I was testing some NSTs today with a Jacob's ladder setup, and am noticed a light mechanical 'jolt' at the upper end of the variac's range. It would cut out between approx 90% and 99%, and would make the vibration most stronly when it 'kicked' back in at 100%. BTW, the variac is a 10A staco model, in nice condition, from a visual inspection, and the NSTs were 12KV 60MA 720 VA Allansons. Eric in Baltimore