mode. In one of the iterations I noticed it was not the meter that caused the noise but the presence of a meter lead even without the meter. Result was the same with a single jumper cable like it acts like an antenna picking up 60 hz. But from what source ? There is nothing on any amp in or outs. The amp is off and not even plugged into ac. I shut off ac power to my garage/shop. There is nothing running on ac within 30 feet. Yet, when I clip a jumper on there is 60hz (with sufficient power to have an effect) showing up in the amp. You may ask how do you know if you have everything (including the scope) turned off ? Which is a good point and if this post weren't already ridiculously long I would go into the dozens of other observation I made in coming to this belief (not to be confused with a stone cold fact). I would be happy to clarify or provide more information if anyone cares to weigh in.