Kit-Building 101: The Triode Vacuum Tube

Vacuum tube amplifiers are prized by many guitar players for their characteristic tone, and many players will only use a tube amp. Many tube enthusiasts have very little knowledge of how the vacuum tube operates or why it glows. That's fine for most, but if you're going to be building an amplifier kit, or modding an amp, you'll want to understand a bit more about how vacuum tubes operate.

In this article we're going to learn about the type of tube found in most guitar amplifiers, known as the triode.

As you may know already, electrical current can be thought of as the flow of electrons through a conductor, such as a piece of wire. The interesting difference between a vacuum tube and most other electronic devices is that that current does not flow through a conductor, but instead through empty space - a vacuum. Yes, if you look into one of those glowing tubes in our amplifier, you are looking at electrons crossing an empty space.

The triode vacuum tube is commonly used to amplify the audio signal within a guitar amplifier. It will help here to understand a subtle difference - that an audio amplifier doesn't actually take a small signal and make it bigger. Instead it uses the small signal's current to control a much more powerful current and turn it up and down at the level and frequency of the incoming audio signal. To make an amplifying device, such as a triode, we therefore need two things - a powerful current source and a way to control it.

In a vacuum tube, we create the powerful current by getting electrons to separate from their atoms through a process called thermionic emission. When a piece of metal is heated to the point where it glows, the electrons near the surface are given enough energy to fly off into the surrounding space. The higher the temperature, the greater the number of electrons emitted. In a vacuum tube the special name given to the piece of metal that emits electrons is the cathode. So at this point we don't yet have a current, we just have electrons jumping into the vacuum but with no place to go.

To create a flow of electrons we need to get the electrons moving. We do this by adding another metal conductor inside the tube and giving it a positive charge. This positive charge attracts the electrons and pulls them toward this metal conductor, called the anode or plate. Since electrons are negatively charged, they will be attracted to the plate only when the plate is positive with respect to the cathode. If the plate were negatively charged, no electricity would flow, meaning that the vacuum tube can conduct electricity in only one direction. This property explains why vacuum tubes are sometimes used as rectifiers, which are needed in the power supply of an amplifier to convert alternating current (AC) to direct current (DC). But that's another article, so we won't cover it here.

Now with our cathode emitting a lot of electrons, and our anode or plate attracting those electrons, we can produce a very large electrical current, which is the first part of what we need to build an amplifying device. But now we need a way to control that current, so that it can go up and down in level and frequency in accordance with our input signal. To do this we add a third element inside of our tube, and if you're starting to make a connection between three elements and the name "tri"-ode then that's good! This third element is called the control grid, or just grid for short, and goes inside the vacuum tube in between the cathode and the plate.

At this point it will help to look at at the diagram, shown here. This tube would be about 3 or 4 inches high. In the center is the cathode, in the form of a vertical rod. Coiled around it without touching is the grid, a thin wire with spacing between the turns. Then surrounding the grid, again without touching, is the plate, a thin, solid metal piece. All of these parts are inside a glass envelope similar to a standard light bulb. During the manufacturing process, all of the air is removed from the bulb to create a vacuum.

As discussed earlier, with a hot cathode emitting electrons, and a positively charged plate, we'll have a powerful current flowing through the vacuum from the cathode, through the open space of the grid, and then to the plate. It turns out that if a small positive voltage is applied to the grid, the number of electrons reaching the plate is increased, resulting in a higher current. On the other hand, if a negative voltage is applied to the grid, it reduces the flow of current to the plate. So the grid acts as a controller, and if we apply a small signal, such as from a guitar pickup, to the grid, it will cause the powerful current flowing from cathode to plate to vary in amplitude and frequency.

Assuming we've setup the tube circuit properly, the large signal flowing through the plate will be a high-quality but much larger version of the small signal being applied to the grid. That, at a basic level, is how the triode is used as an amplifier.