Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx> Hi Dennis,Sounds like spark loading may be in a situation that is problematic for your coil. I've seen the same as your describing. I've had it where at full variac setting, the gap was firing away but the sparks were short and brushy, but at about 70% variac setting, the sparks were normal.
My gap was running into a high bps (multiple firings). The gap width was set fine and heat was not the issue (I know better than to go down that avenue). Tuning had no effect. After a little thought on the cause, I suspected a quenching issue at the gap. I suspected this because bps was higher than the sound of 120 bps, but much weaker in strength. Something odd was happening and it was consistent, but should not have been due to the cap size.
I decided to address the suspect quenching issue after verifying the gap was not heating abnormally. To alter the quench situation, I needed to address the secondary system (breakout level and energy transfer). I began trying different toploads and found a configuration that completely eliminated the issue and ran excellent long sparks at full variac setting. The topload configuration I ended up with was an 18" sphere on top of the toroid I was using. BTW, this coil used a .025uF cap and vacuum static gap with 12/60 NST, which is similar to your specs.
I decided on a larger toroid which I calc'd capacitive loading based on the working configuration. I built an Al-flex tube covered and sanded smooth with a mix of plaster of paris and a patching compound. A messy job, but it ended up very smooth and it's really pretty now (almost double in size as the original). I haven't tested the new toroid yet. In between things, I built a helical, cone, and another flat primary (some other testing going on). The primary is all bent to sh.., so I'm rebuilding the whole it.
Anyway, that is an experience I had with what sounded similar to what you experienced. If you can address the topload in some manner, it may be worthwhile. If your gap spacing is within conventional wisdom, leave it there. I'm not certain of the cause of my own experience (conjecture and theory), but I'm certain of what eliminated it.
Take care, Bart Tesla list wrote:
Original poster: otmaskin5@xxxxxxxI could use some advice. Recently, I'getting a significant drop in output as I dial up the variac. I start getting sparks at the 80v setting, as usual. But then, as I increase the variac voltage, the coil doesn't increase spark output proportionately as it used to.Might even decrease at higher variac settings ~120-130v.I'm running a 15/60 powered RQ type static gap coil with a 0.03uF Maxwell pulse capacitor. The RQ is set at 6 gaps X 0.03" each for a total gap spacing of 0.18" which seems conservative.Went back & verified all connections are tight & no wires near each other or near anything. Couldn't find any signs of arcing or shorts.NST checks out OK - both legs.The only thing I could find was some scorching on the safety gap - both the left & right terminals & grounded teminal. I've never observed the safety gap firing, but it's location makes it a little hard to see. Plus it's near the spark gap which I try to avoid looking at. (I've thought of putting colored glass/plastic over the safety gap to make firing more noticeable without looking directly at the lower deck).Anyway, I set the safety gap to the first adjustment point where they wouldn't fire with just the NST connected.Maybe corona causing premature firing? I'm kind of stumped. Any ideas would be appreciated. ThxDennis Hopkinton, MA