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Vacuum Tube Diode: What It Is and How It Works

In recent times, we’ve seen a lot of technological advances in the area of electrical devices. Nowadays, we can find a bunch of transistors creating logic gates and enabling computation.

But, computers were around before the invention of transistors and direct current. Though this earliest form of a computer was so massive, it took up entire rooms and weighed thousands of pounds in the 20th century. 

The Vacuum tube diode is what played the role of transistors in these behemoth computers. Though transistors run the world of modern electronic devices, the vacuum diode still has its uses.

So, in this article, we will show everything about a vacuum tube diode, how it works, its types and applications for your PCB, and its characteristics. 

Let’s begin!

What is A Vacuum Tube Diode?

Before the transistors Sir John era, Ambrose Fleming created what we know as the vacuum tube—in 1904. Sir Lee De Forest (from the 17th century) is another name to reckon with, especially regarding radios and filament related to vacuum tube diodes.

The vacuum tube is a device that blocks the flow of single electrons from an electric current in one direction (anode to cathode) while allowing the flow of electric current in another direction (cathode to anode).

Now, the vacuum diode is the simplest form of the vacuum tube that produces and controls free electrons. Additionally, the vacuum diode has two electrodes which we know as cathode and anode. The anode works as an electron collector, while the cathode serves as an electron emitter. It also works as a 1.4-volt filament with a one-way valve supporting both heated and slender filaments.

Plus, the cathode can be a positive electrode or a negative electrode during its electrostatic field discharge. In other words, it has great field emission properties.

Anodes are hollow metallic cylinders made from nickel or iron. However, in high power situations, you’ll find anodes with molybdenum, graphite, or tantalum because high power events can damage nickel or iron anodes. Also, Anodes are larger than cathodes to dissipate heat without a high increase in temperature. So, you can’t consider an anode as one with a weak current.

On the other hand, Cathodes consist of a simple tungsten filament or thoriated tungsten. Also, cathodes with field lines can be barium oxide or strontium oxide-coated nickel tubes. Plus, oxide-coated cathodes show better emission efficiency.

How Vacuum Tube Diodes Work

When looking at how a vacuum diode works, it’s important to know the effectiveness of the way electrons escape from a surface.

The number of electron streams a heated material can emit per unit area relates to a constant ‘b’ and an absolute temperature. The constant ‘b’ indicates what a primary electron does to escape the surface.

Thus, from this, we can derive an equation for the current leaving the outer surface as:

I = AT²ε (–b/T)

The equation for the current leaving the outer surface


I – current measured in amperes

A – constant for the kind of emitting material

T – the temperature in degrees absolute

b – work needed for the electron to leave the outer surface

Walter H. Schottky and Thomas Edison’s inventions support the above equation too.

The anode (positive terminal) deals with positive voltage. So, it works on the thermionic emission principle. Additionally, the filament heats the cathode (negative terminal) and allows the emission of electrons. These emitted electrons are then attracted to the anode. However, if the positive voltage the anode receives is not enough, it won’t attract the electrons from the cathode.

For this reason, an invisible cloud of electrons will accumulate in the space between the anode and cathode, creating a space charge. The space charge repels other electrons leaving the cathode. Hence, stopping electron emission and the flow of current through the circuit.

But, if the power supply voltage applied between the anode and cathode is high enough, then the space charge effect will be neutralized slowly. In this way, the flow of electrons to the anode will be free. Hence, electrons can move across the vacuum inside the glass envelope of the vacuum envelope. For this reason, nothing is blocking the emission of electrons, thus allowing the free flow of current from the anode to the cathode.

Plus, as the applied voltage increases on the anode, the current also increases. Eventually, the space charge completely vanishes, and the anode attains the maximum emission from the cathode.


the only way to increase the cathode’s electron emission is by increasing the temperature of the cathode. It also increases the energy of the electrons allowing more electrons to leave the cathode.

Though all areas of the vacuum diode feature a space charge, it’s quite important in the cathode region. Why? Because it determines critical elements—including maximum emission.

In contrast, if the anode deals with negative voltage, there will be no electron flow—because it won’t be hot. Additionally, the electrons that leave the heated cathode-ray tube won’t move to the anode. This process accumulates a strong space charge between the anode and cathode-ray tube. Due to the strong repulsion of the space charge, all electrons move back to the cathode. Therefore, no current flows through the circuit.

Characteristics of Vacuum Diodes

Here are some of the characteristics of vacuum tube diodes.

Diode as A Rectifier

When applying an alternating current to your anode, its polarity will stay positive during a positive half cycle. Thus, electrons can flow to the anode. Moreover, during a negative half cycle, the plate stays negative, which terminates the anode current.

So, it shows vacuum tube diodes will allow the anode current to flow in one direction only and produce a rectified output current. And this works better with a thermionic diode or semiconductor diode with heater voltage or reverse voltages.

Two Types of Cathodes

The vacuum tube diode can use two types of cathodes:

Directly Heated Cathode

Here, the cathode also serves as a filament. So, you can call it the filament type cathode.

Indirect Heating Cathode

Here, the cathode has a thin metal sleeve coated with oxides. The sleeves serve as a cathode, and there is an electrically isolated tungsten wire separated from the sleeve.

Space Charge

The space charge is an important characteristic of the vacuum tube diode. A positive charge appears on the cathode when it emits electrons. It allows the cathode to attract the electrons and create a space charge in the vacuum tube envelope.

Cathode Materials

Here are two common cathode materials:


Tungsten comprises a pure metal, and it has a 4.54 eV work function. You can safely operate this material at 2500®K and use it in a high power tube, Thermionic vacuum tubes, or traveling-wave tubes.


This material works in Directly heated cathodes. It supports electrons at low temperatures (about 700®C to 750®C. You can operate this material with high efficiency and small heating power.

Types of Vacuum Tube Diodes

The Vacuum diode tube type has six classifications which include:

  • Vacuum diodes for frequency range ( radio transmitters, microwave, audio)
  • Vacuum diodes for power rating (audio power, small-signal) with uniform field
  • Cathode/filament type vacuum diodes 
  • Specialized functions vacuum diodes (light detectors)
  • Application vacuum diodes (transmitting tubes or receiving tubes)
  • Vacuum diodes for specialized parameters (low noise audio amplification)


The applications for vacuum tube diodes include:

  • Atomic Clocks
  • X-ray Tubes
  • Radio Sets
  • Grid Bias Battery 
  • Audio Systems
  • Voltage-regulator Tubes
  • Control Electrode
  • Triode Amplifier
  • Electronic Amplifier
  • Consumer Applications
  • High-speed Circuit Switching
  • Klystron Tubes
  • Ion Propulsion Systems
  • Electronic equipment
  • Battery-powered equipment
  • Professional Audio Equipment
  • Radio Communications
  • Solar Collectors
  • Microwave Systems
  • Military Systems with (High-tension) Supply
  • Mobile Phone, Bluetooth, and Wi-Fi Microwave Components
  • Cellular Telephone Satellites
  • Particle Accelerators
  • Photomultiplier Tubes
  • Strobe Lights
  • Semiconductor Vacuum Electronic Systems
  • Vacuum Electron Devices
  • Vacuum Panel Displays

Final Words

Despite the world now being powered by transistors, the vacuum diode still has its uses. Perhaps the most outstanding modern use of the vacuum diode is in the music community. Most audiophiles prefer the sound quality of vacuum tube electronic amplifiers over semiconductor amps.

Single envelope vacuum tubes

Single envelope vacuum tubes

Another notable application is high-power RF transmitters. Vacuum tube diodes generate more power than semiconductor counterparts. So, you’ll find vacuum tubes in MRI scanners, particle accelerators, and even microwave ovens. That concludes this article. If you have any questions, feel free to contact us. We’ll be happy to help.

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Emma Lu
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