A relatively simple modification to the basic rectifier vacuum tube is responsible for many early radio transmitters and more. Here's how the triode, the next step in vacuum tubes, works. The triode allowed for switches and amplifiers to be made using a signal vacuum tube.

Before starting, here's an image of a triode, and the circuit schematic symbol for triodes:

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Review: Vacuum Tube Rectifier

It's first necessary to understand the vacuum tube rectifier. This is the simplest vacuum tube, and I just wrote a post about it here, but if you don't want to read the whole thing the basics are as follows:

A metal filament and a metal terminal are separated by a vacuum gap. A battery heats up the filament until it glows, causing electrons to be ejected from the filament via a process called thermionic emission. Applying the positive terminal of a battery to the metal terminal and the negative terminal to the filament causes current to flow across the tube, bridging the circuit, via free electron charges. Reversing the polarity causes no current to flow, as electrons are attracted back into the positive filament. In this way, the simple vacuum tube rectifier acts as a diode and only allows current to flow in one direction.

The Triode

The triode was the next step for vacuum tubes. Imagine that we take a vacuum tube rectifier and put a metal, conductive mesh in between the filament and the terminal. Now if we connect a battery across the tube with the positive end on the terminal (the anode), current flows across the tube via electrons if the filament is heated up.

Applying a nonzero voltage to the mesh in between the cathode and anode will change how the electrons move. For example, applying a positive voltage to the mesh will attract more electrons, causing more charge to flow across the circuit and increase the current across the tube. Applying a negative voltage to the mesh will repel electrons, causing less to escape through the mesh and bridge the gap, reducing the overall current through the tube.

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In this way, the triode can be used as a switch. If you put a high negative voltage across the mesh, no electrons can pass, and you have effectively shut off the vacuum tube using only an electrical signal. Remove the voltage or apply a positive voltage to the mesh and current will once again flow. Now, with with the triode, you effectively have a vacuum tube diode that can be turned off, down, or up.

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Triode Applications

The simplest way to use a triode is as a vacuum tube relay or switch. Whereas a relay requires moving parts to act as an electrically-triggered switch, the triode can be closed using only an electrical signal. But a more interesting use for the triode is as an electrical amplifier.

Say we obtain a very weak electrical signal and want to amplify it. This could be, for example, input from an AM radio antenna. We can use a triode to amplify this signal into a more usable form. Putting a steady voltage across the triode and applying the weak signal to the central mesh will produce a greatly amplified signal across the vacuum tube - small changes in the mesh voltage correspond to larger changes in the current across the overall tube.

A basic triode amplifier circuit
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Here's a DIY AM radio receiver circuit I found online, with a single triode at its core:

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Triodes are actually very similar to certain types of solid-state transistors. As such, in most applications they have been replaced by transistors, as transistors are smaller, cheaper, and more reliable than triodes, on top of not requiring an internal vacuum. Since triodes can also be used to make oscillators, they are used in radio transmitters that require very high power. Of course, they are also used in DIY amplifiers and circuits. You won't find a bulky triode in your computer, but some transistors that make up the computer act much like the old triodes.

I hope you learned something from this post. Let me know if you have any questions or comments.

I like writing about vacuum tubes because their operation can be explained solely in terms of charges and fields. I might make some posts in the future about other types of tubes such as cavity magnetrons.

Thanks for reading!