Vacuum Tubes: A Historical (Hysterical ?) Overview

The vacuum tube, in its very primitive form, evolved from the light bulb. Invented by Thomas A Edison in 1883, the incandescent lamp, had 3 basic necessities to operate:
(refer to fig. 1.1)

The Envelope
The Filament
The Vacuum


figure 1.1

The envelope is basically a sealed container, a box or jar so to speak, which completely surrounds (envelopes) whatever is inside. The first envelopes were made of glass, however, there was no written law that they must be made of glass. In fact, many modern tubes have metal and/or ceramic envelopes.

The filament, otherwise known as the heater, was the basis of the light bulb. The idea was that if a high enough electrical current flows through a coil of wire, it generates light (and heat). Edison's object, was to create a thin enough piece of wire, that even a very low current could generate a great amount of light. The problem was that he kept burning up the filaments. They would work for a matter of seconds, then die out. He experimented with many different filament materials. Finally he found a metal material that would last - tungsten. Most modern filaments are made up of a thoriated tungsten material.

The vacuum was added along the way, as an attempt to keep the filament from burning out. It was logical, that in order for fire to exist, you must have oxygen. Edison assumed that if all the oxygen were removed from the envelope, by creating a vacuum, the filaments would stop burning up. It helped, but was not the total solution to the problem.


He did find, however, that if a filament were energized within a vacuum, that after time, a "shadow" would be left on the inside of the glass, which resembled the shape of the filament. He surmised from this, that within a vacuum, particles (we now call them electrons) were emitted around the wire, forming a cloud, or SPACE CHARGE.
(refer to fig. 1.2).

This effect became known as the EDISON EFFECT, which is the basic operating theory behind all vacuum tubes.
fig 1.2

During his experimentation on the electric light bulb, Edison found that many metallic substances will emit electrons when heated to incandescence. In a light bulb, these emitted electrons become waste, as they serve no useful purpose. The vacuum tube is, however designed to make use of these emitted electrons. Edison experimented by placing a second ELEMENT, or ELECTRODE within the vacuum along with the filament, but not touching it. He then connected an ammeter to the second element, and attatched the other lead of the ammeter to the positive terminal of the battery. He found that when doing this, current would flow through the ammeter. The second element is called the PLATE
(refer to fig. 1.3).
fig 1.3
The emitted cloud of electrons, bearing a negative charge, is attracted to the positively charged plate. It flows through the vacuum toward the plate and is collected upon its surface. This action was monitored and proven by use of the ammeter. But what happens if the plate is connected to the negative side of the battery? Edison discovered that when this is done, NO current flows through the ammeter. So electricity flows, within the vacuum, in one direction only - from Negative to Positive. This was in direct contradiction to Benjamin Franklin's conventional theory, that electricity, being a fluid (much like water), flowed from positive (a full glass) to negative (an empty glass).

Edison further reasoned that since, with the polarity reversed, the negative particles of electricity didn't flow from the plate to the filament, that there must be some outside force causing the electrons to leave the filament. He discovered that while he was working with a heated filament, the plate was not heated. The heat of the filament caused the electrons to be "boiled" off, and freed from the solid matter of the filament into the surrounding vacuum. Once the electrons were freed from the confines of the solid matter, they could be attracted to any positively charged source within the vacuum.

This is known as THERMIONIC EMISSION, which is the process of the electrons being forced out of the solid metal via thermal agitation.

fig 1.3

This is the basic concept of the FLEMING VALVE invented by J. Ambrose Fleming in 1904. It was noted that since electricity flowed within a vacuum tube in one direction only - from the filament to the positively charged plate, it was as if there was a 'one way valve' placed in the circuit. By this method, a direct current (DC) charge, formerly only available by chemical production through a battery, could now be converted from an alternating current (AC) source. This outstanding development was called RECTIFICATION and the Fleming Valve was known as a DIODE (two element) RECTIFIER. It wouldn't be until 44 years later that the crew at Bell Labs would recreate this effect using semiconductor materials.

fig 1.4

The AUDION came about when Lee DeForest, In 1906, added a 3rd element between the two. This third element, a control grid, allowed one to electronically control the output of the tube based directly upon the input. Along with the ability to control the output, came the ability to AMPLIFY the output as well. A small signal could be injected at the input of the tube, resulting in a very large signal at the output of the tube. Electronics was about to take on a whole new role in life, as radio as we know it would now soon be born. The term AUDION was later replaced by the term TRIODE, as the tube has 3 elements within the vacuum.


Later improvements included the adding of 2 more elements, the supressor and accelerator grid, which allowed higher frequency operation, increased stability, and eliminated unwanted oscillation. The 4 element tube was called a TETRODE and the 5 element tube was called a PENTODE.

The biggest problem in tube design came when trying to reach higher power levels, at higher frequencies. The higher the frequency, the tighter the tolerances became.






In an effort to overcome this problem, the BEAM POWER TUBE was developed. This tube was special, in that it FOCUSED a BEAM of electrons, rather than simply creating a cloud of electrons boiled off the cathode.

The beam is focused by applying a sufficiently high negative potential to repel the electrons being boiled off the cathode. At the same time the highly positive plate is attracting the negatively charged electrons.

This focused beam of electrons places more energy directly on the plate, eliminating losses, and allowing for better heat distribution.



Even today, in the age of the semiconductor, we can not do without tubes. This is why I insist that we still study them. They are still (as of the year 2000) used in Televisions, Computer Monitors, Microwave Ovens, Medical Equipment, Radar, Transmitters, and many other phases of high tech electronics.

We use some tubes, such as the big red ones pictured above, that are as large as a man. There is also a new wave of "nanotube" technology which might be worth riding. The point is, that tubes are not dead, nor will they be for quite some time, and should be taught as a viable technology.

As stated before, this is only meant to be a historical overview. Now we will get into the detailed theory of how each of these tubes operate. 

Tube Theory - "ODE" to Electronics

The "Ode to Electronics" would have to be the vacuum tube. This is because you will learn lots of "odes" in tube theory: Electrodes, Diodes, Triodes, Tetrodes, and Pentodes, just to name a few.

Before we go too far, we'll have to learn what an electrode is:

An Electrode is a conductor which permits current to flow. Remember Frankenstein's Laboratory? It had the two round balls with the high voltage applied, and you saw the electricity arcing across from one ball to the other? These were electrodes, and they allowed current to flow from one ball to the other through the air. Electrodes are always used in pairs. You could say that the big alligator clamps on your automobiles jumper cables are electrodes, as are the little pads the doctor puts on your chest when he hooks you up to an EKG. Although I'd rather that the doctor use the little pads than the jumper cables!

In tubes, the electrodes are elements within the vacuum which emit, collect or control the flow of current within the tube.

The simplest of tubes has only 2 elements:

The Cathode, and the Anode.


 

Looking at the pictures above, the picture on the left is a graphical representation of a "diode" or 2 element tube. It has a CATHODE (K), which emits electrons, and an ANODE (A), otherwise known as a PLATE (P), which collects the emitted electrons, allowing current to flow.

The picture on the right is a schematic representation of the same. The circle represents the glass envelope, and the elements are contained within. The Cathode shown here looks like an inverted "V", and the Plate looks like, well, a flat "plate" of metal.

What you see in the schematic on the right above is actually two circuits combined. We will now break these two circuits down, to simplify and show what is happening in each circuit.

Keep in mind that there are multiple theories on the flow of electricity. We will use the electron theory (negative to positive) first to describe how current flows through the tube. However, whenever describing how power gets to the tube, it may sometimes be easier to think in terms of conventional flow from positive to negative.

In the schematic to the left, arrows show the flow of electrons from the negative battery terminal, through the cathode of the tube, and back to the positive terminal of the battery to complete the circuit. The cathode gets red hot and glows. It gets so hot, that some of the electrons are thermionically emitted into the vacuum space directly around it, as shown by the little specks in the picture.

The Cathode, in this case, is directly heated by the high flow of electron current through it. An electrode which is directly heated in this manner is also called a heater or filament. Not all filaments or heaters are cathodes, and not all cathodes are heaters or filaments.

When a cathode is DIRECTLY heated, as this one is, then it is a heater / filament / cathode. Otherwise, tubes may have separate heaters & cathodes, in which case we say the cathode is INDIRECTLY heated.
Below are illustrations of both types of cathodes, so that you may understand them better.



As you can see from the schematic diagram, the circuit operates in the same manner, except the heater is separate from the cathode. This is done for several reasons, including increased reliability and longer life, less interference between signal and power supply stages, lower resistance, higher frequencies can be obtained, and ease of design.

Understanding the left side of the schematic gives us some insight to what happens when we apply voltage to the plate.

By applying a positive voltage to the plate, a current can be observed on the Current Meter ( also called an Ammeter ), indicating that the circuit is complete between the two terminals of the battery. So by the indication of current flow on the Ammeter, we can assume that there is a complete loop formed via the plate! The electrons leave the negative side of the battery, and are emitted by the cathode of the tube. They are then collected by the anode (plate) and returned to the positive side of the battery. Edison noted that if the polarity of this battery is reversed, so that a negative voltage is applied to the plate, no current flowed. It was surmised from this, that current only flows in one direction within a vacuum - from negative to positive.

Assuming you are craft and oriented, you could actually use the knowledge in this page to begin building your own tubes. Let's face it - the first tubes were ALL hand made. That being said, I do not send you to other people's pages often, but this one may be of interest to those of you wanting to try your hand at building your own tubes. The man's name is Claude Pailliard, and his website is primarily in Italian. However, all the necessary info/parameters can be found there, and at the bottom of his Main Page you can find a video of him actually producing tubes in his home lab. Very interesting.

Tube Theory - The Diode

  

Both of the schematics above show the operation of a diode tube. Now let's study a little about the theory of its use.

While not the only use for a diode, the most common use is that of RECTIFICATION . Rectification is just a big fancy word for changing Alternating Current into Direct Current. A RECTIFIER turns AC into DC, and a diode is an excellent rectifier.

This is because it only allows electrical current to pass through the plate circuit in one direction.

Let us examine what happens when we apply an alternating current to the plate circuit.

We replace the battery in the plate circuit with an AC generator. The AC generator creates electrical voltage which swings from a positive to a negative potential each cycle. When the generator's terminal on the cathode side swings negative, the the one on the plate side swings positive. This energises the tube such that the cathode is negative and the plate is positive. The electrons floating about in the electron cloud are repelled by the negative cathode. At the same time, they are attracted to the positive potential of the plate. For this reason, they travel through the vacuum to the plate. These electrons then go through the Ammeter, making it read current flow, and continue onward to the load resistor. Finally they reach the most positive point in the circuit - the positive terminal of the battery. They are drawn to the positive, much like a South pole magnet is drawn to a North pole magnet, and they will move toward each other until they meet.

When, however, the AC generator reverses (alternates) its polarity, such that the generator's cathode side terminal swings positive, and the plate side swings negative - look what
happens !!

With the cathode being positive, the electron cloud collapses, and no electrons are present to cross over to the plate. Because the plate is not heated to thermonic emission, it does not radiate electrons, and so will not allow current to flow. Current flow stops at the plate.

Therefore, in a vacuum tube, ELECTRICITY CAN ONLY FLOW IN ONE DIRECTION

If we draw a graph, indicating voltages fed to the cathode vs current flow monitored at the plate, we will see a pattern. Electricity only flows on the positive cycle of the AC waveform. While the input swings both positive and negative, the output fluctuates from 0 volts to some positive number of volts. In effect, the input is Alternating between positive and negative (AC) but the output is positive only (DC). We say that the output of the tube has been rectified. It is, however pulses of DC, and for most general purposes, useless until we clean it up. This IS however, the basis of EVERY POWER SUPPLY in every piece of electronic equipment you own.

The Characteristic Curve of the diode is found by applying several different voltage levels, and measuring plate voltages vs. plate current. We note that below a certain plate voltage, ( in this case 0 volts ) no plate current flows. The minimum point at which the tube no longer operates is called the CUTOFF POINT . Above a certain plate voltage, additional plate voltage has very little effect in increasing the plate current. The maximum point where raising the plate voltage no longer increases current is called the SATURATION POINT .