New TC picture added. I increased the size of the top load, tuned the coil better, and managed to increase the spark length from 15cm to about 20cm.
On top of the toroid I put a sharp screw to act as a breakout point so I could more easily judge the length of the discharge. I then adjusted the tap point in until I got the longest sparks.
I also adjusted the spark gap spacing. Finding the optimum distance between the electrodes can be tricky. If too narrow, the capacitors don't have time to charge enough or the gap won't quench; too wide, and the transformer might burn out. I previously had a problem where the coil would run for about 30 seconds, then the gap would simply stop firing and the transformer emit a buzzing sound. Luckily no lasting damage was done. I narrowed the spark gap a little, and the problem seems to have gone away now.
## Bigger Tesla coil ##
I've already started working on my next coil. This is actually the one I started building over 10 years ago. I have the secondary coil, the transformers, and just finished building the primary coil and a TCBOR style spark gap. I just need to order better capacitors, and I'm set. I won't bother building a protection circuit for the transformer, seeing as how it would cost more than the transformer did when I got it!
The primary coil is a flat pancake style coil, made up of 14 turns of 10mm copper tube. Many people say that making the secondary is the worst part of building a TC, but I disagree. Making the secondary is merely tedious, making the primary is hard work! It is quite difficult to get the tube, especially a thick one, twisted into a smooth evenly spaced coil. It helps immensely to have a second person assisting. However, in the end it was easier than making the much smaller primary coil for the small TC, which had an inverse cone primary.
The spark gap is a TCBOR, or RQ, gap. Short pieces of copper pipe affixed to the inside wall of the PVC pipe function as electrodes. A fan attached to the end of the pipe provides cooling. I tested the transformers together with the gap (but without the fan) and they seem to work fine.
It's interesting to note that with just the transformer connected, the gap makes a soft buzzing sound, but when the capacitors are added, the sound becomes very loud. This is of course an indication that the capacitors are doing their job; instead of a continuous spark, the gap is firing and quenching dozens of times a second.