Re: Tube coil project
Dr. Drone, Sat Dec 20 2008, 06:27PM

Re: Tube coil project
Proud Mary, Sat Dec 20 2008, 06:54PM

Good work, Vasil!

Re: Tube coil project
Dr. Dark Current, Sat Dec 20 2008, 06:56PM

yeah, I would point one of the fans on the MOT instead They can run VERY hot and it's perfectly fine, most are rated at 180 or 200C maximum temperature. I'm sure you wouldn't want to see your nice work melt down, would you

Re: Tube coil project
vasil, Sat Dec 20 2008, 07:43PM

OK, added some wood spacers.



I will use some impedance matching (autotransformer primary type), so I hope that the MOT will not be so hot. But a safety precaution more is always wanted.

Re: Tube coil project
Proud Mary, Sat Dec 20 2008, 10:18PM

You could add a time delay relay so that the anode voltage can not be turned on until the valve filament/heater has reached full temperature.

You could also add a soft-start relay on the input side of your mains transformer, if you haven't done so already.

Re: Tube coil project
vasil, Sat Dec 20 2008, 11:41PM

The HV transformer will be fed through a variac (externally). Ading a variac in the already built setup would give discal disturbances, heh. The fillament, anode and coolers will be powered separately through mains power cords. I prefer this way, the space is too small. I put some switches directly on the cords.

Re: Tube coil project
Experimentonomen, Sun Dec 21 2008, 12:49AM

What tube is that ?

Re: Tube coil project
vasil, Sun Dec 21 2008, 07:36AM

GU-81
The most accesible tube in this part of the world.

Re: Tube coil project
Dr. H., Sun Dec 21 2008, 07:45AM


Indeed Maaaan this is a sweeeeet tube. I have 5 or 6 of those and we (with a friend - ham) have built a 80m to 20m amplyfyer, wich easuly takes out 1kW of power in the antenna when we want to be naughty

Cheers

Re: Tube coil project
vasil, Sun Dec 21 2008, 04:44PM

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).

Re: Tube coil project
vasil, Thu Dec 25 2008, 12:36PM

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

Re: Tube coil project
Mads Barnkob, Thu Dec 25 2008, 06:51PM

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

Re: Tube coil project
vasil, Thu Dec 25 2008, 08:10PM

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. :)

Re: Tube coil project
Mads Barnkob, Thu Dec 25 2008, 08:34PM

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

Re: Tube coil project
vasil, Thu Dec 25 2008, 08:43PM

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.

Re: Tube coil project
Mads Barnkob, Thu Dec 25 2008, 08:54PM

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.

Re: Tube coil project
vasil, Thu Dec 25 2008, 09:21PM

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.

Re: Tube coil project
vasil, Fri Dec 26 2008, 08:40PM

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.

Re: Tube coil project
Arcstarter, Fri Dec 26 2008, 11:29PM

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.

Re: Tube coil project
vasil, Sat Dec 27 2008, 07:39AM

NO, the caps on the table were used for something else. The connections in the primary tank are copper strips as you see in the picture:



The feedback is tapable. I tried differente settings, and the best is 21 turns:



What I would need would be a 2400 V MOT...but I am building with what I have in my hands. Big MOTs are rarities here.

Re: Tube coil project
Arcstarter, Sat Dec 27 2008, 08:04AM

Oh, hehe maybe i should have looked better before making assumptions. I wish i could donate a MOT for this project, but the shipping would be just horrible. I can get good MOTs quite often. I do not have a spare big one right now(depends on what is considered big), but i am sure it wouldn't be too hard to get one. Also, i have no way of knowing voltage. I could look at the capacitor, but that means nothing, they could be over rated for all i know.

Re: Tube coil project
Dr. Dark Current, Sat Dec 27 2008, 09:11AM

Afaik the caps almost always have 2100VAC on them, even when they happily survive 4kVac. The MOTs don't vary that much, almost all are in the 2000-2200V range.

Re: Tube coil project
vasil, Sat Dec 27 2008, 11:00AM

This is not quite difficult. Feed your MOT with a known voltage (let say 10 V) and measure the secondary output. Or connect it to the mains in reverse (mains on the secondary) and measure the low voltage output from the primary. You will know the transformation coeficient K = N2/N1 = U2/U1, so multiplying the mains voltage with K you would have an oppinion about the high voltage output.

If I will get a bigger MOT the transplant is easy.

Re: Tube coil project
Phillip Slawinski, Thu Jan 01 2009, 03:46PM


Interesting feedback coil. I like how you have variable taps on it.

Have you considered running two series-wired then level-shifted MOTs? I'm not sure what kind of voltage the tube you're using can take, but this is the configuration I use with my tube coil. Under load the the transformers in my setup do not deliver enough voltage to damage the plate, but they certainly deliver more voltage than a single level-shifted MOT.

Nice job on the coil. It's harder when all the components are visible, as you have to make the wiring much neater to keep the coil looking respectable. For my VTTC I put it in a box, so all the wiring is hidden except for the two leads to the primary, and the tube.

Re: Tube coil project
vasil, Thu Jan 01 2009, 06:18PM

Thanx Phillip

I have only one little MOT. I had a bigger one on my previous PROJECT You can see it HERE , biger core and plastic insulated coils, bigger sparks also.

I will hunt some new MOTs (I need two identical to parallel them in my next GU5B VTTC), maibe I will have luck. If one more MOT comes in my hand I can series them. :)

Specs of the GU81 tube HERE

Re: Tube coil project
Phillip Slawinski, Fri Jan 02 2009, 03:08AM


Well you could always steal from another project At any rate it looks like the tube you're using wouldn't take the kind of voltage that an 833 will take.

I'm not sure if I see the benefit of paralleling MOTs. They can usually deliver plenty of power on their own. Doubling them you get nearly twice the voltage which means four times the energy stored in the tank cap

Re: Tube coil project
vasil, Fri Jan 02 2009, 09:01AM

All the previous projects were given to another hobbysts, just to clear space in my room, so I have to wait a bit until some MOT will appear.

The idea of paralleling MOTs comes from here:
http://4hv.org/e107_plugins/forum/forum_viewtopic.php?60496

The new tube is a GU5B triode with 2.5 kW dissipation on plate. Ther specs here:
http://tubes.ru/techinfo/Broadcast/gu-5b.html

The plate voltage is low enough for a single MOT and a single doubler and I was afraid to not be able to get all the current needed only with a single MOT. I just invented some complicated circuit with a lot of diodes, but the advice from the forum was jut to parallel MOTs to get it better....it will be easier for me too.

-------------------------------------------- -------------------------------------------------- -------------------------------

dec. 02.2011

Over 50 cm sword sparks after some HV PSU and grid circuit modifications:

Youtube link