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
(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...
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