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PhilGood

Fri Apr 08 2011, 05:43PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Hi James,

Thanks for the infos,

I'm glad you managed to improve your coil in such a good way, well done !

It would be nice to see a vid of your coil running, if you can.

650pF caps might improve your coil even further (Arcstarter is using 650pF for both tank and grid caps).

I think this is definitely the 1st coil I want to build, as it is even simpler than any SGTC, and also less noisy on both audio and RF points of view. And I really like how they sound (reminds me some video games :p)

Also, could you please post link of the eBay shop where you bought your 650pF doorknob caps, I cant find any ...

James

Fri Apr 08 2011, 06:29PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

The capacitors I bought are here

I can post a video later. My coil sounds pretty much the same as Arcstarters coil. A nice pleasant 60Hz flubbing sound, it's actually rather quiet.

If you build one, do be extremely careful and don't forget that this is powered directly from a MOT. Contact with any part of the primary tank circuit while in operation can easily kill you. I unplug mine and hold the plug in my hand while adjusting anything. If you screw up with a MOT, there's no second chance.

PhilGood

Fri Apr 08 2011, 06:49PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Thanks for the link !
Woaw thats really cheap, basically the price of one unit at Rfparts !

BTW, what did you type to find them ? I tried "High Voltage Capacitor", "Doorknob Capacitor"... and got no result at all ... ?

Yup, I'll be very careful with safety rules, I know how deadly these nice toys can be... I was even wondering if touching the sparks as Arcstarter does in his video is really safe...

James

Fri Apr 08 2011, 07:09PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

I searched for 650pf and they came up, I think it was as much a matter of luck as anything else.

Touching the sparks briefly is ok, but don't touch the terminal itself or hold your hand in them long or you can get a burn. I got a minor RF burn on my finger last night when it got too close to the pins on the end of a fluorescent tube I was holding. You get a weird feeling of warmth where your fingers touch the glass of the glowing lamp too.

Arcstarter

Fri Apr 08 2011, 09:28PM

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

Yes, i use a typical Armstrong oscillator. It works like the majority of vttc's.

I used no rectification or multiplying, as in Steve's circuit i used AC in. Vacuum tubes conduct current in only one direction like a diode, so AC is fine.

I don't remember the secondary but it was around 300 turns of ~28 gauge wire. The primary was around 20 gauge magnet wire (too small, but the coil still ran

) and the feedback was whatever the secondary was. The coil specs really don't matter though, as long as each one is properly tuned. My tiny secondary (which did get hot) is proof that secondary size doesn't matter :P The grid leak resistors where around 2-5w each, and i think it equaled pretty close to 5K ohm. The red capacitors where a generous donation by a fellow coiler, but they are sprague, and ebay has many similar ceramic doorknob caps. I used mains ground. The power is low and it is not a disruptive coil so it never seemed to inject RFI into all of my electronics.

The circuit is simple yet it works very well running a Tesla secondary, but efficient operation relies on tuning. So what i suggest is try to get the parts and just build it. Even if your parts' values are way off, tuning can fix that as well. Experiment with different values and make a tap-able primary. I even used spade terminals so that i could tap easily. The parts i used where mostly junk i found from assorted thrown out appliances, so it's not like i found some perfect value. I just spent time tuning it. The primary and tank cap should be tuned to resonate at the frequency of the secondary, and the feedback should be tuned to whatever duty cycle works best. Lower duty cycles means less heating, but less power (IE thinner sparks).

As for caps, i just searched "high voltage capacitor" and the 18th item listed was:

And more caps that would work:

But, i did not filter country, so i got results from all countries.

James

Fri Apr 08 2011, 09:52PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Hey what effect did you find altering the number of turns or position of the feedback winding had? That's one aspect I haven't messed with yet.

As for getting an idea of the number of turns needed, I used a program called TeslaMap, it was only 3 dollars which seemed very reasonable. It's intended for SGTCs but is useful for others. It will tell you if it's possible to get the required inductance to match the capacitor you have with the size of the primary coil form

Arcstarter

Fri Apr 08 2011, 10:00PM

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

James wrote ...

Hey what effect did you find altering the number of turns or position of the feedback winding had? That's one aspect I haven't messed with yet.

As for getting an idea of the number of turns needed, I used a program called TeslaMap, it was only 3 dollars which seemed very reasonable. It's intended for SGTCs but is useful for others. It will tell you if it's possible to get the required inductance to match the capacitor you have with the size of the primary coil form

Well any change in the feedback either gives the tube more or less drive. That being said i don't really remember any difference if there was any, other than grid and plate heating.

This is what i use, for any coil. The formula for calculating resonance seems to be impressively accurate, for all coil sizes from my 2600 turn secondary for my drsstc to 10-100 turn secondaries for HF vttc's (plasma tweeter-esque).

PhilGood

Sat Apr 09 2011, 01:50AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Arcstarter wrote ...

But, i did not filter country, so i got results from all countries.

My bad, I was logged on ebay.fr ... :p

Many thanks for your detailed answer and coil specs

Only weird thing : seems your secondary resonant freq. is around 3 MHz (calculated)

Just gotta find all the needed parts now ...

PhilGood

Sat Apr 09 2011, 05:53PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

@Arcstarter : (and others too ofc)

The main thing that makes your design different from other VTTC's is your tinny coils and high fres (calculated around 2.8MHz with TeslaMap), maybe that's the key ...

Is the fres you found using javatc around those 2.8 MHz ?

In a previous post you said your secondary was heating ... was it really heating a lot, or were you still able to run your coil for long times ... ?

Depending on your answer, I'll either stick to your design, or build something a bit bigger.

Fact is, I'm starting with this hobby and actually have none of the needed parts at hand. That allows me to choose whatever I think will be better, thats why I want my design thoroughly thought before I start buying materials... and thats why I'm bothering you with so many questions, sorry for that

A few more ...

- what is the disc between top of the coil and topload ? Seems to be a mica disk... if I'm right, did you experienced any arcing problem on top of the secondary, or is it just to hold the breakpoint ?

- concerning primary taps, is it ok to bend the wire to insert it and crimp it into the spade terminals, or do you have to keep the wire flat and solder terminals on it ?

- are the "Made in China" 811A from eBay reliable ? Or shall I better buy a US one from rfparts ?

Thanks again for your and everyone's help

Arcstarter

Sat Apr 09 2011, 07:28PM

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

I don't mind alot of questions, however you should try editing previous post when you have more questions to ask ;-D

I forgot to factor in that the coil only ran on the positive half of the cycle, and the coil was measured at 1.4MHz with a freq meter. So it is 2.8MHz. I guess i never thought about that

. And yea, it heat quite a bit, but i could run it for a rather long time. I never got it so hot i had to turn it off. The disk is part of the topload, it is an aluminum hard drive... Don't ask why i kept it there

. I thought it looked cool, and obviously those two toploads create an E field that makes the sparks wanna come out sideways.

The primary taps can be whatever, as long as the resistance is low. I soldered them to make sure they made good contact, because i used magnet wire and if i accidentally left some insulation on it could be a bad splice.

That tube was using those cheap Chinese tubes, i got two (unmatched) of them for 24 USD minus shipping (which was around 10USD) from eBay.

And about the performance, like i said, it took a lot of tuning, and i think i could get similar performance with pretty much any secondary (within reason). Perhaps a 'sweet spot' did improve performance, but a larger secondary would have the benefit of higher Q, and the possibility to use tighter coupling.

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