How Does the Vacuum Tube Work ?
Inside The Vacuum Tube ( in the examples below, the SET, or Single Ended Triode Amplifier ) = Owing to the limited space, I will provide only an overview, using images and short description.For a thorough and complete understanding, I recommend the book "Inside The Vacuum Tube", by Colonel Rider.
First, let's take a look inside a typical vacuum tube. The first photo here reveals the thorium coated sleeve, called the "CATHODE"- inside of which is found the "Filament"- a heater wire which causes free electrons to boil off as a result of the orange hot heat. Wrapped around the Cathode, but not touching it, is the spiral wire "GRID".
Thorium, by the way, is a rare earth element, the second step down from Uranium. The element in between only survives seconds...but you can appreciate the sophistication and magic that tube audio possesses....free electrons hurtling through a vacuum on their way to the positively charged anode. I once had a book by aCalifornia Institute of Tech professor who demonstrated, by calculus, and experimentation, that electrons actually accelerate on their way to the anode. It must be understood, that for something as incredibly infinitisimal as an electron...traveling an 1/8 inch to an anode is like a basektball being accelerated to the Moon from Earth by way of rocket engine. Some electrons strike the anode metal plate, hitting a proton dead center, and bounce back. These electrons encounter the other oncoming electrons, and are driven back into the anode. It is my own personal theory after studying that book, that this is what makes the revered "Triode" tube so "Musical".This may be the cause of the abundant 2nd and 3rd harmonics that tubes are famous for, and why guitarists much prefer tube amps...85% of guitarists own a tube amp. This also helps folks understand why tube amps have made such a strong comeback, in spite of the far more costly components, compared to transistors...which literally cost mere pennies ! Hard to compete with that, huh? Since I brought up the subject of guitar amps, and I receive questions abotu what the difference is between a guitar amp and a pure audio amp...guitarists often drivetheir amps beyond capacity, so to speak, and in so doing create peculiar effects. So guitar amps can get by with far cheaper components. Driving their amps to "clipping" is hard on the amp, and especially the speakers, which are having to contend with what amounts to "square waves". If you buy a heavily used guitar amp, don't be surprised that it needs a complete overhaul.
The Audio Signal is felt upon the Grid. It must be underscored here, that the Grid wire must have a "NEGATIVE" charge at all times. The reason for this is the 300 Volts DC POSITIVE that is applied to the "ANODE" or "plate". This is a powerful pull upon the free electrons. If the Grid were positive, that plus the anode, this , then, would permit electrons to rush across from"Cathode" to "Plate" so furiously that the tube and particularly the Cathode resistor would burn up very quickly.
The way all audio signal is made to be a "Negative Sine Wave Cycle" is by a process known as "SELF BIAS". Even with no audio signal, and the power is "ON"...there is a small current flowing steadily across from cathode to plate.This is called the "QUIESCENT STATE". This current flow is felt through the Cathode Resistor. With this resistor connected to ground, or "zero", it is by definition "negative" at the ground connection end/bottom of the resistor.
HOWEVER...at the top of this same cathode resistor, the current flowing causes a certain voltage to be felt across the cathode resistor, which is "POSITIVE" at the cathode end of this resistor. THIS effect causes all the audio signal on the grid to APPEAR to be negative ! Trust me...this works. In the very early days of vacuum tube development, an actual positive voltage was applied to the cathode in order to make the audio signal on the grid "look" negative. Later it was discovered that this "bias effect" could be achieved automatically by the cathode resistor-hence "Self Bias".
Above here is removed = the glass envelope and the metal enclosure called the "anode" or "Plate" to reveal the Thoriated Tungsten Cathode, and around that, the delicate grid wire. This is one of the things that make tubeaudio so interesting...the machinery and manual skills required tomanufacture a vacuum tube are nothing less than a phenomenal human acheivement. For there are tiny wires to solder to pins, and tiny welding to do to assemble the metal Anode enclosure which completely envleopes the cathode and grid...and some 22 rare earth chemicals and diferent metal alloys involved in the manufacture of a tube...not to mention that it all has to be inserted into a red hot glass envelope, and virtually all air removed- hence the "Vacuum "...quite an extraordinary accomplishment !.......Now take a look at the typical circuit diagram, or schematic for a single stage amp. As you examine this drawing, does it look to you like the resistor at the top of the tube,, the "Plate Load Resistor, is "PARALLEL" with the last resistor? It is , but the reason it is not clear is that the ground for the plate load resistor is by way of the center tap of the power supply transformer. THIS is the main reason I wish to make a schematic drawing that makes this more clear. More on this below.
The first resistor is the "Grid Resistor".The incoming signal is felt or"read" by way of this "load". Remember that without a "load" there IS NO SIGNAL. Think engine running without connection to the wheels...and so, there is no forward momentum. The capacitor is there to prevent the killer positive voltage from getting back into your audio source...which would, of course fry it. Capacitors totally block all DC voltage, and allow only alternating signals pass. Incidentally, I forgot this resistor during assembly once, and kept burning up my cathode resistor, and couldn't understand why...so this resistor and the cathode resistor work together to keep the tube from "runaway condition ".
Now a new way of drawing the tube amp circuit below here...click on any image in order to enlarge it in another window, and then use the "back arrow" to return.
Several things become more readily apparent here. First, you do see that the "plate load resistor" IS in parallel with the following grid resistor for the next stage of amplification. Second, note that the 300 volts DC applied to the plate load resistor which I have drawn as a vacuum cleaner to underscore how the current is ALWAYS moving, and always in one direction only...either moving a little... or moving a lot...but always moving...even at rest, or "in the quiescent state"- as I have drawn it here. The quiescent ( no signal ) current flowing through the cathode resistor is also causing a positive 2 volts to be felt at the cathode for "Self Bias". The Vacuum Tube itself serves as a "light speed variable resistor". ( Note the arrow on the 'coil' above the tube...this is a symbol of variable internal tube resistance )
As the audio signal becomes "Less Negative"...more current flows through the loop...and as the audio signal becomes "More negative"...less current flows in the loop - shown in red here. It is also critically important to understand that the AMOUNT OF current flowing through the plate load resistor determines the "Voltage Drop" across that load resistor...this will change with the AMOUNT of current in the loop. This will become more clear with the following images ....
Now take a look again at the Quiescent State - a 'flat line' in regards to no audio signal, yet the amp is "ON". The internal view of the tube I have drawn to indicatethat the filament is orange hot, and there is a small current flowing through the tube, the quiescent state indicated by the yellow color from cathode to anode. The oval represents the "Grid" The current flowing is .00065 amps, or "point 65 milliamps. Thevoltage drop across the LOAD RESISTOR is 130 volts. Remember the formula "E=IR" or Voltage equals current multiplied by Resistance. Thus , if .00065 amps is flowing through the 200,000 ohm Load Resistor, the voltage drop = 200,000 X .00065 =130 Volts
Now let us apply a sine wave signal to the grid. ( the lower image ) Notice that immediately the Positive Going signal, which is "Less Negative" - the grid must remain in the negative "zone" to prevent a runaway condition in the tube. Notice the symbol above the tube which I have have drawn to resemble a variable resistor - has dropped to the tube's lowest resistance, permitting maximum current flow across the tube. I have indicated this condition by use of red and yellow from cathode to anode. Notice also that the arrows, indicating current flow, have "fattened" and are Red. This is to underscore that now a maximum current is flowing in the loop. This time the current has increased from point 00065 to .0011, or"one point one milliamp". To determine the voltage drop across the Load Resistor, again, multiply point 0011 times the 200K Load Resistor = 220Volts. Now this is important...notice that the amplitude of the audio signal is larger in the Loop, than the incoming audio signal. This is an indication that the audio signal has been amplified...and is "In Phase". As the signal goes more 'positive', so does the amplified audio signal go "more positive". HOWEVER....notice the oscilloscope reading across the grid resistor of the next stage, how it is going down ! The signal has changed to one of opposite phase...but still retains the 'amplified state". Think of the "Loop" of the first stage as "blottering up the current"...creating a "fall" in available current flow at the grid resistor of the next stage ...although the increased amplitude remains intact.
Now let's look at the same circuit and watch the "scope" as the audio signal goes "More negative" - effectively increasing the internal tube resistance - indicated by the variable resistor at the top of the tube going to maximum resistance. The current flow through the tube has dropped- indicated by the Blue Color inside the tube-and also by the figure "point 00012" Multiply 200K X .00012 =24 volts. This means that the Load Resistor is dropping only 24 volts. This means that 300 Volts DC in the Loop , Minus 24 volts = 276 volts . This increase in voltage is now FELT upon the following grid resistor, and "read" by the oscilloscope as a "Rise" in amplitude. Note how in the Loop, the opposite reading is seen on the oscilloscope...a DROP in amplitude. This is important - for we are beginning to see that the 300 Volts DC is blocked by the "Coupling Capacitor", in effect, Isolating the DC component from the grid of the next stage - yet allowing thedramatically increased amplitude, the "AC component" to pass to the next stage. Naturally, we DO NOT WANT 300 volts positive DC to be felt upon the next tube, or it would burn up instantly.
I am providing this overview of tube dynamics for there is no tech school that offers a course in tube audio. It would be like offering a course in steam engines..there is almost no demand for steam engine workers-nor is there any demand for tube amp workers. Yet tube amp audio is still revered after its having nearly vanished. Finding information on the internet is relatively difficult. Especially when there are so many variations on tube amp circuits. Here I have chosen the very simple "Triode" Tube for its basic-ness, and simplicity. It also happens to be THE most prized tube audio circuit -thanks to its simplicity. It is this simplicity that allows the original sound recording to be amplfied with a minimum of electronic parts. It is felt by many audiophiles that the Triode Tube is the best. Naturally, opinions differ. And that is why finding information about tube amps on the web can become a "hall of mirrors" - leaving the novice more bewildered than informed.
Above, I just demonstrated, mostly visually, and with a minimum of mathematics, what is going on inside the vacuum tube. Below here, I have just a little space left to offer a little more information about tube dynamics. Just as there are tubes for sale, both new, and "old stock"...there are "Tube Manuals" still available as well. These are very useful for the specific inormation on each tube ever manufactured. For instance, fo the simple 6SL7 and 6BL7 I used in the Blueberry amp described on these pages...the RCA Tube Manual provides the Self Bias voltage required, optimum plate voltage, and also "tube curves". These are graphic illustrations of what the tube does when the grid voltage and cathode voltage is held constant, and only the plate voltage is changed. This is called the "Grid Family" of curves. There are also graphics of the "Plate Family of Curves". This is where the Plate voltage is held constant, and only the signal voltage changes.
There is an interesting dynamic that ties these two graphs together, called the "DYNAMIC TRANSFER CHARACTERISTICS". This is dealt with in the book "Inside the Vacuum tube" in far greater detail; but it bears mentioning here in brief, since it is an important feature of tube dynamics to understand-which relates to holding down "distortion" in the amp, and in so doing cause the amp to behave in such a way as to be ultra faithful to the original recording. This wil be a little tedious, but bear with me on this brief introduction to "Dynamic Transfer Characteristics" or DTC.
Below here, I have drawn a very simplified version of a tube manual's "Plate Family of Curves". The numbers you see relate directly to all the graphics above regarding the voltage swings and current swings in "The Loops" above. If you recall, the voltage "drops" across the Load Resistor swung from 76.7 through the quiescent state voltage of 168 Volts-and on up to 275.6 Volts. At each of those "points" Notice the voltages "Felt" upon the Cathode Resistor..."0", " - 1" " -2" and "- 3 ". Notice also how at the bottom of, say for instance, the100 Volt Curve along the "-2 Volts" of the Cathode...that it curves at the bottom...or "Distorts". The "Straight Line" going diagonally from lower right to upper left is the"Load Resistor" line, or "Load Line". A resistor is a "Linear" phenomena in electronics, while everything else is a curvilinear dynamic. For "Fidelity" we want "linear"= that is to say, an avoidance of the curved portions of tube dynamics. So how is this accomplished?...by way of the Transfer Characteristics mentioned...by projecting the voltage points on the Plate Family of Curves, ACROSS to the "Grid Family of Curves". it may take some time for this to make sense, so contemplate this for a while before you give up entirely...it will sink in. First...the Plate Family of Curves for the tube dynamics explored above....and THEN below that, take a good long look at the Dynamic Transfer Charactereistic...and observe how the line associated with this, the red line which slopes from upper right at approx . 80 Volts,..down left, toward the 280 Volt point...it is "STRAIGHT"..so you see, = this is the "Zone" of dynamic we are aiming for in a vacuum tube, for the best avodiance of distortion.
Above is the"Plate Family of Curves". At the Upper limit, we see that at 76.7 volts on the graph, point 0011, or "one point one Milliamps is flowing through the Loop . The "one point five" line is the same as "1.5 Ma" ...and just below that is the point 0011, " or 1.1 Ma". At the other extreme end, at the 275.6 Volts point, .00012, or "one tenth milliamp", or more precisely, one tenth point 2" Ma....the least amount of current.
Now we transfer the range of voltage and current in the Loop over to the "Grid Family of Curves" seen below.....THIS is called the "DYNAMIC TRANSFER CHARACTERISTIC"
This is naturally a bit bewidlering at first; but in time, it will become clear that the voltage and current swings in the tube circuit follows a Straight Line...and is thus considered "Linear", and with an absolute minimum/reduction of tube circuit operation in any of the 'curved" areas of dynamics. The Red Line sloping from upper right to lower left does indeed start to curve a bit at the lower left end...but the tube, you see, is set up to operate comfortably within and upon the straightest portion of the voltage and current swings inside the tube. This is all accomplished by study of the Tube Manual's graphics and information a circuit designer would need in order to set up the tube with the right voltage on the plate, the right value of Cathode Resistor, and the RightResistance of the Anode/Plate Load Resistor.
Now, I've taken you on a Jog past Robin Hood's Barn to make a point...but this, in a nutshell is what is going on inside a vacuum tube, and more specifically, the voltage and current limiting Resistors that hold the tube within a safe operating condition, with the least amount of anything either added to or taken away from the original recording..thus a "Hi Fidelity" ( faitfhul to ) original recording, and an accurate reproduction of that sound. There's more...but this is all the space I have on this page/website. Again, the more thorough study can be found in "Inside The Vacuum Tube"...a book well worth investing in.
Understanding what is going on in a tube circuit, by the way, will help you understand Solid State Electronics, especially in regards to the FET or "Field Effect Transistor" = which is set up in very much the same manner as a vacuum tube circuit !