AM modulated tesla coils

[kb3ouk]

ok so i know this is the am forum, but this is somewaht am related. I was doing some looking around today and found a bunch of vidoes on youtube of singing tesla coils. these were solid state coils, which can do 100% continous duty, not spark gap coils. after doing some more reading, i discovered the two most common ways of modulating this things is either what basically amouonts to high level modulation, similar to plate modulating a transmitter. The audio amp varies the voltage to the coil which varies the power of the coil. the other method involvess turning the coil off and on at the rate of the audio tone you are trying to reproduce. what caught my interest was that the coils (solid state or vacuum tube, not spark gap) seem to operate around the area of the longwave and lower am broadcast frequencies, which leads to my question, since all a singing solid state tesla coil amounts to is a low frequency am transmitter feeding the coil, could someone build a class e tesla coil? one of the articles i read mentioned that the reason the high level audio modulation scheme wasn't used very often was due to the low efficiency of the whole setup.

[k4kyv]

You should be able to drive a Tesla coil with any high power AM transmitter. 160m would probably be the best ham band to use, but most Tesla coils work at frequencies below the broadcast band.  It is nothing more than a high inductance tank coil, self resonant with its own internal capacitance, link coupled to the output of the transmitter.  Since there is no antenna or other load on the coil, very high voltages may be generated, and the resistive load seen by the transmitter comes mostly from losses in the coil.

[kb3ouk]

You should be able to drive a Tesla coil with any high power AM transmitter. 160m would probably be the best ham band to use, but most Tesla coils work at frequencies below the broadcast band.  It is nothing more than a high inductance tank coil, self resonant with its own internal capacitance, link coupled to the output of the transmitter.  Since there is no antenna or other load on the coil, very high voltages may be generated, and the resistive load seen by the transmitter comes mostly from losses in the coil.

i somewhat suspected you could, but wasn't sure. what i did find was that for better sounding audio, the frequency used needs to be in the megahertz range somewhere, causing the arc to just hiss, instead of buzz. what it then forms is a plasma arc, which is said to be able to produce better audio than any speaker.

[kb3ouk]

ok so i just mentioned class e tesla coils and decided to search tesla coils on ebay. this is what came up.
http://cgi.ebay.com/Solid-State-Tesla-Coil-Kit-Plasma-Speaker-/320696117114?pt=LH_DefaultDomain_0&hash=item4aaafa677a

''This is a Class E based solid state Tesla coil.  The class E switching topology allows for extremely efficient operation with little wasted energy utilizing minimal component count and smaller overall device footprint.  This switching method really lends itself to high frequency audio modulated solid state Tesla coils just like this one.   If you always wanted to build your own solid state coil but were daunted by their complexity, this kit is your opportunity to dive into the wonderful world of Tesla coils.  Utilizing the step by step videos and manuals anyone with basic soldering skills can have this Tesla Coil running in about 1 hour’s time.  This kit makes a great kit for beginners and experienced coilers alike.  It utilizes a very stable, self resonant, feedback based driver circuit paired with a Class E RF amplifier stage.

There countless experiments that can be done with this coil once it is finished.  Some examples are audio modulation of the output yielding a dramatic “plasma speaker” effect, RF energy propagation as illustrated by the igniting of gaseous tubes from a distance and even basic radio theory as can be illustrated by turning on a nearby radio when the plasma speaker function is being used.  This coil will broadcast the audio being heard from the plasma plume on it’s carrier frequency (approx 1mHz).''

[kb3ouk]

and for those who prefer tubes....
http://cgi.ebay.com/Build-High-Power-Vacuum-Tube-Tesla-Coil-PLAN-/370094478893?pt=LH_DefaultDomain_0&hash=item562b59962d

''These spectacular display devices produce arcs and sparks quite unlike the damped spark gap driven Tesla coils. Operation does not require a noisy spark gap that produces copious amounts of RFI radio frequency interference but operates efficiently at the quarter wave frequency of the secondary coil.
Vacuum tubes while large and requiring filament power do offer robust and forgiving operation and are intended as a good starter approach to solid state Tesla coils. This approach is far less temperamental than the solid state MOSFET or IGBT drivers. However once these devices are debugged and tweaked one can obtain some excellent results. Our lab FET coil generates over 6 foot sparks using only 3 kilowatts. Output frequency tracks within 5% of the resonant point using one our proprietary circuits.

The circuit of the vacuum tube device is nothing more than a high powered Hartley radio frequency oscillator tuned to the resonant frequency of the secondary coil. The circuit uses a medium powered 833A triode transmitting tube that inherently has a high grid to plate capacitance and is readily available. The plate section of the coil is wound with 1/4 inch copper tubing. The grid feed back coil is wound with #16 litz wire. The secondary is wound with #24 litz wire on a properly prepped plastic form.


The output of the oscillator is relatively closely coupled to the secondary coil designed for high Q performance and self resonant to the 1/4 wave of the oscillator frequency. The voltage distribution is now that of a quarter section with a current node at these base and voltage node at the top. Input power to the coil is raw unrectified AC at 3000 volts rms with a current of .5 amps. Peak voltage is over 4000 volts and is supplied by a conventional plate transformer being fed by a voltage adjustable variac for power output control.
We also have built coils using 304TH, 450TH and parallel 2500TH’s with a full 5000 watts of plate dissipation. This coil produced spectacular hot flaming arcs up to 8 feet with 8 kw input.

A pulse signal is also shown that controls the grid of the tube allowing a wide range of spark texture variation by changing the duty cycle and frequency.''

[kb3ouk]

ok now i found what would be the perfect experiment, build a class e tesla coil the works at 4 mhz, this one built here does that and runs 400 watts output

http://www.richieburnett.co.uk/hfsstc.html