Playing with Cathode Ray Tubes
Hello and welcome back. If you follow the channel, you know that we just restored a bunch of high voltage supplies, and used them to light up an old CRT for our Xerox Alto monitor and find its color - it was a glorious amber. But I have a whole bunch of other CRTs in my collection, waiting to be played with. Some do light up by simply putting them in front of a UV light, but some do not light up at all. Let’s hook them up properly and see if we can make them work as intended. One note of caution. This video shows us dealing with very high voltages.
Such voltages are dangerous and can kill you. We make it look like it’s easy and we are casual about it, but believe me, it’s neither. We took many safety precautions that are not obvious in the footage. So do not try to replicate this at home. Hello! Today we're back in the lab for more tube mayhem, because we have TubeTime. And we are going to try to light up more of my CRT tubes, which are actually oscilloscope tubes. Now I have the correct high voltage cable, isolated to 10 kilovolts. I figured out that these aren't BNCs, they are MHVs. They look like BNCs except they have a bigger
ceramic protection, longer ceramic tube, on both the male and the female. So with that we are a little bit equipped to attempt a better hookup. So what tube do we have? [Eric] This is the tube out of the Tektronix 527 oscilloscope. [Marc] And it's called actually a T527 or something like that. [Marc] And how does it work? [Eric] Well, we have a little heater in here. So it's just the filament that lights up. There's a cathode, and it generates an electron beam that comes through here, and is attracted to the deflection plates and some other electrodes up here at the front.
And then when it hits the screen, it lights up the phosphor, and we see that as a spot on the screen. [Marc] An indeed, the basic principle of a Cathode Ray Tube is relatively simple. But to make them work in practice, a lot of electrodes have to be added, and it gets fairly complicated. The various electrodes are easier to explain if we follow the historical development path that led from the initial discovery of cathode rays to a CRT like this one, suitable for making an oscilloscope or a TV screen. Which of course will go well with some elevator music. It all started with Heinrich Geissler, a german physicist, who invented the gas discharge tube in 1857. Geissler put various gases at extremely low pressures in sealed glass tubes, with electrodes at either end. When submitted to a very high voltage from a Tesla coil,
the rarefied gas in the tube would ignite and start to emit a pretty glow. This is eventually became the neon tube as we know it. They were popular and used for decorative purposes. The next step occurred when English physicist William Crookes got intrigued that a small region around the cathode stayed dark, and that the dark region would grow as he further lowered the vacuum, up to the point were it extended through most of the tube. By coincidence, he had used a soft German glass for making his tube, and that glass happened to be phosphorescent.
He observed that when the residual gas pressure was very low and the tube mostly dark, the end face of the glass tube itself started to glow green, as if the glass was hit by something. He placed a metal mask in the path, and a shadow image of it would appear on the glass. He concluded that some mysterious rays, which he thought were of molecular nature, were traveling in straight lines through the tube. Since the rays emanated from the negative cathode, they were soon called cathode rays.
He also found that putting a magnet near the tube would bend the rays up or down. There was something electro-magnetic to these rays, and it was hotly debated what their nature could be. It took another Englishman, Nobel Price winner Professor J.J. Thomson, to find out what these mysterious cathode rays were made of. But first, he had to make an even better tube, one with a near perfect vacuum thus eliminating potential collisions between the rays and residual molecules in the tube: today we would say extending the mean free path. One of his most critical experiment was to show that he could deflect the rays between two charged electrodes within the tube, something that his predecessors had failed to achieve, because the vacuum in their tubes was insufficient. After a series of experiments involving many complicated tubes,
he correctly inferred that the rays were made of a previously unknown, negatively charged particle. And he did not stop there. He was able to estimate the size, mass and charge of the mysterious particle. He once again correctly concluded that the particle carried a fundamental unit of negative charge, and was at least 1000 times lighter and smaller than the smallest particle known at the time, the hydrogen atom. He then took the final giant leap, and posited that this must not be an
atom, but a sub-part of it. The year was 1897, and J.J. Thomson had discovered the first sub-atomic particle, the electron. And while he was at it, he started the field of what we now call electronics. Which eventually resulted in this YouTube channel. Thank you J.J. Thomson. The next major step is attributed to German Nobel Laureate Ferdinand Braun. He added an
extra acceleration anode with a small hole in its center to produce a small, well defined beam. He also made the tube extend past the beam shaping anode, and inserted a phosphor covered mica plate, that acted as a proper screen in this front section. With that, Braun had a usable CRT and demonstrated the first oscilloscope. Yet another German scientist, Arthur Wehnelt, would give us the next two critical inventions that made the CRT practical. All the tubes we discussed so far were cold cathode tubes.
Hot platinum filaments were known to be better emitters of electrons, but Wehnelt discovered that if he coated them with barium oxide instead, his hot cathode would emit copious electrons at a dramatically lower anode voltage. Furthermore, he added another electrode around the cathode, known as the Wehnelt cylinder, or sometimes called the grid. This cap-like electrode controls the beam intensity, and is also the first part of an electron lens which we’ll discuss later.
He now had the prototype of a modern CRT, from which our oscilloscope CRT is a descendant. The only major difference is that our hot cathode is separated in two parts, a barium coated electrode that emits the electrons, and a separate filament that heats it. This is called indirect heating. But other than that, it’s pretty similar. So now that you have been thoroughly educated, you should be able to follow TubeTime’s explanation of the electrodes.
[Marc] Right, so in reality it's a little bit more complicated. So I colored the electrodes to show the voltages. But there is a bunch of low voltage electrodes, a bunch of high voltage electrodes, and some in the middle. So, TubeyTimey, tell us which electrodes we have. [Eric] So over here, we've got some low voltage electrodes, which is our filament, our cathode, and we have our control grid which is for controlling the intensity of the beam. [Marc] So that's basically the generation of the electrons, the electron gun, right? [Eric] Generating electrons. And then up here we've got high voltage electrodes,
and that accelerates that electron beam. And then there's also some focusing grids here as well, that shape what that beam looks like. [Marc] Right. And in the accelerating electrodes, there is the first anode, which is right after the gun, which is already at 4 kilovolts. Here, actually there are two of them. And then all the deflection stuff that's over there, everything
that's in the later part of the tube, has also to be at the high voltage. So that means the two sets of deflection electrodes, the interior of the tube which is coated with some conductive stuff... [Eric] Right, the Aquadag coating. [Marc] ... and then there's an extra silly electrode here, that's also at high voltage, that prevents the two deflections from interacting. And we will leave the other ones for focusing later.
Okay, and we have all our supplies ready! However, there is a catch. If you look at the schematics, they didn't do it the way you'd think, with zero volts over here and 4 kV over here. They drove this at close to zero, and put the blue side at - 4kV. So that's what we are going to do here, we are going to get the -4kV (here). It is what they did from the schematics, we got that from the scope itself. And then the forward portion of the tube will be at around zero volts, and it will be easy to modulate and move the move the beam around.
So, on the setup made by Mr. TubeTime, we have the filament. And then there's another catch: we need an isolation transformer. And the reason is, is that now we are going to put our cathode at minus -4kV and the filament needs to be at -4kV too, or it will arc. So what we have done,
I have purchased this little isolator. It's 6.3 volt in 6.3 volt out, but it has 5kV of isolation. This guy takes 5 000 volts of isolation, so I can have one side grounded, and this side, one of the legs will be to our very low -3.8kV voltage. And on the plus side of the power supply, which is now our ground, we have all the other orange electrodes: basically all the anodes. [Eric] Yes. [Marc] So, I have not turned it on. You have, so go for it. I haven't seen it yet, so...
[Eric] Let's turn on the filament first. [Marc] Yep. [Eric] So the filament is on. [Marc] It's back there. Yeah, yeah, I can see. Okay, so the filament is going. [Marc] Let's put some acceleration voltage. Not the full
3.8 kV actually, just... [Eric] 1500 volts! [Marc] 1500, that's enough? Oh, it sure is enough! And there we go! Wow, piece of cake huh? What's the next step? Try to focus it, or try to move it? [Eric] Let's try to move it first. [Marc] Okay! So, to move them, we connect to the deflection plates, which are these little pins that come through the glass on the outside. And here, I have two of my amplifiers. And then we have function generators on each. And since it's now at ground, we just connect one straight to the other, right, that's that's the whole beauty of the thing.
[Eric] Well, let's turn it on! And we have deflection! [Marc] Oh, that's a piece of cake. Yeah, so we're going about + 60V to - 30V, something like that. And of course we are helped that we use a little bit less acceleration voltage, it makes it more sensitive. You have the same on the vertical? [Eric] Vertical? And here's vertical. [Marc] And we have a bouncing ball.
And, same thing, this goes -20V... Oh, actually, this one is more sensitive. It goes -20V to +20V. All right, so the last thing we have to do is to shape the beam then? [Eric] That's right. So we have this de-focussed spot, and we need to sharpen that up to make a dot. [Marc] Right. And that's where we start to touch our middle electrodes. There's one green and one yellow. And the green is the focus, it's at mid voltage, about 500 to 900 volts above the cathode. And then, this one has an extra electrode that not all tubes have, which is called astigmatism.
Man, so many electrodes in a semi modern electron tube! This electron gun from a dual beam oscilloscope tube should allow us to see them better. [Eric] That is the grid. [Marc] Right. [Eric] Inside, where you can't see it, is the cathode and the filament. [Marc] So that's these three.
This doesn't exist in this tube, but instead we have this doohickey. Just as an aside: this weird split anode is because this is a dual beam tube. In our tube, we have another bizarre first anode for blanking. But let's ignore those details for now.
[eric] And then after that we have an anode. [Marc] Anode! [Eric] And then the focusing connection and the astigmatism connection. [Marc] Right. And then each of the guns has its vertical plate: there is one here, there is one there for the other gun. That's the isolation plate from over here, I think. This guy? [Eric] Yeah, between vertical and horizontal.
[Marc] And that is the single horizontal deflection for the two beams. Because it's a scope, it's a specialized instrument. [Eric] Right, so both traces always move together horizontally. [Marc] Right.
[Marc] OK, we’ve finally made it to our last electrodes, the focusing assembly. It consists of a low voltage cylindrical electrode between two anodes at the high acceleration voltage. This arrangement creates potential lines within the tube that very much look like a lens, like this diagram shows. In this diagram, the focusing electrode is anode 2. Note that the Wehnelt or grid is also important, as it focuses the electron beam at the output of the gun. The electron lens
then images that tight focus onto the screen, like an optical objective would do in a camera. So that would be the focus. Okay, I've written it on there. And that's the extra electrode. We had it wired to nothing, right? Okay, with feeling and focus.
Okay, focus. [Eric] Now we have a shape! [Marc] It's better, it's not super good. [Eric] So I'm trying to adjust it right now. But I can't quite turn it into a round dot. [Marc] Were you able to focus it before? [Eric] No, I haven't tried this yet. [Marc] Okay, so this is all new. All right!
So before we focus it too much, we are going to do the control of the intensity. Which will be that light blue electrode here, which needs to be 20 volts below, 20 to 100 volts below the cathode. So it's the most negative point in everything actually. Go for it. Yeah, it did work. [Eric] Now we have brightness control.
[Marc] Can you turn it up and down. [Eric] Brighter. Dimmer. Now it's gone. [Marc] All right, so now we have our intensity under control. And I was arguing that we should run it at - we can go up to 2.9kV, right? [Eric] Here is 2 000 volts. [Marc] Okay.
[Eric] Let me try to fix that focus. [Marc] Yeah, it's not doing so well. [Eric] There we go, 400 Volts.
[Marc] I wonder if, are we seeing some... Is the beam moving around in a loop? Are we seeing some noise somewhere? [Eric] Maybe it's from the power supplies? [Marc] No. [Eric] So it's not from the deflection. It's coming in somewhere else.
It could be these cables. [Marc] No... Careful, it's powered up. [Eric] Yeah, it's near zero volts though. It's not the cathode. As long as I stay away from the cathode. [Marc] Oh ho, that makes a big difference.
Aha! So I think you're focused, but there's some leakage of something in the electrodes. All right, TubeTime's magic fingers. [Eric] Wow, it's detecting all the magic! [Marc] Oh you know what, we don't have the mu shield around the thing.
Right? This tube should be completely shielded in something. [Eric] There we go. [Marc] Yeah, oh, and you can see what's going around right. There you go. Go faster! There we have it. Uh, give me a sine wave. There you go.
It's the other one there. There we have it. All right! Oh, we finally have made an oscilloscope! Ooh! Aah! Can we see the ripple or whatever? [Eric] Yeah, I can see the ripple. [Marc] Oh yeah, there you go, you can see that it's still doing it.
[Eric] That looks like 60 hertz to me. [Marc] Yeah, it might be just that transformer right here. [Eric] Now we can figure it out. We have our own oscilloscope, right? [Marc] Right! [Eric] So let's turn this off.
[Marc] how to make an oscilloscope with lots of HP equipment. There you go! [Eric] So that's a little more than a complete cycle. And we are at 30-ish Hertz. So yeah, this is 60 cycle interference that we're seeing. [Marc] Okay, so we're just seeing a transformer from somewhere.
My bet is that it's this one. So we would need to shield the tube. Yeah, here's our new invention in the vertical oscilloscope! [Eric] Haha! There is a lot of noise in there. [Marc] Yeah [Eric] It's gotta be the 60 Hertz.
[Marc] And that's a problem with tubes: you see every defect. [Eric] Every defect comes through, if the power supply has noise. [Marc] Everything comes through it. [Eric] And if I set the frequency to a multiple of 60 Hertz, then we should be able to see a clean trace. [Marc] Ah, very smart! [Eric] Incidentally, this is why NTSC uses 60 frames or 60 fields per second.
[Marc] Let's do 60 hertz. There we go. There that's clean. [Eric] Oh, that's pretty good. [Marc] Yeah! So now we are very near 60 Hz, and you can you can tell that the noise we're seeing is 60 Hz from somewhere. Ain't it cool? And, Eric, while we were working on the other video over there, making nice exponential plots, you've lit another tube.
[Eric] That's a small one [Marc] It's a teeny tiny guy. Okay, well, this is good! Because I thought that tube was dead, because it had so much, it has such of a blemish in the middle. [Eric] It's got a lot of burn-in. [Marc] Yeah, if you look at the middle, right here, it has this huge line. Because that's what it used to display. It's for the CV-89. But it's plenty fine!
And how much voltage do you need for that one? [Eric] Uh, this is 2000 volts right now. We can actually decrease it to 1000, and experiment with a higher deflection factor. [Marc] Oh, but then then you see the burn-in. Yeah, you can see the burning in the screen. We also tried our bigger round tube, also from a Tek scope, but this one requires a post acceleration voltage of 10 kV, which we do not have yet.
Without the post acceleration connected, the spot was too dim. So, to be revisited later. And for the grand finale, Eric brought his triple scope clock that he built from scratch a few years ago as an exercise: his own power supply, deflection circuits and software design. You get it, we love CRTs. I hope you do too. See you in the next episode!