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Re: Answers/Advice solicited

Hello Larry,

At 10:45 AM 7/5/99 , you wrote:
>Original Poster: Larry M Szendrei <ne1s-at-juno-dot-com> 
>
>Hi everyone!
>
>My name is Larry and I'm new to this list. I have no previous Tesla Coil
>experience, and not a lot of  knowledge in this specific area, but I do
>have considerable experience in designing & building tube-type radio
>transmitters. I am an amateur radio guy from the old school (I build my
>stuff) with callsign NE1S. For my next project, I would like to try my
>hand at a vacuum-tube (CW) fired Tesla coil, in the 600 - 1000W class. I
>have a well-stocked junquebox that can provide most, if not all, of what
>I need for the circuitry. I am thinking of a 4-400 tetrode for the
>"final." I would like the kind advice of those experienced and/or
>knowledgeble in tube coil design to provide answers and commentary to the
>following questions:

As a Tube guy I am sure you should have nearly all you need except the coils.

>
>(1) How does one choose a resonant frequency for the coil? What are the
>tradeoffs involved in this? I would think a higher-frequency coil would
>be easier to make (less wire in the secondary, smaller values for
>capacitors, RF chokes, etc.) Based on spectrum allocations, what is a
>good frequency choice to avoid incidental interference from any stray
>radiation?

I believe most use the secondary coil as the starting point, and aim for
around 100 to 600 KHz. What do you think of as a higher frequency? The
higher F (over, say 1 MHz) would only cause more potential interference
problems, I think.

>
>(2) The tube designs I see tend to use smaller wire in their secondaries
>than spark-driven coils. For example, Dr. Rzeszolask's (sp?) 6146 coil
>uses 27 AWG, and the large tube coil in "High Voltage Project Manual,"
>K.R.Scott, 1987 uses 34 AWG. Most non-tube coilers advocate using #22 or
>larger. What is driving this apparent need for smaller wire in the
>secondary of a tube coil? Is there any reason I shouldn't use #22 AWG or
>larger for a tube-coil secondary?

I have used #22, #28, and #30 gauge. The smaller the wire the easyer it is
to have a higher primary to secondary coupling. For a big coil #22 should
work fine.

>
>(3) How serious is the frequency-pulling (stability) problem when an arc
>is being drawn from the secondary? I am torn between using the 4-400 as a
>power oscillator, or as an amplifier driven by a smaller tunable
>oscillator. The former would be the simplest to build, but the latter
>would have superior frequency stability. The latter configuration would
>also have the advantage of smooth power control down to zilch simply by
>using a variable voltage supply on the screen grid of the 4-400. (whereas
>if one were to change the screen grid voltage on an oscillator, the
>oscillator would abrupty stop oscillating when the screen voltage was
>reduced to the point where the tube gain drops to unity, ignoring
>losses.)

Your losing me here, but I am not very knowledgeable regarding Radio
transmitters. I do know that the corona does indeed add to the secondary
capacitance, thus affecting the tuning, but there are others on the list
that could answer this better for you.

>
>(4) What output voltage could I expect from such a coil? 
>
>I plan on using "shunt feed" (RF choke + DC blocking cap) for the 4-400
>plate tank coil/coil primary which will keep the primary at ground
>potential for DC (safer). This way, all HV at DC and low frequency AC can
>be underneath a ground plane positioned  below the base of the coil,
>eliminating the possibility on an arc to HV DC or 60 Hz AC. 
>As you have gathered by now, I will be using rectified/filtered DC on the
>tube circuitry, as opposed to raw60 Hz AC which seems to be commonly
>done.

Please explain for me "shunt feed". Filtered DC will be more inefficient at
producing a discharge, i.e. more power per inch of spark. One of my many
current projects, is, audio modulation of the corona, so I have been
working with a filtered DC supply, but can only get about 4 inch discharge
(flame like) at about 1700 to 2000 watts. See my web page for some info on
tube coils: http://www.pond-dot-net/~davidt follow the Tesla coil link and then
the tube coil link. You will not find any info on Filtered DC coils, I an
still slowly working on it...

Regards,

David Trimmell

>
>That's all for now. Comments/feedback would be much appreciated. I'm sure
>I'll have more questions as a get further along in the project; right now
>I'm in the conceptual design phase...
>
>Thanks,
>-Larry (NE1S-at-juno-dot-com)
>
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