Well nearly every part of my coil is temporary because of my lack of money. Not only that but the capacitor bank I used which was already too small is now 40% dead. My max spark length is 14" while it should be more than 24" with my power supply. I bought the wrong type of magnet wire so my secondary is to skinny and short.
Other than that, I’m happy with it.
Despite what people say, tesla coils can be made with just about anything. (Mine is a good example.)
Basically you have a power supply, capacitor bank, spark gap, primary, and secondary. The power supply charges the caps, breaks down the spark gaps and drains the caps through the primary inducing a charge in the secondary. Because the bottom of the secondary is grounded it acts like a cap to the ground. The toroid increases capacitance to the ground.
Power supply
So first just find a power supply. If you live in a city there is probably a neon sign shop around. They will normally give you a free neon sign transformer (NST) that is damaged. Try to get one that is rated at 12kv or 15kv. You can also buy one on ebay for about $20 and another $20 for shipping (they are heavy).
Secondary
Then find some PVC pipe and magnet wire, 4" dia. is the best for the PVC. The magnet wire can be just about any size from 22AWG to 28AWG. Just wrap the wire around the PVC tell you have 800-1400 turns. You will need a few pounds of the magnet wire so ebay is your best bet. After you wind it just spray on a few coats of polyurethane varnish to make in nice and shiny.
Primary
The primary can be just about anything as long as it has about 15-20 turns. Most people use copper refrigerator pipe from a hardware store. It is normally wound flat on a board. I made one like that but it only had 8 turns which was not enough so I just wound some high voltage wire vertically on a plastic container that had Chinese soup in it. It’s only temporary but it works.
Spark gap
Anything! Two nails sticking out of a board would work. Although it’s best to use more than one segment so it stays cool. You could also vacuum the spark gap. I get MUCH better performance with a vacuum on my single gap.
Capacitor bank
Ah, now for the hard part. After you build everything else download teslamap . Then just input all your specs and it will tell you the resonant cap size. You should try to make your bank that size for best performance but it’s not a requirement. Also remember that your caps have to survive the PEAK voltage of your transformer. A 15KV NST will have a peak of 21KV; it would also be a good idea to have the bank rated much higher. 30KV would be nice. You can build "saltwater caps" with clear beer bottles and saltwater or you can buy a large pulse rated cap or you can build a multi mini capacitor bank (MMC). I would suggest The MMC because it can be altered very easily. Your best bet would be here. Tesla map has a built in capacitor calculator so you know exactly how many caps in series and parallel you would need.
Toroid
Your toroid can be just about anything too. I prefer aluminum ducting in a donut shape on top of the secondary. Wielding two metal bowls together would probably work too.
Tuning
After you put everything together you have to tune it to get the maximum spark length. Basically you just play with all the parts. The primary has to be "taped" in different areas. The position of the toroid also makes a difference along with the distance of the spark gap. Just move everything around and turn it on and off seeing as to what makes a difference.
WARNING
Every part of a tesla coil circuit can be lethal! You MUST have bleed resistors across EVERY capacitor. Also the secondary of the tesla coil can build up a static charge whish is not directly dangerous unless you freak out and fall on something sharp.
Note my Signature:D
