One of the most unique properties of the vacuum tube involves moving electrons across empty space. This was first observed by Thomas Edison. The tungsten, apparently, emitted particles and deposited it on the clear glass of the bulb.
Later, Lee de Forest placed a wire mesh (with a negative charge in order to restrict or allow the flow of electrons) between the glowing hot metal and a metal surround, called the "anode." This was charged with positive voltage to be a powerful draw upon the electrons being emitted from the red hot filament - called the "cathode." Moreover, a small undulating charge on the wire mesh, called the "grid" released a larger undulating voltage from the anode with the larger voltage on it. Thus was born the amplifier.
A greater amount of electrons will emit from the hot cathode, if treated with the rare earth thorium, or barium. These electrons are held back by the negatively charged wire wrapped around the cathode, and hang in a cloud of what is known as a charged field space. It is critically important that the wire and grid maintain a negative charge throughout the entire 'negative' and 'positive' swing of the sound wave fed to the grid. How this is accomplished will be explained here in an abbreviated, and simple manner. There are scores of websites that go into much greater detail and accuracy. However, there are almost no websites that give a succinct and easily understood explanation for anyone wishing to understand vacuum tube audio amplifiers for the first time.
Without vacuum inside the tube, billiard ball like collisions would take place with electrons bouncing around as though in a pin ball machine. Neither would we want something like the cloud-to-cloud lightning strike to the left there. The key is controlling this beam of electrons which we will attempt to explain.
The tube you see here is the first stage amplifier tube in Bluberry; the 6SL7. Here the glass is removed so you can see the metal plate surrounding the cathode and grid. The 6SL7 is a twin triode; meaning two each cathode, grid and plate. The reason for choosing this triode is for its reputation as the most suitable type of tube for music which is capable of low distortion. That is to say, it adds almost nothing to the original sound picked up by the recording microphone other than a 'second harmonic', which many say is one of the things which makes for a more pleasing sound; but this too must be held to a minimum for true audio fidelity.
The two posts on either side of the cathode are there to support the wire grid, precisely wrapped, and much closer to the cathode than the plate surround. This gives greater control, and VERY effectively contains the cloud of electrons. The barium coated cathode is a tube which contains a "filament". It is this filament that glows bright orange. The cathode type here is known as an indirectly heated cathode; for there are some other well known early triodes that have coated filaments alone giving off the electrons.
The first thing to examine in order to set up a vacuum tube so it can be operated within its design parameters= is to study the manufacturer's rating found in tube manuals from the 50s and 60s. These also be found online. We have zoomed in on the principle info on the 6SL7. Notice that it suggests a maximum DCvoltage for the plate, and the amount of watts it can safely dissipate - in this case, it says "1.0 Watt". . What is important to remember is that although a tube may dissipate, say 12 watts, the inefficiency of the amp system is something more like 30%. Therefore, your amp, with a 12 watt tube, will be providing about 4 watts out. Another very important figure to note is the minus two volts suggested for "biasing" the grid. This "bias" causes any sound wave signal coming to the grid to APPEAR to be negative by comparison. Much more on this below.
Another important feature of tube specs are the "Family of Curves." Certain plate voltages, and certain whole number grid voltages are shown in the graph to the left imdicating the possible quantities of current. These are measured in thousandths of an amp, or milliamps of current which can flow through the tube. There are, of course, an infinite number of grid voltage and plate voltage combinations; but for simplicity's sake, the grid voltages are indicated in whole numbers. Tracing one grid voltage alone shows us how the current, in milliamperes, changes as the plate voltage changes. For the sake of simplicity, let us zoom in on the chart, remove some of the grid lines; and then focus on one thing at a time. The first item would be establishing the 1 Watt limit on the chart , known as the "Power Curve."
We now move on to the Solid State Amplifier section. Mr Anderson's superlative Solid State Audio Amplifier, with less than .01% distortion from 20Hz to 30Hz. 20 watts. This one he calls "SS2"
Now the manufacturer's tube characteristics indicate above that the 6SL7, our first stage tube, can produce one (1) Watt. The red line on the family of curves chart indicates the one (1) watt range. This is found easily enough (find the green line. It is faint, but we need to know if you are color blind), by locating on the graph the 2.5 milliamp point on the left, and the 400 volt DC on the horizontal. 2.5 milliamps is .0025 ( keep careful track of your decimal points =2 milliamps, two thousandths of an amp ). .0025 times 400 volts = 1 watt. This is done with a number of points and the dots connected. The tube must be set up to operate below the curved red line.
The simplified chart below displays the 1/10th watt power curve. The 6SL7 in our amp operates below that. Why so low, one would ask, if the tube is capable of 1 watt? Bear in mind what will be required to drive the power stage. If we overdrive it, we would be "clipping," or the cutting off of the top and bottom of the sound wave. We want to keep it as pristine as when it was created in the sound booth. The simple chart below shows an example. Note the arrow pointing to the intersection of the 200 volt line and the 0.5 milliamp line. 0.5 milliamps, a HALF milliamp, reads .0005. .0005 x 200vdc = .1 , or 1/10th watt. There is no reason to draw a power curve to stay under. Nevertheless, a line has been drawn to show how one sets up an operating range below the power curve. This has been done for illustration purposes only. This shows how the load line is often pushed right up next to the power curve. This brings us to our first and foremost operating condition : The Load Line.
OPERATING CHARACTERISTICS AND THE POWER LIMIT CURVE
IN THE BEGINNING...................
ORDER WITHIN 30 DAYS OF RECEIVING.
To understand "Load Line" think of an engine running without a connection to the rear wheels. To go somewhere we connect the tube to a load resistor. Otherwise, all we are accomplishing is splashing electrons against metal and generating heat.
The Load Resistor is the very thing which enables us to take the signal out of the tube that which is being amplified. I recall being rather confused about this important component. It is understandable, for actually there are three resistors in series; the load resistor, the tube resistance itself, which varies, and the cathode resistor.
Let's take a look at the standard way of representing these components. They are then redrawn in the effort to create, perhaps, a more vivid image in your mind, the dynamics of what is going on with these three components.
The schematic to the left illustrates the standard way of representing the single stage of a triode voltage amplifier. The resistor at the top of the tube, known as the load resistor; and the resistor below the tube known as the cathode resistor. We will cover the resistor connected to the grid, and the following grid resistor of the output for the next stage later. For now let's concentrate on the tube resistance, the load resistor, and the cathode resistor.
The first thing that presented some confusion early on was that the load resistor which is considered to be in parallel with the output grid resistor. It doesn't look parallel, does it? It looks very much in series. In fact, the load resistor is connected directly to the secondary output coil of the high voltage transformer - which is center tapped to ground. We will explain power supplies in another episode, another day. For now, understand that both the load resistor and the following grid resistor both connect to ground return in parallel.
As long as we are required to illustrate a tube and its components symbollically, let's draw some new symbology and arrange the components in a way that more clearly indicates how they work with each other.
The illustration to the right shows a tube as it is first turned on.
Current surges across the tube, for with no signal on the grid, ...being at "zero"..., this is not negative, and thus will not stop electrons from going to the plate with 300volts positive voltage.
However, as soon as the flow begins, the cathode resistor develops its positive charge, and the tube "settles down" to the "powered up condition." Note there is "point" OOO65 amps flowing, but there is no sound from the speaker because there is no undulating sound wave to cause the speaker cone to undulate.The important thing is that with a negative charge at the bottom of the cathode resistor; and therefore, a positive charge at the top of the cathode resistor...the undulating signal arriving at the grid APPEARS all negative! Voila. Mission accomplished= to keep the tube from running away.
The numbers used are only approximations. The numbers chosen are representative, just as the symbology above is merely representative. However, they are fairly close to the operating conditions on the Blueberry Amp. With your indulgence, let's proceed with these approximate numbers and a few more charts . My intention is to introduce tube amplifiers with an emphasis on visuals. Graphs are merely geometric representations of numbers; therefore thoughtful analysis of these charts will reveal some critically important features. For instance, how to take a sound wave from a signal source and keep it as absolutely as faithful as possible. ( Hence the origination of the term "High Fidelity" ) The tube amplifier can be made to reproduce these sounds extremely faithfully.
For guitarists the opposite is true. In this case a harmonic IS desirable. Because it adds a second note, so to speak, the second harmonic of middle "C" is one octave up "C"which tubes are famous for. Indeed, 80% of guitarists still require and use tube amplifiers for this reason. We focus on high fidelity in this thesis. It is easy to add the distortion guitarists like via "add ons"-in particular a guitar preamp; which we have designed, and will make available.
However, we want to operate the tube as distortion-free as possible. One of the first things we do, by a study of the manufacturer's characteristic chart, (note that the manufacturer's specs indicate a minus 2 volts for the grid), is place our "zero signal" right in the middle of the load line so the swing of the sound wave, or pure sine wave, swings comfortably between the two extremes. The extremes meaning a grid voltage of zero will be the absolute maximum positive voltage on the grid. For reasons described above, causing the grid to go positive would cause harm to the tube. We have chosen minus 2 volts as the mid point of our swing, or the all important "operating point." Now the question is, "Will this operating point allow us to amplify a wave with minimum distortion?"
To answer this question, we now introduce another chart, known as the "Grid Family of Curves". This chart is the result of measuring the current flow with the plate voltage held steady, and the grid voltage varied . We can place these charts side by side as each of them indicates current flow on the vertical scale. We can project the grid voltage, say minus 2 volts, horizontally over to the left chart, by first finding the plate voltage by coming straight down from the intersection of minus 2 volts, and the load line. On the right, we can see that it is 175 volts DC. Now.. second, find the location of the 175 volts on a real graph, which will be between the 100 and 200 volt curves.
If the "operating point" is carefully placed so it operates the tube on the straight portion of the DTC we will have a faithful reproduction of our original signal source.
It may be apparent by observation that the operating point is placed towards the upper limit of the tube parameters, namely, as close to the "zero" grid volts as we can safely come. In this case, we place our upper limit at about minus point 3 volts."
WHAT VALUE OF LOAD RESISTOR TO ACHIEVE MINIMUM DISTORTION?
The chart to the right is a collection of dynamic transfer curves, DTC, all grouped together. It should be readily obvious that the higher the load resistor, the straighter the Dynamic Transfer Curve. Notice that the 320.000 ohm line is the straightest. Any higher than that, and we would not get all the amplification we need; or rather would not be operating the tube very efficiently.
The rule of thumb is that one should choose a load resistor which is a value about 3 to 7 times the rated "tube Resistance." The manufacturer's chart at the top indicates a tube resistance for our 6SL& at 44,000 ohms. Thus the 320K Load resistor would be pushing it. Hence the choice of a 200,000 ohm Load resistor for our amp; which is plenty straight enough.
THE DYNAMIC TRANSFER CURVE
This Dynamic Transfer Curve, DTC, is the dynamic we are interested in. Operating our tube so towards the upper end of this DTC, we have the best opportunity to maintain a faithful reproduction of our incoming signal. This DTC is quite straight and only curves toward the bottom, or tube cutoff, at the zero current flow end.
I trust this has helped you to visualize all that goes on in what is known as the "RC coupled Amp." By using a few capacitors and resistors, we can have all we need in one tube. There is of course, the output stage or "Power Amp Stage." This not loaded with a resistor, but rather an output transformer. For example, an inductor/capacitor coupled stage which operates in similar fashion - it is just a bit more complex. It is a fascinating subject we will save for another lesson.
Congratulations on making it this far. You are the rugged individual we are looking for; one with a healthy attention span. You are the one who is most likely to succeeed, have the required focus, and presence of mind to keep yourself safe through this entire process.
Very important= How do we determine the value of the cathode resistor? Again, Ohm's law....solve for "R" ...R = VOLTAGE divided by Amps..thus R = 2 divided by * 00065 = 3076.923 ohms..or.a 3K resistor plus or minus 5 %
SO HOW DO WE AMPLIFY A WAVE WITH MINIMUM DISTORTION?
For now let's depart from standard symbols for tubes, and components the goal of which is to create a more favorable mnemonic image of tube dynamics that will remain with you. Here the resistors are drawn in relative size to help you remember that the tube resistance is like a variable resistor, or rheostat seen at the top of the tube. If the grid is significantly negative, over 4 volts negative, the tube has "cut off," or has what amounts to infinite resistance. Likewise, when the grid trends more positive, or less negative really, the tube resistance drops.
For a "Power Supply", I have drawn something like a vacuum cleaner. This is a visual indication that one directional flow is established, through the tube, the load resistor, and returns to the cathode. The 300volts DC is constant but ALL of the voltage is dropped by each of the three resistors. The powerful DC "pull" on the electrons remains within the circuit defined by the yellow arrows. The spirals represent capacitors; two foil membranes separated by a dielectric insulator. DC cannot pass through them; and this is good. Without them, DC could get to the CD/DVD source and damage it or DC from the source could get to the grid, overconduct, and ruin the amp.
The spirals are called "Archimedes Spirals," and relate in an interesting way to how galactic dynamics operate. It is the function of capacitors to pass the AC component ONLY to the next stage. In the following, we try to make it clear how a small voltage is increased with these simple components, known as an RC coupled amplifier.
NOW ON TO THE PROCESS OF AMPLIFYING
Returning to our three resistors... When three resistors are in series, the total resistance will drop all the voltage supplied by any power supply. In this case, we use the recommended 300VDC.
As we learned previously, the circuit powered on has some current flow, known as the quiescent current. This current is at *00065 amps. (asterisk so you won't miss the decimal). To determine the voltage drop across the load resistor of 200K, E =IxR, *00065A times 200,000R =130 volts. The load resistor drops, absorbs, or dissipates 130 volts. The cathode resistor at 3K X *00065 = 1.95, or ca 2VDC. This is the all important "self bias" we will go into in much more detail, next. This sets the "operating range" of our tube ideally to keep the sound wave coming in to be amplified, unchanged as much as possible. 300 vdc minus the 132volts dropped thus far, leaves 168 volts felt at the plate of the tube. Keep an eye on this as we apply a signal to the circuit . We will return to this, but for now, let's focus on arriving at that quiescent current of *00065 amps, and the positive two volts on the cathode. This is known as setting the operating point.
The load Line has been drawn from the 300 vdc point to the one point five Milliamp point. Ohm's Law is E=R x A, or Voltage equals Resistance Times Amperes. In this case, we solve for Resistance, R = E/A, or 300 divided by One point Five Millimaps, which is 300 divided by .0015 =200,000 ohms. The load line is straight= for resistance is the one thing in electronics that is linear.
But the tube being a variable resistor of sorts, is non-linear. This chart is representative of holding the grid voltage constant, and varying the plate voltage. This is known as the "Plate Family of Curves." As the voltage is decreased the current in thousandths of an amp falls to zero at the bottom of the chart.
You can see that as the current approaches zero it curves.
This means that a pure sine wave fed to the grid will exhibit some distortion in wave a the result of this non-linear function of tube dynamics.
It will be extremely worthwhile to not only study and understand this chart to the left; but to be able to construct one of your own whenever you want to set up a tube stage.
For now, the load line is set at 200,000 ohms. We will explore the ideal load line below. Note that minus 2 volts is about halfway between the "zero" grid volts, which we must not go above, and the minus 4 volts at the lower end of tube conduction. This is where the tube will make its swing comfortably.
We take a signal from a CD or DVD, which is normally, about one volt or less. This one (1)volt is known as the root mean squared voltage, or RMS. The "peak to peak" value of, say *90 volts, is 2*828 times *90 = 2 1/2 volts, approx. You see we are considering a grid swing voltage, "peak to peak" of 3*6 volts; so this range will accomodate most CD and DVD players.
Note the voltage swing on the plate of the tube will be caused to swing almost 200 volts peak to peak... with 3*6 volts peak to peak on the grid! Remember the 'power on' quiescent current flow is *00065 amps. Now you can observe the change in voltage at both the plate and the load resistor, as the current is allowed to increase to its maximum, and then is reduced to its minimum. by the signal on the grid, or the "excitation voltage" as it is known...note the *00012 low current; and the *0011 upper limit currrent, and pay close attention to how the "oscilloscopes" detect the difference in voltages at the plate, and on the load resistor.....
"POSITIVE" GRID SWING TO MINUS point 3 volts
NOTE: With voltmeter or oscilloscope measuring from plate to ground return, you will read 168 volts at the plate.
NOTE: Current increased to max of *0011; upswing of O'scope at plate matches upswing of grid voltage - plate voltage drops to 76.7 volts DC; and observe output scope which displays a downswing!
The grid voltage drops to minus 3*9 volts, reduces current, and therefore reduces voltage drop across the load, thus the plate voltage rises to 275.6 volts. Note that the output is indicating the rise in voltage at the plate.
"NEGATIVE" GRID SWING TO MINUS 3*9 volts
GRID SIGNAL RETURNS TO NEUTRAL/QUIESCENCE
NOW LET'S LOOK AGAIN AT THE OPERATING POINT CHART ONCE MORE:
HOW A VACUUM TUBE AMPLIFIES...IN PARTICULAR, THE FIRST STAGE OF AMPLIFICATION, KNOWN AS THE "RC" COUPLED AMPLIFIER......
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