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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, so my VTTC is working all right now, but the sparks don't look how I thought they'll look. They are branchy and fire in almost random directions, just the ends are straight. How do I get the super straight sparks like on Steve Ward's 833A coil?
A video of my coil can be seen here:
I use a triac power controller but I turn it up full power at times.
Coil data: Power supply: doubled MOT with 2 MO caps in parallel Primary coil: 40 turns (tapped at turn 39) of 2.5mm O.D. PVC insulated wire, 110mm form, tank capacitor 1000pf Grid coil is under primary coil and has 25 turns of 0.6mm enameled wire Secondary coil: ~840 turns of 0.3mm enameled wire, form diameter 63mm, winding length 252mm, form length 272mm Resonant frequency of unloaded secondary coil with a topload is ~600 kHz, BUT the primary tank oscillates at 370 kHz.
Registered Member #1034
Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154
I'm not sure about this but I think the picture of Steve Ward's 833a coil you posted a link to is a time exposure of his coil running in staccato mode. When running in "continuous mode" the sparks tend to build on each other.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I found out that most likely my tube starts oscillating just when the plate voltage reaches a certain level, so it runs in a kind of pulsed mode of operation and that may be causing the branchy sparks with straight ends. I'll try adding some positive bias to the grid circuit to help it start up.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yes I have a resistor with a wire twisted around it connected to the plate. I also have a 1uF 250V film cap on the filament.
Just tried adding some positive bias and that didn't help... Hmm, guess I'll have to live with it. Here is a photo showing the weird sparks which are somewhat non-defined on the bottom but very straight at the ends...
PS. Using 1 MOT with 2uF in the doubler and 4 microwave diodes in 2x2 configuration. The rf bypass cap is around 0.03uF.
I may add that the coil produces a loud buzzing sound at full power, not the soft hum I'm used to with half wave solid state coils.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Dr. Spark, I tried a lower f0 secondary, at around 500 kHz. The primary circuit tuned to 300 kHz for optimum output power... And the problem with strange buzzing spark was the same. I also tried removing one cap from the doubler but that didn't help either.
I observed the phenomenon where primary tunes much lower than secondary on other VTTCs I built, I guess it has something to do with the very high voltage per turn induced and high coupling, the optimum power transfer happens so far away from secondary resonant frequency. Maybe with reduced coupling I could go higher on the frequency, but that would mean the primary winding would be huge...
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How to get straight "sword like" sparks from a tube coil?
