Mini Tesla Coil design

I was daydreaming about building a miniature Tesla coil at some point as a weekend project. Like an 8-10" secondary with 500-700 turns of 28 gauge magnet wire, maybe.

I thought it also might be fun to make it a hybrid design, using lil power triodes or tetrodes for primary drive, driven from an xtal oscillator or SPI-controllable oscillator like the DS1077.

I'd imagine having a bifilar-wound center-tapped primary with B+ on the tap and each side going to an LC-tuned plate circuit tank would work, yeah? Could also drive the grids through a center-tapped transformer to have negative bias on the grids for class C, and match impedances from the generator to the grid circuit.

My question is that I don't know what kind of load the secondary would present to the plate circuits in that arrangement - would you have to do something different with the secondary as well to achieve balance?

Tuning could be difficult; I guess you could match the primary resonance to the secondary resonance by dynamically adjusting the clock frequency. Or a VCO/PLL to ensure that the primary voltage and current is always in phase. Bleh this is already getting complicated.

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Most small vacuum tube Tesla Coils (VTTC) work quite well with a simple single-tube or push-pull Armstrong oscillator. Either circuit uses a tickler winding for feedback. You really don't want to use a crystal-controlled or other fixed-frequency source since the optimal self-resonant (and optimal) frequency of the secondary changes significantly with loading and proximity to nearby stuff (you included).

Sounds similar to push-pull Armstrong power oscillator circuit with a center-tapped tickler winding:

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You may want to make a small toroid-shaped topload on the secondary to protect the top of the winding and coil form from corona damage. A foil wrapped polystyrene foam toroid (craft store, for flower arrangements) should work.

Using an unfiltered FW rectified B+ supply will give you longer, sword-like sparks. Using filtered DC will give you hotter, hissing, flame-like corona/sparks.

Tuning the primary LC circuit to be "close" is usually good enough. The high Q secondary and moderate coupling (k typically between 0.2 - 0.35 for a vacuum tube-driven coil) tends to pull the primary LC circuit along for the ride.

Tune the primary to the same F frequency as the secondary. You want to operate the coil at the lowest resonant frequency for the secondary and topload. The secondary frequency can be measured by using signal generator to drive the base of the secondary, and looking for a current peak. This should be done with the secondary in situ.

The primary circuit should be independently tuned by first removing the secondary and moving it far away before tuning the primary LC circuit. Trying to tune the primary when coupled to the high Q secondary will be an exercise in futility.

Using a simple feedback winding eliminates virtually all frequency control complexity versus loading...

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Bert Hickman

Tesla coil plus capacitor is self resonant. I'm not seeing any advantage in trying to externally control that.

They're trivial to make. All you need do to drive it is throw it a low frequency square wave. Chopped mains (or battery) will do, and was the basis for all violet rays from the 1890s to 1950s.


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Great information, thanks. The topology you posted looks like a winner.

I have a box of 6N7G/GT power triodes; I wonder if they'd work for a small coil. The coke-bottle shaped ones look suitably mad-scientist.

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