How to Buy Thermionic Valves

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How to Buy Thermionic Valves

Thermionic valves (or vacuum tubes) are used to magnify or alter an electrical signal, and are used in radio transmission. They utilise a heated electrode and were invented in 1904 by John Ambrose Fleming. Thermionic valves are a very basic instrument but have been utilised in a broad spectrum of devices. Although they were ubiquitous in electrical devices for decades, the digital era and the advancement of technology have decreased their use. However, they are still useful for radio equipment and some older electronics. If replacing a valve, check its labelling to make sure the correct replacement part is purchased.

Buyers should understand how thermionic valves work, as well as their basic structure. They should also know how to read the valve's labelling. This helps buyers purchase or order the correct part and make sure it functions appropriately with the device into which it is being installed. An improper valve can damage the device, so buyers should be cautious.

To find a thermionic valve, buyers can check electronic stores and specialty shops. Thermionic valves are also available at a variety of electronics websites and other online marketplaces. For instance, eBay has a large selection of thermionic valves available, for many uses and in many price ranges.

Parts of a Thermionic Valve

Thermionic valves' basic structure includes the outer glass casing, a cathode, and an anode. They also usually feature one or more "grids". The cathode, the anode, and the grid are all electrodes. An electrode is a device which allows electrical current to pass through, and which is used to amplify or restrict the current.

The cathode, anode, and grid work together to transmit electrical current. Different thermionic valves have different numbers of electrodes, and therefore different power levels and uses. A diode is the simplest form of a thermionic valve. The diode consists of one cathode and one anode held within a small evacuated glass tube. When the cathode is heated, electrons from the cathode become increasingly active. They leave the cathode to enter the space around it. This process leaves a positive charge that is opposite and equal to the charge of the electrons. Because different charges attract, there is a force that pulls the electrons back.

The other electrode in a diode is the anode. If the anode has higher potential than the cathode, the electrons are attracted to it as they leave the cathode. This creates a "space charge", and the circuit completes itself with the current flow. Although thermionic valves started out as simple diodes, they quickly advanced as designers added more electrodes.

As mentioned above, there are now several types of thermionic valves in addition to the diode. The type of each valve depends on the number of electrodes present. For example, the diode has two electrodes, while a triode features three electrodes.

The table below lists names of each thermionic valve, the number of electrodes it has, and the number of grids.


Number of Electrodes

Number of Grids



















The size and materials used to create these components vary with the power and size of the thermionic valve. Buyers should understand the different parts of a valve, and should be familiar with the purpose each serves in transmitting and amplifying electrical currents.


Thermionic valves use hot cathodes to operate. Hot cathodes are an electrode that allows electrons to enter and subsequently power a device. Cathodes are negatively charged, and thus they attract positive electrons. There are a few different types of cathodes used in thermionic valves; the main difference is in the way they are heated or powered.

The first cathodes were heated by passing a current through a wire, called "direct heating". Direct heating cathodes were not efficient because they required a connection to a heat supply. A heat supply was also needed in the actual cathode, so two heating sources were necessary. The extra heating source sometimes caused a slight humming, which could be picked up in the electron signal.

An indirectly heated cathode is another, more advanced type of cathode. In an indirectly heated cathode, the heater not connected to the cathode, but rather heat is radiated to it. This means the thermionic valve takes longer to warm up, but this type of cathode is commonly used because it isolates the cathode circuit from the impact of the hum of the heater supply.

Another type of cathode is the dull emitter. A dull emitter cathode is directly heated, but provides more current emission and requires less heat, making it very efficient. Dull emitter cathodes typically have a long life, but are very fragile. An oxide coating is used on some hot cathodes that use indirect heating. The oxide coating reduces the temperature needed to operate the cathode. The oxide coated cathode is a form of nickel in the shape of a ribbon, of small cup shape.


The anode is another type of electrode; it surrounds the cathode and other electrodes. Valves are typically shaped like a tube so that the anode can pick up the maximum number of electrons present. In smaller valves, which are often used in radio receivers, the anode is made of nickel or nickel-plated steel. For higher powered applications, the anode must be able to dissipate more heat, so materials like carbon, molybdenum, and zirconium are used.


Grids add more refinements, and work as a filter between the cathode and the anode. The grid assists with a thermionic valve's performance. The grid is another electrode that allows the flowing current to pass through. Grids are typically mesh- or net-structured, and are made of nickel, or a nickel alloy. It is important that a grid does not overheat. If the grid overheats, the entire valve sometimes fails. To avoid overheating, the grid is sometimes carbonised, and sometimes has fins attached to assist with cooling.

Thermionic Valve Labelling

Thermionic valves are labelled with different letter classifications to make their differences clear to the user. The first letter on the valve label indicates the heat supply. The second letter indicates the type of valve. And the final letter indicates the base type.

The first table below shows the labelling for the heater supply.

First Letter

Heater Supply




200mA AC/DC


Battery Supply (1.2 to 1.4V)


6.3V AC/DC




2V Battery Supply


300 mA


100 mA AC/DC

Keep in mind that the classifications above are for European valve labelling, and are not worldwide standards. The next table lists the types of thermionic valves, and indicates which letters represent each type.

Second Letter

Valve Type


Single Diode


Double Diode




Triode Output Valve






Hexode or Heptode




Pentode Output Valve


Tuning Indicator








Gas Filled Full Wave Rectifier


Half Wave Rectifier


Full Wave Rectifier

Make sure that the valve purchased matches its intended use and purpose. Be sure to research any unknowns before purchasing the valve. Different valves work more efficiently and have different power levels that should be considered before making a purchase.

How to Buy Thermionic Valve on eBay

To begin a search for thermionic valves on eBay, enter "thermionic valve" into the search box on the eBay home page. Look over the options available. Try using alternative terms to broaden the search results. For example, searching for vacuum tubes, another term for thermionic valves, displays a lot of additional results.

Once you have found one or more valves that meet your needs, be sure to read the detailed item listing for any additional important details. Also carefully examine any included photographs, to make certain the item is what you want. If it is, make sure you read the seller's detailed information, especially feedback from previous buyers, to see how satisfied others have been with the seller's performance. Also check the seller's other listings to see if they have any other items available that you may be interested in. If you find multiple items that you like from one seller, you can sometimes combine the items into one package, which may reduce the cost of postage. If you are concerned about the postage cost, you can limit the sellers to individuals with listings close to you. To do this, specify your location in the detailed distance information on the eBay website.


Thermionic valves have supplied power to various electronic devices for decades. They've helped power radios, televisions, computer monitors, and many other important technological tools. They were invented in the early 1900s, and have truly been a fundamental building block of the modern world.

When purchasing a thermionic valve, buyers should know a bit about their basic structure and parts, as well as how each component works. The thermionic valve includes several parts; these include the anode, the cathode, the grids, and a glass casing.

Labelling on thermionic valves is important. Buyers need to know how to read and understand the letter codes to make sure they buy the correct type of valve. Buyers should remember that labelling of the power and parts of the thermionic valve is not universal, and it differs from country to country. Be sure to be familiar with foreign labelling if buying thermionic valves from abroad. Thermionic valves are sometimes difficult to find. Try searching for other common terms that they are known by, including "vacuum tubes", a term commonly used to describe thermionic valves in the United States.

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