Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>Hate to be bearer of bad news Steve but that was not really a 250 kV coil. It hit around 105 kV output max. This article was written before good techniques were developed to measure RF potential. People were equating spark length with essentially air plasmas and trying to call it higher potential than it really was.
Also, the PE mag article used a NST and classic spark gap --- it was not a vac tube coil. I believe you are referring to the vac tube coil in Radio-TV Experimenter by Harold Strand that used a pair of 811A's.
I have the mag and actually built the coil in 1964. Contact me off list. Dr. Resonance
HiI'm trying to track down an on-line copy of the article in Popular Electronics (July, 1964) for a 250KV Tesla coil. The article I'm looking for was a TC using 2 x high-power transmitting tubes. There may have been a similar article in Popular mechanicas around the same time. I found a copy of the article some time ago in GIF format but needless to say I've lost both the original link and the image files :-(Many thanks in advance Steve