How A Tube Works

hi there and welcome to another tech tips Tuesday in this episode are going to take a look at the vacuum tube and how it works so we'll take a look at the diode the triode the tetrode and the pentode so let's get started the first tube that we're going to take a look at is the diode die meaning - ode meaning electrode - electrodes this is the symbol for a diode vacuum tube that's indirectly heated this is the symbol for a diode vacuum tube that's directly heated and I'll explain this here in just a moment so whenever you look at a vacuum tube you want to remember this the upside-down T is the plate or the anode okay so you want to remember that this is the cathode it looks like the letter C turned kind of like this all right the cathode on the data sheets is marked with a K you'd think they use a C but it's a K same with this plate now these are the filaments inside this tube because this is a directly heated vacuum tube but the filaments are the cathode so these are usually marked with h4 heaters but they are the cathode as well now the difference between these two diodes is this is an indirectly heated tube so this means that there's a filament structure inside of a pipe inside that vacuum tube and the filament structure has to heat the pipe up first before electrons will flow so they the turn on time for these tubes or you know the time to when they actually start to work is between 12 10 and 20 seconds or something like that usually for a directly heated vacuum tube it's usually about three seconds or so and they're ready to use now directly heated vacuum tubes are not only dials or you know trials and tetras and pen toads as well you have a directly heated vacuum tube in your house right now if you have a microwave oven the magnetron tube inside your microwave oven is a directly heated tube so when you put a piece of food or whatever in your microwave oven and you turn it on you know you usually hear the fans start and it starts to hum now if you listen very carefully to that microwave oven in about three seconds after you turn it on you'll kind of hear the microwave oven sound like it's bogging a little bit you'll hear you know the tone go down that's because the filament structure inside the magnetron tube is heating up and it's you know obviously starting to draw our current that's how that works so most houses that have you know microwave ovens obviously have a vacuum tube in them so an indirectly heated vacuum tube is used for many many different things they're all different shapes and sizes of actual tubes they're used as detectors and and rectifiers and DC restores and all sorts of different kinds of things in in radios and televisions and all sorts of different kinds of test gear this kind of tube here is usually only used as a rectifier yeah there are some cases where they use them for other things as well but a directly heated rectifier tube is you know power supply stuff usually that's the most common place that you're going to encounter something like this a good example of a directly heated rectifier tube would be a 5 u 4 5 r 4 5 y 3 5 y 4 even in the old Rogers radios the 2 X threes things like that a 2 X 3 would actually look just like this if you're to look at the schematic symbol of a 2 X 3 a 5 y 4 has two plates so that you can use it as a full wave rectifier this would be considered half wave right here you'd be using this and a half wave situation ok so keep in mind the upside-down t is the plate that's one you want to always remember that and this see that you see is kind of turned like this is the cathode all right very important things to remember now the electrons flow from the hot surface to a positively charged surface all right so which is the plate so the anode is has a positive charge on it and the electrons will flow from the cathode to the plate inside of a vacuum tube which brings me to a conventional flow and electron flow I've had a few people mention I see that you use conventional flow and of course vacuum tubes and you know circuitry uses electron flow and things like that whenever you're working with a schematic it beneficial for you to think with the conventional flow because that's pretty much the way everything is drawn and it makes things quite a bit easier to think about especially in vacuum tube radios they have you know negative supplies buy supplies and all sorts of you know different kind of voltages here and there and tapped off of different resistors and you know they lift the the centre tap of the transformer to create a bias supply and a lot of the radios if you were to think of the entire schematic as electron flow it would make things quite a bit more difficult to work on so whenever you're using a schematic conventional flow is much easier to work with no matter how you've learnt it whatever you're good with of course is whatever you're good with for myself whenever I'm looking at a schematic conventional flow 100% looking at the vacuum tube and picturing it as its own entity and the electron flow going in there is absolutely fine I do it all the time whatever makes things easier for you okay so it's the same thing with this directly heated vacuum tube the electron flow from the hot surface to the plate alright so whenever you put a positive charge on the plate the electrons will flow from the hot surface to that positive to that positive charged aperture which is the plate or the anode whatever you want to call that inside of this tube alright so this is the diode here and we're going to go over to the bench in just a little bit here and we'll talk a little bit more and I'll show you the difference between a directly heated tube and and an indirectly heated to both try and get my camera nice and close and and point out the parts inside the vacuum tube you can even figure out the pin out of a vacuum tube just by looking through the glass and looking to where the lead-in wires connect to and I'll show you how to do that as well so next we're going to take a look at the triode in order to control the electron flow inside of a vacuum tube really use the vacuum tube as an amplifier we need to put a grid inside it alright and this is how we draw a triode this is indirectly heated as well tri meaning three ode electrode three electrodes again we have the plate here this is the grid and this is the cathode now by putting a negative voltage on the grid of this tube will stop the electron flow from traveling from the cathode to the plate as we make the grid more positive it kind of acts as a shutter for electrons it opens and it allows more electrons to flow to the plate hence we have an electron valve now we also have directly heated triodes and you see these mostly in transmitter service you know things like a 3 - 500 Z or something like that and you know in the very large amplifying tubes for audio use in things like you know home stereos and working on radios and test gear and things like that it's rare to see a directly heated triode again basically used in amateur service and in transmitting service so what we're going to do is we are going to focus on this triode here and we won't really look at this too much right now again the there's so much with vacuum tubes there are so many topics and so many different kinds of tubes and installations that I could be here for a very very long time so I'm only going to focus on the really popular stuff today again this is an indirectly heated tube we haven't drawn the filament in here if we wanted to draw the filament it's shown underneath the cathode of the vacuum tube so in order to picture how an actual triode works what I'm going to do is use a light bulb a venetian blind and a wall to make this a little bit clearer and you'll see what I mean here again understanding how all of this stuff works really is about visualization so what I'm going to do is clean off the board here and in just a second I'll be right back with that drawn alright so this may make understanding how a vacuum tube works just a little bit easier we have a light bulb of any blind and a wall alright so the light bulb being the cathode kind of represents a cathode it glows right the venetian blind is the grid and the wall is the plate when you put a negative voltage on the grid the venetian blind closes when you put a positive voltage on the grid the venetian blind opens the light shining out of the light bulb you would look at as being the electrons coming off of the cathode of the tube ok so by putting a positive charge on the plate it wants to attract the elect the electrons from the cathode so it pulls them towards the positively charged aperture inside the tube and whenever you look at a vacuum tube the outermost aperture that you see it's usually gray or a dark color that usually is the plate inside of the tube in some cases there is a shield on the outside but that's very rare so most of the times like if you look at a 6l6 you look right through the glass you can see the plate the part that glows in the tube is the cathode okay so again if we put a positive charge the venetian blinds open we have electron flow all right so technically you know if we were to put positive here and negative here you know we would be drawing current at this particular time you put a negative charge on the grid the venetian blinds closed the light can no longer shine on the wall anymore because the venetian blinds are closed right so the electrons aren't getting through that's really what the grid is inside of a vacuum tube you can picture it like a venetian blind that just opens and closes when you put a positive or a negative charge on it now depending on the tube and the biasing they'll turn on and off at different rates and you can kind of almost set that where you would like that to turn on and turn off all right and of course that's in the circuitry that surrounds the actual triode itself and that's really how all vacuum tubes work if you have a tetrode or a pentode you're just adding more venetian blinds is really what you're doing all right so what we're going to do now is we're going to look at a tetrode that will be the next tube we check out in order to improve upon the usefulness of a triode we can introduce another grid that extra grid makes it a tetrode so tap being for tetra beam for and owed electrode for electrodes all right i'll draw the symbol for a tetrode and that's what a tetrode looks like so again we have the plate we have the cathode we have grid 1 which is the control grid this is grid 2 so whenever you're looking at a tube in your counting grids up from the cathode it's always grid 1 grid 2 grid 3 grid 4 and so on as you're counting towards the plate this is now grid 2 and grid 2 is called this screen grid you want to remember that all right so grid number 2 is classified as the screen now the advantage of inserting this extra grid in the tube lowers the inter electrode capacitance inside this tube by creating an electrostatic shield now whenever you are looking on a schematic you see your troubleshooting a radio or you know any kind of test gear or an amplifier whenever you see a screen grid there's usually a positive voltage on it all right so by putting a positive voltage on this grid creates an electrostatic shield between the control grid and the plate lowering the inter electrode capacitance if you're to look at a triode you have the cathode the grid and the plate so the cathode is right in the center all right the grid is looks like a spring that's kind of put around that cathode it's not touching it in any way shape or form but that's the grid it's like spiral wound wire there all right and then the outermost aperture inside the vacuum tube is called the plate and that's usually what you can see right through the glass now between the grid and the plate we have Kappa that's right because they're you know they're close to each other whenever you have two metal surfaces that are close to each other technically you have a form of capacitor all right by inserting this screen grid between the control grid and the plate inside that tube adding that extra grid creates this shield and lowers the inter electrode capacitance allowing this tube to oscillate and amplify at much much higher frequencies and that's a very major advantage of the tetrode all right now we have all sorts of different kinds of Tet roads if you're used to playing in the audio world you'll notice the KT series tubes the KT 66 kt88 and the K T stands for King plus Tet road and really what that means is they've lowered or virtually eliminated the kink in the plate curves all right there's a kink in there and they've done that by aligning the grids so basically you have the cathode you have one grid another grid the grids don't touch each other they're basically just you know spaced away from each other but the wires of the grid are perfectly in alignment with each other so that they don't obstruct the electron flow it doesn't have to do that through it it basically just flies straight through the grid and by doing that we really dramatically lower a thing called secondary emission now you can view secondary emission inside of a vacuum tube as the electrons flowing towards the plate is basically a fire hose being shot at a wall so if you're shooting water out of a firehose at a wall you're going to get lots of spraying when it hits that wall all right so picture that water as electrons in that spray is now secondary emission those are electrons hitting the plate and bouncing back into the tube causing issues by aligning the grids greatly reduces secondary emission in Tet roads all right and then of course we have another scheme a way of doing this in a pentode as well and we'll cover that when we talk about pen Toad's all right bye also hi anode slope resistance we also get quite a bit more gain out of this tube and that's by introducing this screen grid here and again we would need to look at an actual chart to compare that but we'll keep it simple for now so grid one again control grid grid two the screen grid remembers you're counting towards the plate you're always counting up in numbers we have pen toads we have Penta grids we have hex ODEs we have all sorts of different kinds of tubes with all sorts of different numbers of grids inside them this could get extremely long because the difference in tubes and there's so many different kinds of tubes so what I'm going to do is I'm just going to stick to the popular tubes right now and next we're going to press on to the pentode another very effective way to stop secondary emission is to add another grid to the tetrode and that grid is called a suppressor grid now we have a pen toad alright so I'll draw a pen toad now so just like all the other tubes we have a plate we have a cathode we have grid 1 which is the control grid we have grid 2 which is the screen grid G 2 here and now we have grid 3 which is called the suppressor grid and this is G 3 now you'll notice in a lot of tubes that they tie the suppressor grid to the cathode all right that's effective and you'll notice in some tubes they don't like a 6 a u 6 you can actually put the suppressor grid where you want to actually attach it all right the suppressor grid is a grid that stops secondary emission because it's tied to the actual cathode of the tube or it's tied to ground or some variant of ground and any of the electrons that want to hit the plate and rebound back into the tube or pretty much stopped by that suppressor grid now this is called a pen toad right but we also have a thing called a beam power tube and they are also called pen toads as well but they could be looked at as a tetrode with beamforming plates what I'm going to do is clear the board off here and we'll talk about those in just a moment a really good example of a beam power pentode or they call it a beam power tube and it's classified as a pen toad is the 6l6 and the 6l6 is probably one of the most common tubes on the earth aside from the 12ax7 and possibly the 6g h8a so the symbol for that tube is very much the same as a regular pen toad except a beam power pentode like the 6l6 really is a tetrode with beamforming plates and they called it a pen toad they classified that beamforming plate as you know another grid or whatever they're calling it grid 3 and I believe that had to do with patenting way back in the day a so really I guess we're dealing with politics at this point a really good example of that if you'd like to look at this for yourself is if you have an RK 39 vacuum tube and you have an 807 vacuum tube if you look inside the two of them the electrical structure is identical the RK 39 is ruggedized it as you know larger spacers and there's you know much more glass inside the tube and support structures but electrically the RK 39 and the 807 are pretty much identical inside you can see the beamforming plates and everything inside the tube yet the RK 39 is called a tetrode and the 807 is called a Penta it's the same of the 1625 the 1625 is the 12 point 6 volt heater version of the 807 so so in order to eliminate a little bit of confusion here alright in the beam power pentode a lot of the times they draw that third the beamforming plates inside this tube here they draw a connection to the cathode not to be confused with a regular pentode because in some of the pen totes they have a connection between the suppressor grid in the cathode as well you'll also know in note in a beam power tube that sometimes they eliminate that one center line they're trying to show that it's a beamforming plate you can kind of picture a beam going through there and some engineers that draw 6l sixes in their schematic just completely eliminate that grid altogether and just draw it as a tetrode because really that's what it is it's a tetrode with beamforming plates this tube here is an 807 it's classified as a beam power tube or pentode this tube here is an arc a 39 and it's a tetrode with beamforming plates in it so this is the rugged version of this tube this tube was designed to fly around Royal Canadian Air Force right there all right so if we look inside this tube this outer aperture that you see the dark aperture and here is the plate or the anode inside this tube and these two silvery things that you see here just below this play is the beamforming electrodes all right now if we look in this RK 39 you can see the beamforming electrodes in here as well this is the plate and as you can see this is you know made quite a bit more rugged than the other tube you can see the big thick porcelain spacers and you can see how they've you know got these support posts going into the glass down here so it's a really really tough version of the 807 is really all that this is and it's quite a bit heavier to a porcelain base on it you know or this is just a standard base on the bottom looks to be made out of bakelite or some like material so if we look in the bottom of the tube here this one is relatively easy to see inside if you see these two copy like looking posts those are the screen grid posts so there's wire wrapped around those posts all the way to the top of the tube right in the centre here is the cathode and that white kind of squiggly wire you see there is the filament that goes inside the cathode that's what heats the cathode up right next to these copper looking posts are the screen grid post and they're kind of hard to see in the bottom here they're covered up by these two tabs which are the beam forming plates underneath here and then on the edges here you can see these wires running down into here all right those hold the beam forming plates and make the connection to the beam forming plates on the top this wire that runs up to the cap attaches to the plate so the reason they put the cap on the top of this tube here is so that they can use a really high voltage and not risk any kind of arc over now the 807 really is just a beefed-up 6l6 so here's a 6l6 alright and you can see that they're really the same tube you know inside they're virtually identical except this one here it does not have the plate cap the plate wire is led to a pin on this octal base right here so virtually no difference between this tetrode and this beam power pentode here again most likely they name them differently for patent reasons and stuff like that way back when here's a nice little trial that displays how it works nice and clearly so you see the Greek connection here runs to this caller and that is the grid structure inside now the grid in this kind of tube is a little bit different than like a 6 l 6 or 6 p 6 or something like that they're more of a rod fashion and they've got rings that hold those rods in place and right in the center these two leads right here attached to the filament that goes in the center of the grid this outer aperture you see here again is the plate and comes out of the top of the tube here so this was intended for some transmitting service or something like that you can see the little rods of the grid here running up into the tube so the signal goes in here and of course the amplified signal would be present up here alright let's take a look at what an indirectly heated vacuum tube looks like when it's warming up so I'll just apply some filament power here now that glow you see is the filament there that's heating up the pipe now if you keep an eye on that little piece right in the center there you'll see it start to glow orange here in just a little bit when that starts to glow orange this tube is ready to start working you see how it's slowly starting to glow orange in there so that filament structure that you see in the bottom that's lit up really bright has to heat that pipe orange hot in order for the cathode in this to start emitting electrons this tube here is a directly heated rectifier so it's pretty much ready as soon as you apply filament voltage to it so what I'll do is I'll just put filament voltage on the pin here and you can keep an eye on the top of the vacuum tube and you'll see these start to glow almost immediately so here I go I'll clip this on here and now the tube is ready to go just that fast so this is how five are four and a five you four and a five y 3 + 5 y 4 and all of those particular vacuum tubes work and this is the plate aperture the outer aperture there you can kind of see the filaments glowing in the bottom there this one here is designed for full wave rectification there's actually two diodes inside here with a common cathode which is the filament structure in the very near future I may be introducing some tear downs into tips Tuesday's I'm not quite sure if I'm going to call it something else yet maybe I'll name it a different series at this point I'm really not quite sure you can leave your comments below so I do believe that that would probably be beneficial to learning electronics there's a lot of really neat things to discover by looking inside of you know well designed devices like Tektronix oscilloscopes and all sorts of various test gear so if you enjoyed this episode on vacuum tubes you can let me know by giving me a big thumbs up and hanging around will be many more episodes just like this in the very near future and maybe some tear downs as well alright take care see you next time you

Channel: Mr Carlson's Lab

Views: 567,808

Rating: 4.9433327 out of 5

Keywords: creativedesigncomponents.com, Creative Design Components, How a diode works, how a triode works, how a tetrode works, how a pentode works, vacuum tubes explained, electron valves explained, 807 tube, RK-39 Tube, 6L6 tube, KT66 explained, KT88 explained

Id: oHjZs0bNwEs

Length: 27min 7sec (1627 seconds)

Published: Wed Mar 09 2016