This is a very unusual tube. It's what i call the m tube and it's made of clear glass and it's got a fill that is composed of neon gas, but with mercury in it as well. Now, normally neon tubes are either just purely the name gas, which makes a clear tube light, bright red or it's uh, either neon and mercury or argon and mercury, or a mixture of both, but with mercury. The mercury vapor is important, but and the mercury vapor gives off a lot of ultraviolet light and it gives off some visible light, not much visible light, which is why this tube isn't very bright.

However, what they normally do is they coat the inside of the tube with phosphor the mercury vapor inside the ultraviolet wavelength stimulate phosphorus, and it makes for bright neon tubes and fluorescent tubes in this case, neon. What's unusual about this, one is that i'm running on dc and it's i got a neon gas fill with a droplet america. You can see little beads american a little bit of staining as well, but it's also got this little dip here. The dip is why i call it the m tube.

It's got that little sort of m bit at the top, and that is designed to trap the mercury, because when you run a tube in dc, the mercury gradually migrates from the positive electrode over to the negative electrode. It's very slow i'd like to have shown you this much further up the orange tongue here, but i can't because uh it takes a while eve when you've had it off for a while. The mercury vapor settles back down and it happens an absolutely microscopic level that just gradually migrates up. I also have to be careful not to bump it there's tiny little beads of mercury here.

If one of those finds its way down, the red will disappear and it will just go into a completely blue tube. But when, when it's been up for a while, it does go completely red at this side and then transitions with this little tongue into blue. At the other side - and that is because it's basically separating the mercury out in the tube with dc, i suppose in a way migration of the mercury. Now i could give you a big long story about this: how it came about making one of these in the first place, but i'm going to put that down in the description, because you guys want to see the circuitry and that's exactly what we're going to do.

So i'm going to carefully tilt this back because it does operate at 240 volts, which is bumped up rapidly to a much higher voltage, and i'm going to show you what's going on in here, um i may have to focus down onto here. I shall focus down onto here. What we have in here is a voltage multiplier and that converts the 240 volts coming on here into a fairly beefy supply, based on a couple of electrolytic capacitors at the bottom. But then it's got the blue capacitors behind that are for uh, multiplying that up to actually ignite the tube with high voltage and the tube is simply in this case the circuit board slots into this case and uh the tube the electrodes just fold over and tack Onto some connections at back, so i shall show you the schematic and i'll put this out the way before i stick my fingers in the back of it.

So here is the schematic which is going to be horribly out of focus, but that's okay. We can fix that. I could also take the exposure off i'm not going to take the exposure off because that will just make it go very dull, very quickly. I'm going to put this down carefully, i'm going to leave it on, though precarious.

Oh, oh, just balance. This high voltage thing off camera right. What should we zoom down a bit? Let's zoom down just a little bit. This is probably swamping out horribly, but that's okay.

I could tame it down, but it would it. I just know: what's going to happen, it's going to go extreme tame down. Then it's going to look a bit odd right. I don't have a pen here.

I shall use a screwdriver uh. I could have done with a pen i'm going to go and grab a pen. Just give me a second i'm gon na grab that pen bear with me. I have a wireless mic phone on.

You can follow me as i run around the house. Looking for pens, i have found the pen return back to the workshop yeah. That makes a change having a microphone that can go with me asunder, so the circuitry is based on a dual voltage multiplier. The reason it's using a dual multiplier is because it's more efficient that way - and it's got one one microfarad electrolytic capacitor at the bottom - and then a series of 10 nano farad capacitors just to boost the voltage up to strike the tube and effectively the use of Two electrolytic plasters, the fact i've got a positive side and a negative side means that this capacitor is charged up to a peak of say about 300 or so volts, and this one's negative, 300 or so volt.

So it gives about 600 volts, which is enough to sustain a short length of tubing, typically about 2 feet or twelve millimeter tubing and the way the multiplier works. If this end goes positive - and this end goes negative, current will flow through this diode and charge. This capacitor up, then this n goes negative and this end was positive, because this is in the ac of the mains and because that was already charged about say, 350 volts on the uk mains. This rises up to another 350 volts and it pushes the current through this diode and charges this capacitor up, because this end is negative and that end is receptive to the voltage being pushed up.

So that ends up with double the voltage across it and it keeps doing that it pushes up adding that doubled voltage each time. So you end up with quite a high strike voltage that can break down the uh, the electrode to gas barrier, the electrode. What's the best way to describe that cold cathode barrier, not really sure the best way to describe that i'm just spotted i've got the flash on the camera, it doesn't really matter, but it breaks down the uh, electrode drop and uh strikes the tube and the tube Will then be sustained? There are two 4.7 000 ohm resistors in series, quite high power ones. They don't get too hot and the reason for those.

If, without them, it would just instantly try and dump all that current through the tube and just make like popping noises and flash this limits, the current but uh in doing so, it drops a small amount of the voltage and dissipates a bit of heat. This little resistor here is 33 ohms. I just added that because it felt like you should um and basically that's it now. This is designed to operate on roughly 240 volts.

If you swap this capacitor here out for uh, say a one microfarad capacitor or two microfarad capacitor, but not polarized, then you can go you can it will multiply it up and the original power supply this is based on, did actually have the one microfarad and then It had a film capacitor there and it drove much much longer lengths of tube because once you've broken over, this voltage drop across the electrode. The actual drop through the gas isn't too bad and i think it drove five or six feet i'll say feet because, instead of meters, because i even though i've said it's a 12 millimeter trip just because they tend to measure neon tubes in feet, because it's an Old technology, but that is it it's a very simple circuit um, when you initially power a tube up a new tube with this circuit, the tube may not quite strike easily at first, it seems to take a while to get conditioned and that if you touch the Tube, it will help it strike just by capacitive coupling to ground, but it will flicker initially, but then it will gradually stabilize and then it will work and the power consumption is very low. Things worthy of note. The positive end stays called the negative end.

The cathode gets hot, it's doing all the work um and also the mercury migrates in that direction. It goes from the positive to the negative, so if you're actually making these tubes up and they're likely, you know if they're going to be in long-term use. If you mounted the tube vertically, for instance, it would make sense to have the bottom positive and the top negative, because, as the mercury migrates up, it will just drop back down to the other end. Naturally, um other versions of this been made for picture frame lights.

Uh, where there was actually name tubing, pictures that used this multiplier a circuit and they actually had a switch. A voltage selector switch that actually swapped the polarity, but they were kind of pushing these components for the the rated design voltage in this. But that is it. How to use a capacitive multiplier to drive a short length of neon tube? I think i've featured this in the past, but it's worth featuring again and the unusual effects you can get uh when you actually apply it to a tube with neon with the mercury in it where the actual migration of the mercury can be seen as a the Red neon, if this was an argan mercury tube, it would just be going pretty dark.

At this end, it would go a deep properly colour uh, but the mercury would then dominate later on but interesting stuff. It's a very neat circuit. I've had a lot of pleasure. Making a lot of variations in this, i'm just trying to think i've got another one knocking about here somewhere that i could show you where i could show you, but i've just propped the knee in on it.

Just give me a second, this version a smaller case, i'll just unwrap the cable. I actually spread the circuitry across either side of the tube, instead of just being up the middle and also added an extra stage just to encourage it to strike easier, but read the description down below, because the story of the way i discovered this circuit is actually Quite interesting um and it's led on to many variants since, but that is it uh how to drive a neon, tube or well an argan tube, not pure neem. It doesn't work with pure knee. It tends to flash or work initially and then just not work it.

It needs the mercury but how to drive it from just basic circuitry on a circuit board with no transformers.