Re: Tube coils

[cwillis-at-xxxxxxxxxxxx (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

Hi Steve,

Tube coils are a great deal of fun and provide many challenges that are
different from disruptive coils.  My website at:

www.angelfire-dot-com/electronic/cwillis

has some info on a small 811-A tube coil as well as a larger 833-A tube
coil.  The info is by no means complete or nicely organized, but it should
give you an idea of what works for me.  I also try to explain the operating
theory of the class-C oscillator in each case.   

Almost any tube can be turned into some kind of tesla coil apparatus.  But
the favored types are going to be big ones with high voltage ratings.
833-A's are a great deal new for only about $100 at www.rfparts-dot-com.  But
used they sell for $10 to $50.  Try e-bay or a hamfest.  There are very
frequently some 833's on the e-bay scene.  811-A's, 810's, 813's, 4-500's,
the list goes on- there are lots of useable tubes out there. Triodes will
make the job easier from the standpoint of calculations and circuit design.
 Also be thinking about how to get high voltage for your tubes.  Microwave
oven parts have a reputation for doing this job.

For efficiency purposes, tesla coil oscillators are run in what is called
class-C mode.  This means that the tubes have a very small conduction angle
and depend on high-quality resonant circuits to turn that pulse of current
into a sinusoidal wave.  The grid leak components (usually a resistor and
capacitor) set the voltage on the grid that biases the tube for this mode.
An oscillator is just an amplifier that has constructive feedback.  This is
what the grid coil is for.  You don't need a grid coil to get feedback- the
Colpitts oscillator uses a capacitor divider to do this.  But by far most
tube coiling is done with the feedback grid coil tightly coupled to the
tank circuit.

Good luck!!  Let us know how it goes.

-Carl