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vasil

Sun Dec 21 2008, 04:44PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

OK...

I finished today the coils and the primary capacitor. I will play with a tapable feedback coil under the primary to find the number of turns needed by the tube, then I will make a definitive feedback above the primary (I leave enough space for it):

I put the last stage on the top of the coil:

Because the bumps on the secondary I am using an intermediary pipe:

The primary capacitor: 4 of 2.2 nF@6 kV 35 kVAR in series-parallel for 2.2 nF @ 12 kV:

The tapable feedback coil:

I have some minor connections to make, but I think I will fire on Christmass day (we have a little vacance then).

vasil

Thu Dec 25 2008, 12:36PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

OK, the first light.

I had too more primary capacitance so I took off two of my ceramic caps (I am runing now only 1.1 nF). Grid 1 and 2 connected together. 100 W light bulb as grid leak resistor. I tunned, added a little toroid on top to have some more current in discharges, tunned again. 21 turns for feedback is just allright, one more or less = no advantage. The same primary connection for the cap and tube (no voltage amplification on tube). The plate is OK, but the MOT is hot after 6 minutes of play (have to make a bit of impedance matching). I will wind a new feedback above the primary trying to get some sword sparks (now they are long, very hot, but forked).

Two movies on youtube:

MOVIE 1

MOVIE 2

Mads Barnkob

Thu Dec 25 2008, 06:51PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Nice sparks, do yourself a favor and add a staccato controller to it, that way you can vary between sword like sparks and huge bucket flower like sparks.

How high input voltage do you put on the mot and how many primary turns? I use 19 and 20 feedback turns too on my dual 811a vttc

vasil

Thu Dec 25 2008, 08:10PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

220 V input (we have 220 V @ 50 Hz here). For higher input not more spark lenght gain, but hoter MOT. 26 turns on primary, 21 turns on feedback coil.

I want to extract the best spark possible (have two ideas about), so I would experiment a bit. Staccato is one of the last options because I have a problem...all my solders suxx. Thats why I am using only mechanical contacts, even for FETs in my SSTCs. :)

Mads Barnkob

Thu Dec 25 2008, 08:34PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

You should try to put a voltage doubler after your mot then, that's what I did so I can run it with a input voltage of 160-180vac on the mot so i completely avoid heating from it and I got a headroom so over voltage my tubes

vasil

Thu Dec 25 2008, 08:43PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

JC...did you look at the first picture? It has a voltage doubler already. Of course I can run at lower voltage for extended periods of time, but the point is to get the maximum spark from it. I will see tomorrow what improvements can be done....I am thinking to one more tunned circuit on primary to get a bushy discharge and a flat feedback coil...the proximity of the toroid can be dangerous. A flat feedback coil will be low coupled so it would be equivalent with a feedback coil more above primary....will see.

Mads Barnkob

Thu Dec 25 2008, 08:54PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Sorry, I see it now, but the best addition to longer sparks is mostly higher input voltage, especially if you are not running over tube specs.

vasil

Thu Dec 25 2008, 09:21PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

For the full wave doubler I would need 8 microwave caps to get the same value of 2 microF. I only have two caps, so the doubler I am using is the rational choice.

I am thinking to build a more potent circuitry, but this will be discussed on a next project. :)

I am within the specs yet.

vasil

Fri Dec 26 2008, 08:40PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Today I was experimented a lot.
I tryed the autotransformer setup on my primary, as I used in my old vttc. Well...doesn't work, the spark lenght was almost the same, but the tube and MOT run cooler. I think that the power input does matter. The MOT in the older setup has a bigger cross section of the core, the actual transformer is smaller, even it has the same output voltage (2200V). Well I dont have another MOT...
I built the flat feedback coil:

I built it on a flat surface and solidarised the turns with cyanoacrilate. Smells awfull and is iritating for the nose and eyes. It looks ok......but it is not worthing at all. The results was dissapointing.

I built a classical feedback coil with 0.4 mm magnet wire and tryed a few positions on the primary pipe....but severe arcs from the toroid to the grid coil forced me to take it off, sand the primary pipe and to build another primary. The feedback coil didn't perform better than my grid coil under primary.

After a day of work:

I renonced to make another modifications and tryes to tune fine the secondary system adding little pieces of metal on the topload...this way I achieved a few cm sparks more than all the trials before. Besides all the modifications I think that the voltage input is essential, I would have some more centimetres with a few hundred volts more.

I decided to let the setup in this stage. The streamers in free air are 35-36 cm, sparks to grounded things up to 40 cm (for 240 V input from variac, MOT start to saturate).

MOVIE

P.S

As quick as I find some MOTs I will start another project with a beefier GU5B tube (2.5 kW on plate). I have some MOCs, caps for primary, fillament transformer (13.4 V @ 27 A) and the tube. I am hoping for bigger sparks.

But I am afraid that my HV source will be too weak.

Arcstarter

Fri Dec 26 2008, 11:29PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Are those tank caps on that table a few feet away from the coil? Shorter wires can help with better performance. Lower inductance and capacitance of the wires is good.

Did you tune the feedback? You should mess with the coupling and taps of the primary and feedback. Just in case you haven't done that already.

I like the coil as it is, the sparks seem good :P.

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