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Tuning a VTTC primary - Is a variable capacitor a good idea?

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LAN_403

Cool Dude Clem

Tue Feb 10 2015, 03:30AM

Registered Member #53981 Joined: Tue Dec 16 2014, 02:23AM
Location:
Posts: 7

Recently I built a new VTTC, I'm also a little new to the whole VTTC thing, I built a few in the past but this is the only one that's given me any good output. I'm using a doubled MOT as the HV supply, a GU-81 pentode, a 60W light bulb with 3nf capacitor across it as my grid leak, I'm not sure how many turns my secondary is (something like 300-500 turns on a paper towel tube (about 30 cm long)), my feedback is about 12 turns of single strand (telephone) wire wound onto the bottom of the secondary, and my primary is about 44 turns of regular insulated wire, wound onto a 10cm form. The trouble is that I'm a little concerned about what I'm using to tune the primary. Since I've had no luck with using a fixed capacitor and a primary with multiple taps, I decided to do something insane and get an air-gap tuning capacitor from an old junked radio, submerge it in oil and use that. It does work and I can tune the primary to where it is needed, and even running off a doubled MOT I haven't had any arcing across the capacitor plates yet, but is this wise? Or should I find the ideal capacitance and invest in some pulse-rated caps if I ever find any (here in the UK they seem impossible to find)?

I'll add some pictures to give you a better idea of the specs and stuff:

The whole thing: (BTW the primary was wound from 2 pieces of wire joined together)

As much output as I dare to tune it (I could tune it a little better to get more output, but I think that might destroy my secondary)

The tuning capacitor:

and finally the feedback:

I would include a schematic, but it's still in my head and I haven't put it down on paper yet. anyway it IS the same standard VTTC schematic we've all seen like 100 times already!

TwirlyWhirly555

Tue Feb 10 2015, 06:53AM

Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122

Regarding the capapcitors I don't know much about variable capacitors in TC use but you can get ceramic RF transmitting caps from eBay that would probably perform better and they don't cost to much .

For my VTTC I used one 1.5nF capacitor with my 811a and tuned the primary by adding / removing turns .

The red thing just behind the secondary is the primary tank capacitor .

SAM_1012 by TwirlyWhirly555, on Flickr

GrantX

Mon Feb 16 2015, 09:24AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

I think I've seen another VTTC system on Youtube that also used one of those open air variable capacitors. As far as performance goes, the oil submerged cap should provide fairly low losses. A large RF ceramic capacitor and a coil with many taps (several per turn) could be even better. Before investing in an expensive capacitor, I would try and shorten all the connections and use the heaviest gauge cable possible, that will help lower system losses even more than switching from oil to ceramic dielectric.

The only thing that worries me about the variable capacitor is the amount of insulation between the plates and the dial. I hope you don't adjust the capacitor while the MOT is energized! :)

By the way, I enjoy your youtube videos. It seems like the majority of people who upload HV and Tesla coil videos end up on this forum eventually. It really is a small world.

Perezx

Wed Mar 18 2015, 08:14AM

Registered Member #54263 Joined: Thu Jan 15 2015, 09:54AM
Location: Perth
Posts: 35

The best idea is to find a resonant frequency using this oil cap, then get it out of the VTTC and measure its capacity. So you will know which cap you should buy.

Bud

Fri Oct 16 2015, 06:36PM

Registered Member #54930 Joined: Tue Apr 28 2015, 01:38PM
Location: pittsfield ohio
Posts: 25

I have built and rebuilt the same 833 VTTC many times. Shortening the secondary to find the optimum sword like sparks. The best way to tune is to find the resonant freq. of the secondary using a scope and signal generator. The seconday cannot be close to the primary for this test! It must be away by several feet!

Once the freq. is found, then find a cap you think you may want to try and combine with the primay circuit and tune the primary by itself by changing the taps. This is the easiest method that I have found. Once these two match freq. you are ready to go. One of the problems you may run into is not having enough turns on the primary.......this I found the hard way. To change taps I cut away just enough of the insulation of the primary wire to solder on some short taps every turn or so just so they do not all line up in a row. They stick out sideways. This allows very easy and quick freq. finding with any cap.

I was just down to the midohio teslathon and Roger Smith had around 8 VTTC's there and they all had this tapping arrangement. It works well and allows you to change caps at any time. The connection from the cap to the tap can be as simple as a alligator clip to allow quick changing of the taps. Try it and you'll be happy.

Bud

Dr. Drone

Sun Oct 18 2015, 07:22AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

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