Re: Answers/Advice solicited

to: Larry

A coil with smaller dia wire (28-30 AWG) provides a better impedance match
for a tube type coil than the larger dia wire, hence, better output.
550-750 kHz is a good operating freq for a vacuum tube coil.  Expect 20%
less output with a DC type coil over an AC xmfr driven coil (rule of thumb).



-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Monday, July 05, 1999 8:30 PM
Subject: Answers/Advice solicited

>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:
>(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
>(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?
>(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
>(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
>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...
>-Larry (NE1S-at-juno-dot-com)
>Get the Internet just the way you want it.
>Free software, free e-mail, and free Internet access for a month!
>Try Juno Web: http://dl.www.juno-dot-com/dynoget/tagj.