Hi. What we're looking at here on my bench is a Hiller Packard 493 AC Band Traveling-wave Tube amplifier. This one is a working unit and I did a teardown with a few years back and I will provide a link in the video description down below for those who are interested. Traveling-wave tube is a kind of vacuum tube, but unlike most of the vacuum tubes that have largely become obsolete in an era which is dominated by solid-state devices, these specialty vacuum tubes are still in wide use in radars and satellites due to its high bandwidth, high gain, relatively low noise figure, and extremely high reliability.

Theoretically speaking, semiconductor devices should have higher reliability as they do not typically age over time, whereas in vacuum tubes, the filament and the cathode material are prone to failures as the H, but in high frequency and high power applications devices heat this plate dissipation and power supply reliability become more challenging and they hinder the overall reliability of power amplifier systems, especially in harsh conditions such as those found in outer space applications. But we have seen more and more Silicon Carbide, and Gallium Nitride based semiconductors used in high frequency power and high power applications over the years. Nevertheless, these microwaved vacuum tubes such as Magnetron, Klystron, and Traveling-wave Tube still have their place in high-power microwave applications. I Have always been fascinated with these devices and although I studied these in my university years, but I had never seen an actual device close up before.

so I picked up another one from eBay and this one is not working and also I had tested it with parts swapped from this working unit so that I know that the problem of this the other unit besides missing a couple of tubes is that it had a very weak travelling ways too so we can take that one apart and see the magic inside. Oh by the way, before I do the teardown. Not sure if you have ever seen a traveling wave tube amplifier in action before, so let me actually first fire it up and let's take a look and for that I plugged in this unit and how this works there. So first we need to let it on standby and after it warms up, the high voltage relay would start kit would start engaging and you should hear a click and after that we can start our measurement.

So right now let me just fire this up. You're gonna be a little bit loud. so while is a powering up, let's prepare the unit for our measurement. I'm just going to put a adapter on here.

now you're just here. You just heard the clicking noise and that's when the the power really actually engaged. To know this, a traveling wave amplifier is ready to be used and let me briefly show you the power to set up I have here. So for that, I'm going to put place this unit backward a little bit so you can see the full picture.

Okay so here is our set up and towards the top over there is a Wave Technology microwave signal generator and it's a expand signal generator. So it ranges from Seven roughly seven, Hoover's to eleven Reapers and now we're outputting about six Point Nine, Five who Hertz at Minus Thirty-one DBM And so the signal from that generator comes into the input of this microwave amplifier because as 493 A is only from four in Hertz to 80 Acres, which is sitting at the seabed. so I can only use the lower band of that 907 output and the output from this 493 A, It goes to a detector and then goes to the wave technology. Our meter and that power meter is capable of measuring up to about eighteen gig fun a frequency range.

So this is while women's is range. So what I'm expecting is that after I power on the 493 A, you should see a jump in that DBM Reading Right now I'm sitting at a minus 65 64 so let's turn it on and as you can see right now, we are sitting at minus 5 DBM So which means that - 31 DBM signal get amplified roughly 25 26 DBM and this is actually on the lower end of what this 493 A is capable of when it was new. but right now because it's a age and the emission if this efficiency of the cathode of that tube WT is not as good as when it was new. So that's why the amplification drops below 30 ppm.

But in general, this one should be capable of delivering 130 DBMS amplification. So just to double-check and now we can see that that's indeed a roughly 25 DBM increase in signal strength. So now let me actually connect the input from that wavetech generator directly to the detector just to test for reference purposes. So for that, I'm going to cut the RF power and I'm going to disconnect the sensor and I'm going to connect the output from the generator directly to the detector and as you can see, it's more or less 20-30 EBM And given that, our setup has some variations and also the cabling and so that is a pretty good reading.

So roughly speaking, we had about 25 DBMS application. From this traveling wave to the amplifier, we can also put the output signal on to the special analyzer to take a look. So for that let me up our opti a spectrum analyzer and we should be able to is getting quite noisy here, but we should be able to connect. So I'm going to connect that back to the input to the 493 eight and me selected.

First of all, this is like the First Man which is ooh - 20. So let's do a start start freaks to forgive Mr. Fogg hit and let's do the stop frequency. Okay, that's good enough so let me.

So right now, you do not see any anything on the spectrum yet because I haven't powering on. Actually, let's change the resolution that would download it. do when I hurt. Okay so now let me turn on the key double-team and I can close up the that's close to that little bit so you can see the tone that is roughly at 0 DBM And what is interesting is that when I power down the Twp you can see that the noise floor also rises.

So let me turn it off and so you can see better. So now it's off, now it's on. So this is some inherent noise from this amplifier. Anyway, So now you guys have seen this Twt in action and I'm gonna power it down and that's opened up.

The other unit that I bought was not working and we'll see what is inside now. I Suspect that this unit is much older than the the working unit and you do see there's some design differences. for example, the switch instead of turning up and down, it's moving left and right. and also this meter is faded quite a bit and from the serial number it is.

It seems that this one also is much earlier, but the construction should be the same and we will power it down. And also there's something stuck on the meter you can see actually this is not connected at all so the meter is just moving by itself anyway. So this unit is and not working as I mentioned earlier. So let's proceed to the teardown.

So now we have just open the top and we can clearly see what is inside. So how does the unit works is not that complicated. Basically we have some high voltage rectifier section this provides the voltage needed by this traveling wave tube and then we'll have a section that handles and modulation and input signal amplification. So really the core of this unit.

it's just that this tube lying down there which is our traveling wave tube which is what we gonna be taking it out and take a good look at this. So my plan for this unit is actually just to an extreme teardown and keep the parts so that I can service the other working version and I have not powered on this for a long time. so I know that this is not charged but you really need to make sure that the caps are discharged before trying to poke around in these kind of a vacuum tube devices because most of the vacuum tubes have a very high anode voltage applied being applied. And for the traveling wave tube we actually have voltage up to a couple of thousand bolts floating around.

So you really need to be very careful when working with these kinds of devices. But right now we're not connected. So a lot of these old units use this kind of a modular design, which this actually make maintenance much easier And up here you can see that we have this: three pen totes. These are the 82-68 They serve as the voltage regulator see in series and then we have these two OC - sorry Oh A - tubes.

These are the reference tubes. so what what they do is. they basically are kind of similar to the Zener diodes about. We've seen lower voltage applications except these are gas discharge tubes and here is something that you actually don't see very often is a delay relay.

So basically these are the relays for the 90 seconds startup when before the high voltage was applied and we have some adjustments. And the by the way, this unit actually misses a two of the relays so they don't work. So now let me remove the board on the left hand side. Now I just turned the whole thing around 180 degrees and this board is the modulation board.

Basically you can input a signal to modulate to microwave signal and you in essence it's just a differential amplifier implement it using vacuum tubes. So again, this is nothing spectacular here. So these are really the the two main boards of the remaining circuitry is basically just II monitoring the traveling wave tube and also the traveling wave tube itself. So let's proceed to removing the that traveling wave tube.

and for that, I'm going to okay, I'm not sure you can't read this, we're not, but it is actually quite nice that some people before me and put down all the markings on that so you can clearly see which color corresponds to which terminal. So that's very nice. But even though that information is in the manual. Okay, so now that I'll screw all these and we want you to remove the traveling wave tube.

And for those who haven't seen this before, this is the reverse side. and I need to unscrew these bolts of these screws to free the T double. But take a look at how beautiful the circuit boards are engineered here and look at all these resistors. And back here we have some transistors that the casing seems to be a gold-colored not sure if those are gold plated or it's just the color, but I think this might be actually going plated to improve the long-term reliability so it doesn't get corroded and now we have freed that probably waves - let's loosen the RF connectors so we can actually take out the tube.

And now we have freed the traveling waves tube out from its casing. And of course we're going to take this further down. but here let's take a look what can see Onsen and it's a 4 to 8 gigahertz and there's some thermal conductive material at the bottom and you can see that the voltages are actually printed on it and these are the operating voltages. So the idea is that you adjust the trimpots on the high voltage supply and various biasing circuitry to make sure that it is in line with what is printed here.

Now, each of these tubes are individually characterized so that printed out values are actually all different. But before we before I move on to take that apart, let's just take a look at this source used here and all these as you can see at the bottom are the two important ones here use. These are two Germanium transistors and this one SK 309. It's another transistor, so nowadays we can't find these such as a source anymore and these are awfully expensive to to get.

But from the date code here, one is from 73 the 12th week and the one is from 83. So this unit is at least after 83. which it's interesting because the other unit I'm pretty sure was made around 82. but the serial number of this unit is actually earlier than the other one.

so I'm not sure what the DOAs maybe they had done some repair to this I don't know and now we have this traveling-wave tube removed I Thought: Before we take that further apart, let's at least briefly review the working principle of a traveling wait tooth and the actual mechanism is pretty complex and I would strongly recommend you to read a book about it and the book I can think of. It's a Principles of Electron Tubes by Tomatis Ki and Watson published in 1965 and that book has a very detailed description and mathematics behind how these kind of things work. But in general a traveling wave tube is just a so-called velocity modulated tubes. And for the traveling wave tube, it's usually constructed with a cathode which we draw here and we have some filament here.

and basically this one generates electron. So here we'll have an opening and the electron get shot out and through the length of the tube. So we have a the tube is somewhere here and we have a target here which is our annulled. so this is our collector.

basically the voltage applied between the here that's our acceleration voltage. Now inside the tube we have a structure called a helix that's a slow wave structure and I'll explain very briefly. So this is kind of encasing the electron beam and ID At one end we have the RF coupled in so there's RF in and at the other end we have the RF coupled out and we quite a slow wave structure because when RF go through this helix, the actual speed propagation speed in this direction is much slower than the speed of light and because the RF signal has to travel a relatively longer distance. So imagine we're traveling at the speed of C which is speed of light along the helix and at the actual speed Giving from the electrons perspective, it's actually much slower than C.

So Long's we're short is by adjusting the electric field applied here and so that the electrons it is just slightly faster than the traveling speed of this RF field down the helix. That's when the energy exchange happens and you will hear the word bunching of the electrons and also the electron. The electron would transfer the energy from the electron field into the RF field and you get a amplified RF signal out. So it's really quite magic and the the mathematics behind it is quite complex, but it all boils down to a Maxwell equations for those who are studying RF Anyway, so I thought I would just give a very short primer on that.

and now let's move on to take the actual tube part. So I'm very excited to see what is part of what is inside this thing because I have never seen one before. So I think I'm going to remove and you can see that this is a sealed because they don't want you to monkey around here. So I'm going to remove the top.

Oops! remove the top screws first and we'll see what we get because I have no clue whether or not we can open it this way where I have to assemble the whole thing. Oh by the way, so I omitted the fact that when the electron beam is traveling through the tube, it has to be a focused so there would be a typically a quadrupole structure of the magnet around this tube to help focusing the electron beam. So we should see a magnet in sight or some magnets inside this traveling wave tube. and I don't think I have seen a tear down of a child in wave tube before so this could be the first of his kind.

And all these wires are silicone wires, which is quite unusual given that it was made back in the 80s. So now we have four of these screws removed. Sure, can we get me now? and I don't think so. So I think we have to further remove all the screws at the end.

Oh by the way, here we have two more. Sorry about that I'm not sure how much these tubes cost when they were nude, but it must be costing a fortune because lease has to be individually characterized and and also they don't have a volume to to get the cost down. So alright, so it seems I still have a few more schools needs to be removable for I can actually open this up so let me do that off the camera and I'll be back. When I managed to open it, it looks like I still have three more more but I right now I Just so far I have just removed the at the top to commerce and one either site and I don't see any movement yet so I'm going to keep removing.

Let's remove the 2 RF couplers and see what's inside here. and ok I just removed all the screws to this output SMA connector and you can see that inside. We have a very thick rigid coaxial connecting to the output so it looks like we have two further dissemble this unit this two parts first but I'm not sure if you can see it, that is the coaxial and we're looking at seen in there yep pipe and I just removed input and output coax the cover for the at the coax and you can see that it's nicely machined piece of aluminum and it's beautiful. Look at that and now we can see clearly the rigid coax used for this input and output.

Interestingly this one is actually pinched. I'm not sure if it's deliberate or not, doesn't look like it's deliberate I did not pinch this so I'm not sure if this would would affect the other performance anyway. but now I can see that the top pieces lose but it's still not able to totally open it. So okay so now I can see.

Oh wait, we might just be able to it here we go. Alright, so let me lift this up a little bit further and we will clean it up and come back. And I think this might be as far as I can take this apart for now because the tube inside seems to be either glued on to the the main casing or there's some or it's just extremely tight. Unfortunately, if I try to pry it out, it would shatter and the break.

But for now we can actually started appreciating what is inside now. I did not totally remove the coaxial. it seems that that coaxial is actually soldered after this coaxial being put in through these two slots. So and let me take a little zoom in here and I'm not sure if you see this so we know that the outside is well the inside.

We have some magnets. so these each section of this is actually magnetic and you can feel that it was a screwdriver. But also we have some shards of the some magnets here. so I'm not sure what the deal is and these are clearly magnets but I don't know why would they be floating around inside? So we have one piece here and we have one piece here.

Okay, there's another piece and we I think I have one more piece here. So the only thing I think of it's they did some kind of calibration afterwards and the certain magnetic field is not strong enough so they have to correct it by putting some of these magnetic pieces material in there. I'm not totally sure but that's certainly the only explanation I have as everything is meticulously put together and you can look at the machining of these beautiful casing and so this must have cost a fortune to to make. And if you look at the N on the side you will see that if I can focus here, we also have some adjustment here to move this way.

so we also have some adjustment screws. We can do some fine adjustment of the analyte presumably here. and so this section would be the section that I might earlier join here. So that section would be where the helix is okay and so the input and output would be directly coupled into and out from that helix.

a slow instructor, and other than that, this is just a vacuum tube inside and so that is a really amazing and we have some costume writing here for 62. Not sure what that means either, it's a QA number where it's some specifics that we don't know of and here also we have some writing 35 see - all right after some try and I was able to finally free this from this casing. Actually it was quite tightly fit inside. Basically I can I had to a poet from one site like this so that whole unit, the whole assembly actually comes out from this.

so it's like that now. In the meantime, I also disorder the SMA connectors so that you can see the rigid coax here. So this is actually quite compact this tube and considering that this tube would output at least one watt of RF power while the input is only one million. and so when I was the wiggling here I and I notice that this part actually has a lot of this silicon material.

Yeah, and after I removed that, I can pull this out and check this out. So this is the electric gone version of the of the Twt and it's just like any other vacuum tube. Basically inside we have filament and we have some coated Catholic material and also we have certain plates here to help forming the electron beam. Yes, I'm not actually quite certain how does this connect to whether it's a vacuum seal.

it has to be vacuum sealed towards the other end. So I wonder if I somehow broke the vacuum? but not entirely sure. The only way to find out is to further open this portion part. so let me try that to see if I can loosen this up and open it apart.

Okay, after a little bit of an investigation, it actually appears that I did manage to break this tube and which is evident. you can probably see there's a little crack on the top of the glass. So so that's why this piece came off and in order to see a little bit more I had to use a dremel to cut off one end because this is the end where the electron gun was housed in and I couldn't see anything from the sent. So I cut it off and now you can see that that actually did break off right here and basically or recently this tube.

It's like this: Okay, basically the electron beam get formed inside this electron gun and there are various voltages similar to your vacuum tubes that helps beam to focus and after that the beam enters the helix. Actually you can see portions of it right here. so this is kind of like spring you structure that's I believe a part of the helix as you can see that from also from inside this tube depends on the light. you might get a glimpse of the first few turns about a eighth of inch down into that hole.

So and as you can see that we also have this magnet material for the casing. As mentioned earlier, this magnet material basically creates a magnetic field which is a quadrature magnetic arrangement. so it's an as an S. So basically the field helps the electron beam to stay focused and racing down through this tube.

and in the meantime the RF signal gets passed in from this one end and so travels along the helix and through the down the tube. And when the electron beam as I mentioned a little earlier travels down slightly faster then RF field down the helix, then that's when the energy get transferred from the beam to the RF field and thus we get a magnified signal coming out from this end. So this tube is certainly a very precise piece of engineering and you know of course I broke it but it it was actually made a very robust. When you think about this, this is a metal tube.

Well, the tube itself probably is class, but it's encased in metal and there's no. it's very rigid so it probably can't withstand a lot of vibration and hard in harsh environment. So anyway I hope you have enjoyed the teardown and also hope you learned something new. And if you liked the video please give it a big thumbs up and do remember to subscribe share and also remember to like I will catch up with the next.