The Quick & Dirty VTTC
Via the internet, I discovered that to my surprise and delight, the "Ionophone" referred to elsewhere on these pages is very far removed from defunct. Several versions have been produced around the world by different manufacturers. One man, Ulrich Haumann, even went so far as to build his own. He in turn inspired others to follow suit. It was from Ulrich's plans that I swiftly (and I mean in 2-3 hours and very leisurely at that) threw together the coil which appears here. First, the circuit diagram, which is a little changed since I have at present no modulation capability for the plasma:
Ulrich reported minor problems with the rf choke RFC in the above. With a little experimentation I found that the inductance of this component is not important, but that if a second Tesla coil TC is wound, this serves perfectly as the choke. This Tesla coil/choke should be say 25-40 turns of thick (0,9mm - 2,2mm) wire wound to a diameter of 25 - 35mm, the thing is completely non-critical, so long as both are the same. Apparently the self-resonance of this choke at the operating frequency performs admirably to keep the oscillations from escaping into the power supply (and no, the HT side isn't earthed to anything, I simply threw it all together breadboard fashion!) The first choke I used was 164uH and physically rather large. It can be seen lying horizontally behind the Tesla coil and valve (EL37) which are at the front of the layout. It was not resonant at the operating frequency, and though it is made of 1,6mm wire and carries only 160mA, it got warm in use! The second Tesla coil I wound, which I used in place of this choke, remains stone cold. A later photo shows this component.
At the rear left is the HT transformer, on top of which is perched the 10H choke, which is essential when using a mercury rectifier. This valve, type 83, is to the right of the HT transformer, centre rear. At the right rear with its LT centre tap sticking straight up is a homebrew filament transformer for the 83. Despite only putting out 5V at 3A, this secondary needs high voltage insulation because it forms the positive HT terminal and can be at up to 400V. In front of that, right midfield, is the LT heater transformer (6,3V) for the EL37. Left midfield is the 4uF smoothing capacitor on top of which is a very large 30k resistor. The front row shows the Tesla coil on the left, atop which is the plasma, barely visible against a steel band of the HT transformer (bad siting on my part for the photo.) Near the valve base of the EL37 to the left is a parallel pair of 4,5nF 4kV ceramic capacitors. The value isn't very important, but a high voltage rating is, as the "hot" side is connected to the loop L, seen at the top and outside of the Tesla coil as a thick brown wire. In front of the valve base is the 15k grid resistor (read it right to left - brown, green, orange) and to the right of the valve base the 0,5uF screen bypass capacitor. The white blob apparently hovering in mid air at the left shoulder of the EL37 is the screen resistor of 4,3k (actually a 3,3k resistor in series with a 1k which is out of sight).
One important feature of the circuit is that the heater/filament volts can be switched on independently of the HT volts. With a semiconductor rectifier this doesn't matter, but with a mercury rectifier it is vital, as mercury rectifiers are easily damaged electrically, something rare amongst valves. The filaments must heat for long enough to evaporate any mercury droplets from within the electrode structure before the HT volts go on, or there will be a flashover with damage to the valve. "The book" says thirty seconds. Mercury rectifiers are getting hard to find, and I give mine two minutes. Even so, I have had the occasional blue flash at switch-on, fortunately with no damage to date.
So why use them? They're expensive, delicate mechanically, delicate electrically, generate waste heat, generate interference, take up space, demand a separate and very well insulated filament transformer with a centre tap (this centre tap actually matters if you want them to last long) demand a choke coil to restrict the peak amperage drawn, require you to wait a while for the filament to heat before you can switch the main power on and when they finally give up, disposal is a real problem (which so far I have not had to face) as you obviously can't throw them in the household rubbish (though I bet that's exactly where most of them went years ago.) So why? Well, they're a lot more interesting to look at than a 1N4007. Even an exploding 1N4007 isn't half as pretty as a mercury rectifier.
The blue glow from the mercury vapour inside the two anodes of the 83 contrasts nicely with the bright orange filaments, though the filaments haven't really shown up too well in this photo. The unaided eye sees them much better.
The plasma has to be "ignited" by allowing a well-insulated screwdriver to brush the tip of the Tesla coil, the stray capacitance of the metal being enough to incite the arc to strike. On no account should a bare piece of metal be used, as not only the radio frequency power but also the dc HT positive will short to earth through the object (and you if you are holding it). On the other hand, a well-insulated object can be inserted into the plasma. Here, a glass tube (supported on a rubber tube, supported on the well-insulated screwdriver tip) is being heated to white heat by the plasma. The glass becomes electrically conductive, and a yellow rf arc is drawn to the glass, though the yellow doesn't show up too well against the brilliant white-hot glass:
Out of focus, behind the Tesla coil and at an angle is the second Tesla coil which is used as the rf choke, now replacing the very large horizontal choke shown in the previous shot. Whilst a slight glow of the EL37 heater is barely visible, these things are much better seen in the dark:
The plasma is now clearly visible as a bright white "flame" to the left. To the eye it appears blue with a green tip from the copper wire. Far right is the orange glow of the EL37 heater, and to its left a blue light from the 83, plus an orange-red blur above from the 83 filament. The blue fluorescence of the EL37 glass (due to very high vacuum in the valve) is also seen, thanks to another photo-shoot with faster film (Jessop's 200ASA). Mid-way up the coil is the bright light of a neon bulb held nearby, the gas is ionised by the rf field of the coil.
In this photo, a small filament lamp connected across a few turns of wire is being held at the top of the Tesla coil. Rf energy from the Tesla coil is then induced in the coil across the lamp and lights it, demonstrating the presence of inductively-coupled energy. The lamp used to perform another trick, until I got it too close and cooked it . . . the gas in the lamp would light up blue and it was possible, once the filament had lit (electron emission produced thermionically helps ionise the gas filling) and the gas had ignited, to raise the lamp with its coil just above the plasma flame, rotate the plane of the coil on the lamp through 90 degrees to reduce inductive coupling to near zero, and the blue glow from the gas would remain due to capacitive coupling of rf from the Tesla coil. Unfortunately, as I said, I got a bit carried away with this neat trick and it wouldn't do it for the camera! Another shot on Jessop's 200ASA.
Here's a closeup of the plasma flame, on Jessop's ASA200, the blue glow from the 83 is visible in the right background. This has been an interesting illustration of the differing sensitivities of the eye and film to different wavelengths at low levels of illumination. The plasma flame is distinctly blue to the eye, but less so to the film, also the tip of the flame is greenish, whereas the film sees it as yellow! I've also noted that weak Tesla streamers from the induction coil setup elsewhere on these pages are very blue to the film, but purple to the eye! This coil has now been dismantled and the bits recycled for other experiments. Most of my projects tend to go that way, only a very few things end up as keepers. It's interesting to note that, despite the resonant frequency of this coil being around 29Mc/s the amount of radio interference was low - nice one for the law of conservation of energy - you can either radiate it as radio waves, or use it to ionise the air and make it hot, but not both at once.
All photos Olympus OM1 plus Kodak Gold ASA100 unless otherwise stated.