Ceramic "Doorknob" capacitors. These are primarily used as RF bypass caps in tube coils and are used for NST filtering applications in spark-gap coils.
Perfect for the tank circuit on Tube
Coils. This type of capacitor was designed to handle high RF
currents.
The second picture is of a Reconstituted Mica Capacitor. It doesn't
look like the typical Mica Capacitor but was put in here to prove a point
- Always read labels on anything that you don't immediately recognize.
There are also tell-tale signs to look for - Porcelean Insulators
are always a good sign. The raised plastic rib on these capacitors
were a good hint that they may have been designed for high voltage.
You will see a modified version of this on the Maxwell cap shown below.
These make a good companion for the Mica Transmitting Caps shown above in the tank circuit of a tube coil. They can handle large RF currents because the only dielectric is the vacuum. They are adjustable. The larger one in the photo can adjust from 10-400pF and the small one is good for about 100 pF. The crank on the bottom either raises or lowers the bottom cylindrical plate. Actually, both plates are composed of multiple meshed rings.
These are the absolute best find out there. They are perfectly suited for disruptive Tesla Coils.
THE best choice for most small hobbyist Tesla Coils. They are made from metallized polypropylene film. You want to be sure to get metallized polypropylene instead of metal foil because the metallized PP tends to "self heal" after a failure. The metal foil caps become totally useless the second the dielectric is punctured.
This was the standard hobby sized TC cap a few years ago. They work great but are big, heavy, oily, expensive, and require lots of time and skill to manufacture. No wonder MMCs have made them obsolete!
Error in txt of photo, should read "(2) at .018 uF"instead of "(2) at 0.027uF"
These are rumored to be acceptable in the tank circuit for lower voltage tube coils. I suspect corona would cause lots of problems at anything beyond several KV. They are commonly used in old tube amplifiers.
It isn't much use for Tesla Coiling but is a great toy in other ways. This is the type of monster cap that crazy people use to build coin shrinkers and exploding wire devices. (See my Pulsed Power Work)