Powering a TC with a stereo amp

From:  Jim Lux [SMTP:jimlux-at-Earthlink-dot-net]
Sent:  Monday, June 01, 1998 1:54 PM
To:  Tesla List
Subject:  Re: Powering a TC with a stereo amp

Tesla List wrote:
> ----------
> From:  Jonathan Bazemore [SMTP:jpbazemo-at-ouray.cudenver.edu]
> Sent:  Saturday, May 30, 1998 10:52 PM
> To:  Tesla List
> Subject:  Re: Powering a TC with a stereo amp
> Dear Jean-Marc, and Tesla list Members,
>         This sounds facinating--please elaborate, if you could.  Let me see if
> I
> grasp this concept--you could use a function generator at the stereo amp
> input,
> and, at the correct frequency, get a regular-sized telsa to resonate,
> producing
> sparks at the secondary output?
>         Thanks very much for replies from experts, commentary, etc.

Sure...Think of the secondary as a LC resonant system, coupled by the
primary. If you put power into the system primary, it will wind up in
the secondary. In a "conventional" TC, you get the power by hitting
another resonant circuit (the primary L & C) with an impulse. The
"tuning" process consists of making the primary circuit resonant
frequency match that of the secondary circuit. This process is
complicated (!) by the fact that the two tuned circuits are linked and
interact quite a bit.  Part of the tuning is finding the optimum amount
of coupling between the primary and secondary. Overcouple, and all the
energy of your excitation impulse goes into the secondary, and the
primary never really "rings". Under couple, and the energy stays in the
primary circuit and is eventually dissipated as heat.  This is
complicated by the fact that the secondary tuning is not constant,
because the sparks created by the high voltage have a significant
capacitance, and cause the resonant frequency to change, which in turn,
causes the coupling to change.

If, instead, you drive the primary with a source (either a voltage or
current), you don't have to worry about the primary resonance, because
there is none. And, you can have a coupling of 1 (if you could get it),
because you aren't worried about the interaction of two tuned circuits
anymore. The more power you pump into the primary, the more power comes
out the secondary.

A tube coil uses this principle. The tube is essentially directly
driving the secondary resonant circuit as a tank, combining the idea of
an oscillator and amplifier and power driver all in one component.

The real problem is efficiency. It turns out that a spark gap is a
pretty efficient switching device in terms of power lost in the gap
compared to the power being switched. A tube coil, operating Class C, is
going to be 60-70% efficient. A Class AB stereo amplifier is going to
be, at best, 50% efficient. A spark gap scheme could easily have an
efficiency of 80-90% (although it has other problems, like exponential
decay, etc.)