New & Improved Junk Box Coil Sparkz Again!
After a layoff of several months, I decided to refurbish the 4" x 24" New & Improved Junk Box Coil. I had stolen the tank gap, spark gap, and toroid from it to get my 6" x 30" coil going, and sort of lost interest in the smaller coil until recently. I made some improvements to the NIJBC, including a new perpendicular flow air blast gap, and a 34nF MMC for resonant charging with two 9kvac/60ma NSTs in parallel.
First light was pretty satisfying. I got sparks in the 36" to 45" range, and the experimental MMC remained cool as a cucumber. Unfortunately, I noticed a racing spark near the bottom of the secondary, as well as the occasional secondary to primary arc over between the racing spark and the innermost primary turn. I was afraid the spurious arcs and sparks would ruin my secondary, so I shut it down.
I decided to repair and modify the coil to clear up the problems. I cleaned the secondary with paint thinner to remove fingerprints and dust. Then I gave it several coats of clear gloss oil based urethane varnish, allowing it to dry thoroughly between each coat. I also removed the innermost primary turn to gain a bit more clearance. The surgery left me with 9 turns.
2nd light was as good as first light, and the pesky racing sparks and arc-overs were gone. However, the top-turn corona and sparking was bad enough to cause me some concern. I took the 3.5" x 13" toroid off of the 4.5" plastic cup it had been perched on, and sat it atop a 2" wide roll of Teflon tape instead. This banished the top-turn corona and also produced a solid increase in streamer length, up to about 48". I never knew these things were so sensitive to toroid height!