TTT081 How Tubes Work

during the restoration of this sx60 to a I received a number of comments questions and suggestions about the level of detail especially having to do with vacuum tubes a lot of people have never even had a course on vacuum tubes most vacuum tube courses ended 40 years ago or so and so one suggestion that I got was to perhaps do a little bit of a tutorial on vacuum tubes the theory and applications and by theory I don't mean complicated math I mean just a simple straightforward explanation of how vacuum tubes are made how they operate and then under applications some of the typical circuits that vacuum tubes are used in and once again I'm going to take a focus that is one that I taught many many years ago and it's the way I learned vacuum tubes it was very non mathematical and so this is intended for people who are either beginners that is have never seen of course on vacuum tubes or studied them in any detail as well as those people who haven't looked at vacuum tubes for many many years but would kind of like a refresher so let's go on and get started with that I first learned about vacuum tubes from a mentor if that I met in high school who was a friend of my uncle his name was Glen and my uncle his name was Jimmy conspired to get me some training because Glen had some and so I used to go over to his house and he would teach me about radio and about vacuum tubes and and so on my uncle would go to garage sales and estate sales and buy me old radios and TVs to work and so between the two of them they managed to get me to a point where I was very interested but but not as well educated as I would have liked to have been and so a little later I wound up teaching a course in on electron tubes that was that came from this technical manual this is an army technical manual and the place I was teaching was the Army Signal School at Fort Monmouth and in New Jersey now this particular technical manual was published in 1952 so the descriptions were intended to be used in training army radio repair technicians and that's the approach I'm going to take basically I haven't talked this way in over 50 years a lot of this stuff I learned 55 or close to 60 years ago so I hope I'll be able to recreate some of the thrill that I found at the time of learning about electron tubes how they worked and at the time I'm only going to be using simple mathematics add subtract multiply divide that's it electron tubes began as an accident waiting to happen what I mean by that is they the first experiments that produced an electron cube were actually experiments to try to prevent the filament of light bulbs from burning out and some of those experiments were done at the Edison lab this is an experiment that they carried out in which they heated the filament they evacuated the air out of the glass and then they put a plate inside the evacuated envelope near the filament and applied voltages to the plate now in this case the they only have a galvanometer connected to the plate and what they were trying to determine was whether they could by putting a plate here they could absorb some of the electrons that gathered around this filament now they didn't know they were electrons at the time they actually thought that it was just impurities that were being boiled off but nonetheless they stuck this together they put a galvanometer on it and they discovered that there was a little bit of current that flows through this galvanometer now notice here that the battery only supplies the filament with energy to get hot you could actually do the same thing if you've ever used a magnifying glass to get something hot you could actually focus the sun's energy on this filament and get the same effect in other words you could eliminate the battery and just have the galvanometer between one lead of this filament heat up the filament and connect the galvanometers to the plate and you'd get the same result later they discovered that what was going on a man by the name of JJ Thompson figured out that electrons were being emitted by the filament also sometimes called the cathode but nonetheless the emitting source and were being carried over to the plate and that was what was actually being measured on the galvanometers were electrons now up until that time nobody really knew what electricity inside a vacuum was one of the next things that someone did was insert a battery in the plate and what they discovered was if they put a positive voltage on the plate that is positive relative to the cathode that the current would increase dramatically and what was going on here is when you heat up any surface electrons collect above the surface they're boiled off this even happens in a campfire but only in a vacuum are the electrons free to move any significant distance without colliding with something like air molecules or whatever so if you put a vacuum here and heat this up to produce this cloud of electrons kind of like a mist over a lake in the morning when when the lake is warm and the air is cool some of this cloud of electrons will be attracted to this plate the more positive the plate since electrons have a negative charge the more positive of the plate the more electrons will be attracted and so that produced a device that was called a diode a diode simply means it's two electrodes the old comes from the same word as electrode and the die means two this is an example of a simple plot of plate current that is the amount of current flowing this way versus the plate voltage and as you see as the plate voltage increases the plate current increases also it's not a perfectly straight line but it's fairly linear eventually diodes were produced in an envelope like this with the filament in the center the plate surrounding the outside this is a half wave in other words a one plate diode of course there's a glass envelope to hold the vacuum and there are glass stems inside that hold things like the filament supports the getter is a material that is flashed that is attended from the outside with usually with an induction heater and it actually flashes kind of like flash powder and the purpose of that is to heat up the rest of the oxygen that might be in the tube even after they've evacuated it then of course they're a base was installed with pins at the bottom so you could make contact to the circuit now as we saw when you put a positive voltage on the plate you get current that is electrons flowing from the cathode and here the heater and cathode are shown separately and I'll explain that in just a second but the electrons are shown flowing to the plate and then through a meter and to the battery that's when the plate is positive when the plate is negative however no current flows the reason is that when the plate is negative and the electrons are negative like charges oppose or repel so the electrons that are being boiled off here collecting a cloud but they're being pushed down by this negative electric field against the negative field of the electrons there are basically two ways used to produce electrons one is to coat the filament with a material that is very active in other words will emit electrons when it's heated up that is in that case normally the filament is not called the cathode it's simply called the filament and you have to make direct electrical connection to this in order to attract electrons there's a second way of doing it which is to put a sleeve of emitting material around the heater in that case the filament is called the heater and the sleeve is called the cathode I know it gets a little confusing but you'll soon discover that most tubes that you will see are done this way a few power tubes are done this way where you want maximum heating obviously some heat is lost in this situation and so these tubes aren't quite as efficient but in normal radio and television applications that's all you need in high-power transmitters for example sometimes they use this structure but nonetheless they coat a material on to this cathode put the cathode around the heater but insulated from the heater now the interesting thing now is the voltages on the heater and the voltages on the cathode can be quite different and that can be an advantage that we will see later on this is also what is known as an indirectly heated cathode one of the obvious applications for a half-wave diode is as a rectifier circuit a rectifier circuit is one that changes alternating current into direct current the way it does that is when the input is positive on the plate relative to the cathode electrons flow up through the capacitor and through the load to the cathode and then from the cathode over to the plate and back to the source that causes the bottom end of this capacitor to be negative and the top end to be positive when the current reverses so that the plate is negative with respect to the cathode as we saw in an earlier picture no current flows so during that time it's as though this was an open circuit and during that time charge that's been stored on this capacitor can feed the load so basically the load is fed through the tube for the positive half cycle and is fed out of the capacitor during the negative half cycle thereby changing alternating current into direct current if you have to dot diodes and a center-tapped transformer you can set up this kind of circuit it's a full wave rectifier by full wave the idea is to keep current flowing through this capacitor on both the positive and negative cycles but to make sure that it always flows in this direction so the top is just like the previous circuit that is when this is positive current flows up through the capacitor from the cathode to the plate and back to the transformer when this is negative from transformer theory you know that when one end is is negative the other end relative to the center is positive well when this one is positive current flows through the capacitor and through this diode to the plate and back so you may have noticed we are charging this capacitor on the positive half cycle and then we are also charging it on the negative half cycle in the same direction this produces a steadier DC current that is one that is more constant over each cycle diodes are also used as AM detectors am meaning amplitude modulation in this case the input is tuned to some frequency in this case let's say that's the radio frequency all of the station you're listening to the diode causes current to flow in one direction and basically cuts off the bottom half of the RF in doing so it produces a wave that you see down here whose amplitude varies with the amplitude of the incoming RF signal so if you have an AM signal here and you tune this secondary to that frequency across this capacitor and this capacitor is usually a fairly large capacitor relative to this one across this capacitor you will get an average voltage and the capacitor will essentially short out the RF part of the input leaving across it a slowly varying audio signal and then that audio signal isn't applied to your audio amplifier you'll see these detectors a lot in am radios and a similar though slightly more complicated circuit is used to detect FM we will not talk about that here a specialized kind of diode is also used as a voltage regulator now by specialized kind of diode what I mean is it still has two active elements in this particular case the cathode is cold that is there's no heater or filament in this tube just a cathode the space instead of being filled with a vacuum is filled with a gas that has the property that the gas breaks now have a particular voltage that is the the pressure of gas in this tube and the spacing between the cathode and plate are carefully adjusted in manufacturer so that at a particular voltage for example 150 volts the space between these will break down and electrons will flow from this cold cathode to the plate so if you put an unregulated DC let's say DC that is varying from say a hundred and seventy volts up to two hundred volts and back down again and you put a resistor in series what happens is this tube conducts more current the higher the voltage so as this point tries to go up it starts at a hundred and seventy volts but because the tube is conducting it drops twenty volts across this resistor so 170 minus twenty leaves a hundred and fifty now when you put 200 volts here the tube will still be a hundred and fifty and the 50 volt difference will be dropped across the resistor so the effect is that the output is a regulated direct current that is fairly stable after the invention of the diode a man by the name of Lee de Forest began experimenting with the idea of trying to control the current between the plate between the filament and the plate or the cathode in the plate in a way without varying the plate voltage that is trying to put some kind of structure in between that would either increase or decrease the current flow this was called a grid and so DeForest created this multi element tube with a filament it's also the cathode a grid in between and a plate the way it works basically is you remember I told you that the plate is positive the electrons are negative so they're attracted to the positive plate but if you put a negative grid that is if you put this grid in and apply a negative voltage here relative to the filament that negative voltage will repel those electrons and fewer often will get through now this isn't a solid plate so it doesn't stop all the electrons and the more negative you make this grid the more it cuts off the electron flow this type of tube was called a triode once again because it's three electrodes the battery used to supply the filament began to be called the a battery the battery to supply the plate was called the B battery so the battery to supply the grid was called the C battery later this was called the bias battery so here you see is a circuit to test how a triode works you'll notice that there is a voltage that is applied to the grid that is called the C bias and that can be varied it can be either a negative value or a positive value there is a plate voltage and of course not shown here is the filament voltage the a battery well when you do that what you get is a curve that looks like this this is the negative grid voltage this is positive grid voltage this is plate current so as you see as you drive the grid very negative the current will go to zero as you bring the grid to neutral the tube becomes a diode it's simply a diode with zero volts on the grid it's as though the grid we're not there then as you add positive voltage to the grid it does the reverse instead of repelling electrons it actually attracts them and speeds them up and so the current in the tube actually goes above the diode point up to some saturation level and then once you reach saturation level the tube becomes relatively flat actually at some points it can even fall off a little bit because what happens is some of the electrons instead of flying through the grant and hitting the plate start collecting on the plate on the grid and coming out into the circuit so here is how you might use a triode to produce an amplified signal notice here the C battery is supplying a grid bias but there's also an input resistor and across that resistor we can put a varying voltage that varying voltage will add to and subtract from the bias similarly on the plate we have a B battery supplying plate voltage as the plate voltage varies that will cause more or less voltage to develop across here thereby producing the output voltage that varies in place with the electron flow or the plate current so we have produced a an amplifier if the plate current is substantially more than the grid current because for example suppose this input resistor and what resistor are the same and let's suppose we develop one volt across the input resistor well since these are the same if the current through this is higher than the current through this the voltage across this will be greater let's suppose the current through this is five times the current through this resistor and they're equal resistors that means we have a gain of five one volt across here five volts across here here is a cutaway view of the construction of a triode now notice that the plate is on the outside inside between the plate and the cathode is the control grid that's that spiral structure here and then on the very inside is the cathode and inside the cathode is the heater making this an indirectly heated cathode and because it has three active elements a plate a control grid and a cathode it's called a triode I mentioned the spiral control grid in the triode we just looked at there are a number of ways that you can construct control grids this is a more or less uniform control grid that is even spacing this has variable spacing and it's used in a special kind of tube called a variable you tube or a variable amplification tube we'll talk about that a little bit later here is a flat oval type there are other types this is the ladder you have two insulators and you wrap the wire around the insulators this way this is a circular helix this is similar to the one we just looked at and then there are some special types that are used in microwave tubes and other things of that sort normally when you are looking at the description of a piece of electronic equipment using electron tubes you don't see the cutaway view instead what you see is what all the schematic symbol this is the schematic symbol of a six c4 triode at the top is the plate that is the thing around the outside below that is the control grid that's that spiral structure below that is the cathode that is the thing that gets heated by the the heater and then below that is the heater around the outside or shown the pens so for example the heater is connected from pin three to pin for the plate is connected to pin five and you may notice the plate also comes out on pin one it's not uncommon for there to be more than one pin the control grid is on pin six and the cathode is on pin seven pen two has no connection if you look in a tube manual you may see a picture similar to this this is a six a three triode it's very similar to the 64 we just looked at here is the schematic symbol and this may be somewhat intimidating to someone but it really is very simple it is it is simply a graph of the plate current and the plate voltage so for example this is 250 volts on the plate this is a hundred milli amperes of plate current so okay what are all these curves well these curves are curves that show you how the two will operate at a particular bias voltage so this is with no bias voltage with voltage of zero if the grid is 10 volts negative the tube will operate along this curve so for example if you have a hundred and fifty volts on the plate and minus 10 volts on the grid you will have just under 125 milli amperes a plate current so don't be intimidated by these they simply are a way to give you a picture and also a way to do some graphical design we aren't going to talk about amplifier design here we're just talking about operation and basic circuits but you can actually use this chart to design an amplifier shortly after triodes began to be developed the attempt was made to control the characteristics off trials even more by inserting a third grid I'm sorry a second grid called a screen grid and it was inserted between the first grid now called the control grid and the plate normally the control grid in those days was connected to a top cap that was done to reduce capacitance because by bringing the leads of the control grid out through the bottom of the tube it was very near all the other elements and in those days it was hard to control the capacitance of those leads so they brought it out the top that resulted in this kind of tube that is a tit Road or four element tube on the left is one used for receiving on the right is one that was used for transmitting of course the only difference being it was intended to carry a lot more current the reason that that extra grid was called a screen credit was that it's its purpose was to reduce the coupling between the plate and the grid at this time a lot of the radios were called tuned radio frequency or t RF receivers that meant that the plate was tuned to the same frequency as the grid and what they noticed was as they tried to increase the amplification the tube would begin to oscillate so a screen grid was placed between the two to cut down the control grid to plate capacitance which is where the the feedback was occurring causing the oscillation for the screen grid to operate effectively it has to be operated at a potential that is higher than the cathode but lower than the plate and so as you see in this depiction a battery is tapped now of course normally in an AC powered receiver you don't actually have batteries instead you would have a resistive divider that the plate supply would come from a higher voltage point than the screen supply this helped trf receivers to operate at higher gain however it had one unfortunate consequence and that was the characteristics were really messed up down here when the plate voltage drops down near the screen voltage you'll notice this incredible non-linearity here now as long as the plate voltage is very high relative to the cathode and the screen grid the tube operates almost like a triode and everything is fine but when you apply a large voltage to the input the plate swings a lot so for example the plate might swing from 250 volts down to a hundred volts or maybe even 50 volts and if it does all of a sudden instead of this nice linear characteristic you start getting this incredible distortion so an attempt was made to find out why that was occurring well it turns out that what was happening is that as electrons would hit the plate they would bounce off some of them back into space well if the screen create was nearly as positive or in some cases even more positive than the plate those electrons instead of flowing out of the plate would go out through the screen grid so this so-called secondary emission at the plate was made worse by a screen credit particularly if the screen grid voltage was close to or above the plate voltage that's what was causing this incredible non-linearity so the next attempt was to correct that and as you may have guessed the way they corrected that was by handing yet another grid you'll notice at the top that there are three grids an integrate the control grid a second grid the screen grid a third grid now called the suppressor grid suppressor because it suppress the effect of secondary emission so let's look at that in a circuit and see how it did it so this is a pen tote that is a cube with five active elements a cathode a control grid sometimes called grid on a screen grid sometimes called grid 2 and a suppressor grid sometimes called grid 3 and of course the plate so how did it work well by putting cathode potential on the suppressor grid it basically blocked the electrons remember the the cathode is very negative relative to the plate so if an electron bounced off the plate due to secondary emission it would be less likely to be attracted to grid 2 if in between there was an element that was very negative and that's what g3 or the suppressor grid did because these grids were very open electrons flow generally through them very easily and particularly with high enough plate voltage the electrons acquired enough velocity that they would slam into the plate that actually was the reason you had secondary emission was the velocity of the electrons was kind of high but now you could gain the advantages of a screen grid tube that is isolating the plate from the grid a little bit better and also not have the nonlinearities associated with the tetrode so if you look in a tube manual under a pentode what you find are a set of characteristics once again this is various values of bias voltage this is plate current and this is plate voltage here is your pentode but you'll notice that now the tube is considerably better behaved that is as the plate voltage drops down at some point if it reaches a point at which the plate current drops begins to drop off quite a bit but at a much lower voltage notice that at 50 volts here this tube is still operating on a relatively linear part of its curve as you saw earlier the pentode or the tetrode that we looked at by the time you got down to 50 volts had a big dip in the plate characteristics or the plate current so now you have a cube that not only provides isolation between the grid and the plate but also doesn't have the nonlinear charities of the tetrode here is a cutaway of what I would call a modern Pinto that is modern a nineteen late 1950s early 1960s pentode this is the 6c b6 it was used a lot in I of amplifiers and even in some RF amplifiers of radios and television sets once again it has five active elements plus a heater and it provided a superior tube not only was it less likely to oscillate but also it provided more gain more amplification and also a higher plate resistance which we will talk about in just a few seconds a second approach to the problem of secondary emission was what is called a beam power tube now this is a top-down look at a beam power tube and what is unusual about it are these beam forming plates which we'll talk about in a second you may notice there's a cathode in the center this is the control grid this is the screen drip notice there's no suppressor grid this is actually a tetrode but it operates like a pen tote and the reason is these beam forming plates force the electrons into a narrow beam between the cathode and the plate and that beam in essence sweeps up secondary electrons let's talk about that an electron leaves the cathode with pretty high velocity because it's being attracted by the plate it hits the plate and bounces off now you may know then remember in a pentode there's a suppressor grid in here to repel it back in a beam power tube by focusing the it's the electrons themselves that is those that are coming off of the cathode at high speed in this direction that produce an almost impenetrable cloud of high velocity electrons headed toward the plate so this electron that bounced off it's swept up by those high velocity electron cloud and gets carried right back to the plate it doesn't have an opportunity to get all the way back to the screen grid you may recall that we looked earlier at a grid structure in which the grid is fairly close here and very widely spaced in the middle this is a special grid structure used in a pentode to produce what is called variable you will talk about mu in a little bit but basically what it does is it produces a tube whose characteristics change with grid bias this is a 6s j or 6s k7 there's a cutaway view the grid it's a little hard to see but it's more tightly wound at the ends than it is in the center and it produces a characteristic like this that is this is for a given plate current and a given grid voltage I point out this is a different kind of graph than the one we looked at before this is the amplification factor for example of a 6s k7 you'll notice that as the grid voltage gets more negative the amplification drops off here it's amplifying a lot you apply a little negative voltage and amplifies less and so on why would you want that well in radios and in TVs you want to be able to vary the amplification so that as the signal varies the loudspeaker doesn't go from whispers to booms but rather holds the volume constant that is called an automatic volume control we will talk about those circuits specifically here but they are used in radios and TVs in TVs they're often called AGC automatic gain control whereas in radios they're usually called automatic volume control or AV C so variable mu pentose were developed as I have amplifiers so that they would respond better to automatic volume control so now the dreaded math here you see the formulas for three constants or three values that are used to represent the quality of a tube don't get upset by the math this is simply a division you're dividing a change in base voltage or B voltage by the change in C voltage and that produces the amplification constant that is the amplification factor how much is the output changed by the input a second calculation is the plate resistance and you may see these plate resistance once again is simply a ratio in this case of a voltage to a current that is the plate voltage EB to the plate current IB and finally there is this term transconductance transconductance just means it's like the the effect of the grid on the plate current not on the plate voltage here you see is the grid voltage on the plate voltage here is the grid voltage on the plate current IB is called GM and that's measured in Mo's mhos it's called a mo because it's actually ohm spelled backwards and in case you're scared by the term transconductance conductance is just one over resistance in other words it's what is mathematically called the reciprocal of resistance so transconductance is the same as one over the trans resistance in other words the effective resistance of the tube from its grid to its plate we won't use these very much but you may see them in a tube manual and if you just remember that they are simply ratios that is it's simply one number divided into another you you won't be as timid or afraid of them as I was when I first encountered them if your interest has been piqued by this presentation that you might want to download a copy of this technical manual here is the number TM 11 six six two and then /t M stands for technical manual the slice tío the T o stands for technical order 16 - 1 - 255 I believe if you just enter TM 11 6 6 2 into a search engine you'll find a PDF version of this book it's a wonderful source it contains all of what I've talked about today it also has a lot more and once again I promise you because I taught from this book or for a period of time at the signal school there's no math beyond simple add subtract multiply and divide and yet if you read the whole book you will become as knowledgeable on electron tubes as anybody from that time or since so I hope you've enjoyed this little introduction to electron tubes and how they work I'm going to do another section on electron tube circuits but I thought I'd separate this so that it doesn't run too long once again I hope you enjoyed it I hope you'll stay tuned for the electron tube circuits video but in either case have a good day and we'll see you in the future

Channel: tomtektest

Views: 23,092

Rating: 4.9358287 out of 5

Keywords: Vacuum Tubes, Electron Tubes, Tube Amplifiers, Vintage Radio, Early Electronics

Id: E71qCufzb38

Length: 45min 1sec (2701 seconds)

Published: Fri May 06 2016