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Registered Member #316
Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212
Hey, I just got an old 811a and some other parts from a hamfest and I'm working on my first VTTC. I bought a nice cap to use as my tank cap, but when I reread the data on the cap it's 20nf, not 2nf. Would I still be able to use it?
Registered Member #223
Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125
What kind of capacitor is it? Doorknob? You sould be able to but you have to design your TC around that value. Use JavaTC or another tesla calculator and play around with the secondary and primary specs until you match the primary and secondary frequency.
Registered Member #316
Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212
I'm pretty sure it's a mica capacitor. Anyway, I thought VTTC's didn't need to have a tuned primary and cap because the vacuum tube switches the current on and off at the correct frequency. Is this right, or do I have some more reading to do?
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
The tube is switched on and off by a reaction winding (grid coil), driven by a LC resonant circuit (the primary tank). Even if you use forced oscillations (signal from a separate oscillator) you need a resonant cap to get the maximum output from the coil. For low power VTTCs I would stay under 2 nF. For higher power under 10 nF. 20 nF is way too high.
Registered Member #316
Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212
Well, I got it working a little bit. with a sixteen turn grid feedback coil and 4nf of bottle caps I got sparks up to 3/4", but the plate got orange within ten seconds My secondary is a 1.75" PVC tube wound for 14" with 23awg (all I have right now) and no topload yet. I have a 3nf feedback cap and a 39kohm grid leak resistor. This seems very odd to me from what I've read about VTTC's and grid leak resistor values. I started with 6kohm but the plate heated up really fast. Then I added a 33k resistor, which made it run very nicely for a bit, but then something happened. Also, the surface of my feedback winding glows with heavy corona in places and looks kind of like it's arcing to itself. I'm pretty sure this shouldn't be happening... But at least I made some sparks
PS it would be great if someone would be willing to donate a single doorknob capacitor to the project, since I have absolutely no money left. Just thought I'd ask
Registered Member #316
Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212
I made a new feedback coil that slides easily. I started with another piece of the tube I wound the primary on, and then wrapped about three layers of masking tape on it. Then I wound one layer of plain paper on top of the tape. I wound the feedback coil on the paper, smeared the coil with glue, let it dry, and then pulled the paper out from under it. Slides very nicely. Christopher Robin, I thank you from the bottom of my heart! I love this forum and its members! In a few minutes I'll put up pics of the coil as it is now.
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VT Tesla Coil
