The following is an article I wrote for the "Tesla Coil Builders Association" newsletter that publishes unique information on Nikola Tesla and his discoveries, high frequency-high voltage experimentation, and other related topics.

VACUUM TUBE TESLA COIL

This project actually began when I was in the 6th grade in 1965. I saw my first Tesla coil at our school's science fair, and the next day, I ran to the library and got a copy of the science projects book that detailed the construction of a vacuum tube Tesla coil. Eventually, the plans and the amassed parts were lost in the black hole that existed in my closet.

Many years later, while leading an archeological expedition into the boxes that my parents had so thoughtfully packed with my accumulated debris, I found those plans.. After 25 years, I decided to finish my project.

The original schematic was from Science Experimenter, titled "New and Improved Tesla Coil". It was designed to be simple in construction, low cost, and of small size. Because I wanted the device to be displayed at a science fair (here I must admit that I have a son who wants to follow in my footsteps; he wants to enter it next year), it had to be visually exciting in order to compete with the many contemporary projects to be seen at any modem fair. We built it on a wooden top, and spread the circuits out so they could be identified. Legs were added and casters, so it could be moved around in our electronics laboratory.

In the original schematics, a standard, low cost plate transformer is called for. I obtained a 2000 volt transformer direct from the engineering department at Thordarson. The familiar TV door-knob type capacitors were very expensive, as well as the two 811-A triodes called for (I paid $72 for just the tubes -- the three high voltage caps were $45). 

I stuck with the design in the prints for the construction of the coils. The primary is 18 turns of two #14 wires wound in parallel. The grid coil is 18 turns of #20 wire, and both coils are wound on a 4-1/2 inch PVC form. The secondary is about 1900 windings of #32 magnet wire on a 2 inch O.D. form, 17-1/2 inches long.

We wanted to make sure that the components were isolated from each other to prevent arcing and feedback, but also give the board a kind of antique, sci-fi type of effect, to help capture the attention of the folks that might like to ask questions and to promote the experimentation that Tesla founded. This seemed to be a perfect time for us to combine the two hobbies that we enjoy ---  Tesla research and insulator collecting.

Electrical insulator collecting has it's own following, and it is really amazing how much effort goes into the hobby in the U.S., as well as around the world. I have been collecting insulators for some years, and I own a sizable number of them. They are proudly displayed for all to see and there are also quite a few stories behind many of them. Insulators were (still are in some places) used to separate the electrical wire carrying telephone, telegraph, or line voltages and currents, from going to ground through the support poles or suspension brackets. They come in many shapes and sizes, and almost as many different colors, with the variations in their manufacture due to end use. (For further information, see the publication "Crown Jewels of the Wire", P.O. Box 1003, St. Charles, IL 60174) [Editor: Thanks for the plug, Mike.] 

From the photo on the previous page, notice the high potential terminal on the secondary. It is a rubber insulator from a very old telegraph line. The two high voltage capacitors in the power circuit are isolated on a porcelain insulator that I picked up in Palma, Spain while I was in the Navy in 1985. Referring to the schematic diagram, you see the grid circuit has a 2500 ohm 50 watt resistor paralleled to a 500 picofarad cap. I have these suspended between two glass high voltage mid-span transposition insulators for heat dissipation. There are also two old telephone insulators that are holding the ground connection cable and clamp, located on the project's left. 

The antique appearance comes from the addition of two old meters circa 1920, that I removed from an old power station being tom down. The meter on the left is an AC voltmeter, and the right is an ampmeter which incorrectly gives readings up to 180 amps when I engage the coil. This was a real surprise to a few friends who didn't understand electronics but who were impressed by the layout of the project. Mounted underneath is another electrical display as an attention getter -- a 12,000 volt neon sign transformer and a Jacob's ladder. It cranks up when the power transformer for the Tesla coil is on. It's basically for the smaller kids to see.

Next to this is an antique phase angle meter, and some day the meter will be hooked up to measure the phase shift between the grid circuit, and the plate resonant circuit, when I can figure out how to do it.

To complete the project for display at the science fair, I constructed a large poster, and listed the dangers involved when the current is on and the Tesla circuits are engaged. I purposely overstressed the high frequency field generated, but the warnings I added about not bringing any electronic equipment within 20 feet of the energized coils was a real danger. I burned up my Timex digital watch and shorted out my father-in-Iaw's digital multimeter the first few times I had the coil on. Also, a neighbor a few doors down is not real happy with me. He claims he was operating his ham radio, talking to a friend in Moscow, when the high frequency field effects shut him down. Do I believe him, or not?

If my mathematics can be believed, the coil resonated at approximately 500 kHz, with a .0025 microfarad, 10,000 volt pulse industrial capacitor across the primary. The high potential terminal can demonstrate a 10 to 12 inch discharge, with the apparatus in a darkened room. Output is around 75,000 volts.

Nikola Tesla (1856-1943) was an American inventor, a prophet of science. He is a forgotten genius, who revolutionized the science of electricity, and in a burst of invention, he created the polyphase alternating current system of motors and generators that power our world. He gave us every essential of radio, and laid the foundation for much of today' s technology. The Tesla coil project is but a shadow of Tesla' s giant Wardencliffe transmitting tower, still partially standing in New Jersey. It would have provided free electrical power around the planet had it been completed. 

As an interesting footnote, when Tesla died, the F.B.I. removed all of his diagrams and schematics from his office safe. The United States government is even now using his original research in the generation of high voltage plasma fields for nuclear fusion reactors. (If you have further interest, read "Tesla, Man Out Of Time", by Margaret Cheney)