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Registered Member #561
Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230
I've been building a 833A powered VTTC based on Steve Ward's single tube design. Got to first light yesterday with a crude lashup and today I pulled the shunts out of the MOT so I can start tuning it.
First test run after deshunting the MOT I noticed some corona coming off the top of the ticker wire connected to the grid. I (foolishly) ignored this and gave it another run. Not long into it I got what sounded like power arcing off the tickler, across the PVC pipe and onto the secondary. I couldn't tell for sure because it was on the opposite side of the coil to me.
I turned it off very quickly and went to inspect the damage. I tested the resistance of the secondary and got 35KOhms (pretty sure it should be much less than that!) and I need a magnifying glass to be sure but I think the wire melted and broke.
Any idea what was the cause? I had 2nF and 17k on the other side of the tickler coil between that and and ground. My secondary was connected to ground via a steel I-beam cemented into the floor of my shed for mounting a vice on. Ask for any other details, not sure what you need to know.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
I don't have experience with VTTCs yet, so I cant really give any advice on your grid coil and primary tank circuits.
With the corona you saw, was it coming directly from the tickler coil and the grid terminal on the vacuum tube itself? Or was there noticeable corona around the secondary coil near the site of the flashover? The tickler coil might be positioned too high and the secondary shorted out to earth through your grid leak circuit. If the corona was strictly coming from the tickler coil and its tube connection then perhaps you have too many turns and are getting several kilovolts induced on the winding? I'm not sure if either scenario is accurate, but that's all I can think of at the moment.
EDIT: Just checked your photos again and it looks like you have enough distance between the secondary and primary/grid windings. What sort of output were you getting before the flashover? I remember having a similar incident when tuning a spark gap coil. If I recall, the primary was accidently tuned to a much higher frequency than the secondary and I ended up with some corona around the center of the secondary, with some inter-turn sparking.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
The voltage across the entire feedback winding can actually be quite high, I have had arcing between turns on the feedback when using side by side winded copper magnet wire.
Normal machine tool wire is usually better because of the large distance between the conductors.
It could just be a matter of too high potential difference between windings of the feedback coil or between feedback coil and secondary. The solution is more insulation or everything or lowering coupling.
Registered Member #2018
Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74
I would place the feedback coil below the primary coil. I've had similar problems in some of my vttc designs and it worked better with lower feedback coil.
Registered Member #54263
Joined: Thu Jan 15 2015, 09:54AM
Location: Perth
Posts: 35
This looks like bad resonance and/or too high coupling. The best idea with VTTCs is to run it at low voltage (500V for example), get a small streamer, find the resonant frequencies, interrupter ratio and so on and only then switch to full voltage. Usually you will have to make 2 feedback windings - more turns for low voltage, less turns for high voltage.
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[VTTC] Ticker to mid-secondary arcing.
