Re: Vacuum Tube Tesla Coil Design (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

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Date: Fri, 05 Oct 2007 13:41:35 +0200
From: Martin Damev <mdamev@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Re: Vacuum Tube Tesla Coil Design (fwd)

Hello Brad,

Some two weeks ago you posted something about
building a VTTC with your 4-400 tetrodes.

There is a very interesting and efficient oscillator
circuit you could use for your VTTC. If you are
interested I will help you with the necessary calculations.
I'm talking of the  "push-pull oscillator with capacitive
dividers for feedback". This design uses two 4-400 tetrodes
but in return you get approx. twice the RF output.
Using capacitive dividers for feedback you can avoid
the "tickler coil" and you get better driving conditions for
the tetrodes. The circuit may look  more complex at first
glance, but it is not more difficult to build. Basically it
is similar to an astable multivibrator circuit.

I designed such a circuit for a coiler-friend. This small
but very nice VTTC uses two 7843 tetrodes by RCA that we
pulled out of some military communication equipment.
These small tetrodes are metal-ceramic design and conduction
cooled. The VTTC works very well though  it will need larger
primary capacitance to take full advantage of the tubes capabilities.

I personally do not recommend using tetrodes as triodes by
connecting both grids together. First you can be sure that you will
easily exceed the control-grid's dissipation rating in most cases
(especially when the oscillator is not loaded). Second a tetrode
allows you to pulse the oscillator through the screen grid voltage.
John Freau has shown that there is a lot of potential in using this
method.

Two 4-400s should give you 25-30 inch sparks in a good an optimized
setup.

- Martin