Just wanted to share this.
I have an AMI JAL in the shop. One of the many problems was an intermittent static like noise in the speakers. I thought it had the sound of a 60 cycle arc, if you can imagine that. It didn't do it on the bench, but it was intermittent. It was very loud and would go up and down with the volume control. I pulled the inputs and it still did it. Started pulling tubes in the pre amp, then in the power amp. It didn't completely go away until I pulled all 4 output tubes. Somewhere along the line I had even unplugged the pre amp from the power amp and only had the output tubes in it. (they tested good) It still had the noise but very, very quiet. I had to have my ear close to the speaker to hear it.
To make a long story a little shorter, it was a bad turntable motor cam switch arcing and causing interference. When I pulled it out, it had about 80 ohms resistance. (Even with the bad switch in, it still played records at normal speed.)
It now makes me wonder if I had had an arc fault interrupter circuit breaker in my electric panel, if it would have tripped.