A Guide to Buying Thermionic Valves

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A Guide to Buying Thermionic Valves

Thermionic valves, also referred to as vacuum tubes or electron tubes, is a tube that controls electrical currents through a vacuum sealed glass container. The devices have been key to the development of electronic technology since the days of inventor Thomas Edison. Thermionic valves contributed to advancements in broadcasting, television, radar, sound production and recording, telephone communication, and analogue computers. It was the abilities of the vacuum tubes to amplify electronic currents that made all of these advances in technology possible.

Today the valves are not as widely used as they were in the mid 1900s because they have been replaced by new technology. However, they are still used for some applications such as radio and high-power transmitters because their performance still cannot be matched by the solid-state devices that are now used in most things. The vintage thermionic valves are still highly sought after, though, because older equipment cannot run without them so they have become somewhat of a collectors’ item.

History of Thermionic Valves

The development of the thermionic valve began in the 1880s when Thomas Edison discovered an electrical current flowing between the heated filament and a wire inside one of his light bulbs. Though he did not fully understand his discovery, he patented it as the “Edison Effect” in 1884 because he believed that it may be a way to control the flow of electrical currents. Later, a man by the name of John Ambrose recognised that this way of conducting electricity could possibly be used to send and received radio waves. He patented his theory in 1904 and named it the “oscillation valve”. The valves were in inconsistent though because they held residual amounts of gas that affected their performance, uniformity, and reliability. No two valves were the same. In 1913 Harold Arnold from the American Western Electric company invented a high-vacuum hard valve that was much more reliable and by 1925 thermionic valves were used widely in radio transmissions. In the 1940s and 1950s the valve became miniaturized and then it was eventually replaced by transistors. However, they continued to be used in military equipment, television, and radio.

Understanding Thermionic Valves

Thermionic valves consist of two or more electrodes that are inside of a vacuum casing. The enclosure can be made of glass, ceramic, or metal, but they are usually glass. When the filaments get hot, they release electrons into the vacuum sealed case. The released electrons then go to the second electrode, creating a positive charge. The electrons then pass through a grid which controls the current. They then hit the anode and are absorbed.

The Number and Lettering System of Thermionic Valves

The naming systems for thermionic valves generally vary based on the country they were made in. In one commonly used notation, the first two digits in the name represent the filament voltage, the two letters after than represent the model number, and the last digit represents the number of internal elements. For example, the 12AX7 valve runs on 12 volts, is the AX model, and has 7 internal elements. Some of the valves have more letters after the sequence which are often meaningless and simply refer to the different editions of the model. Other common notations may refer to the same valve with a completely different system; the 12AX7 may also be called an ECC83 or an 7025, depending on location and use.

Types of Thermionic Valves

Thermionic valves were made in four main versions. Diodes, Triodes, Tetrodes, and Pentodes were used throughout the 20th century. The vintage valves are one of these four models.

Diode Thermionic Valves

Diodes were invented by John Ambrose Fleming in 1904 as a result of the Edison effect. It consisted of a plate, two electrodes, a cathode, and an anode. It has the ability to send currents in one direction as well as convert alternating currents to its set direction. The cathode would emit electrons through thermionic emissions which would then flow to the anode. Later on the valve was improved by using a crystal set. The diode had to be adjusted to perfection in order for the crystal to make contact but this made the diode vibration resistant so it became ideal for use in the military. However it was replaced in the 1960s by silicon rectifiers.

Triode Thermionic Valves

Unlike diodes, triodes have three electrodes rather than two. A cathode and anode are still there and a grid is present to filter the surges and manipulate the flow by varying from positive to negative. Lee De Forest is credited with the invention of the triode in 1907. The result of the varied charges of the grid creates the ability to amplify the electrical output. It is the amplification ability that made triodes idea for audio enhancement. However, triodes were found to be unsatisfactory for radio use because they had a tendency to oscillate.

Tetrode Thermionic Valves

Tetrodes were created by Walter H Schottky as an attempt to find a better performing vacuum tube for sound amplification. In 1919 he added a second grid, called the screen grid, to the tube that was placed between the anode and the original grid. In addition to being more reliable, it ended up being capable of producing more voltage than the triode. The tetrodes had four electrodes, two grids, a cathode, and an anode. Tetrodes were not perfect though. The secondary grid caused problems because it would sometimes act as an anode and capture secondary emissions of electrons. This would cause the voltage to drop due to negative resistance. They named the issue the “tetrode kink”.

Pentrode Thermionic Valves

Bernard D. H. Tellegen found the solution to the tetrode kink in 1928 by adding a third grid into the the tube and increasing the number of electrodes from four to five. The third grid suppressed any secondary emissions from the screen grid. The pentrode design proved to be much more proficient.

Whether to Buy New or Vintage Thermionic Valves

When purchasing thermionic valves, people can choose between new valves, vintage ones, or new old stock (NOS) (new old stock). NOS products are old items that were never sold. It is essentially like purchasing a brand new vintage product. The thermionic valves sold to the public today are generally considered to be of lower quality than the originals. Most people search for vintage vacuum tubes for their amps because they have a better sound quality. However, the risk of buying vintage or NOS is that the valves may not work anymore. It is essential to make sure that they have been tested and are functional before buying any.

Where to Look Thermionic Valves

Those who are guessing that thermionic valves are fairly rare and difficult to find are correct. Music shops may have new valves that are sold with the amps but they do not have vintage or NOS versions. Second hand shops and antique shops are also not likely to have these because most people do not know what they are so they throw them away rather than sell them or give them to a shop to be sold. Because of this, the only place people can really find them is online. It is easier for companies and private people to sell these online because they have a wider customer base. Websites like eBay operate as a central hub where people from all over the world can come to buy and sell stuff.

Find Thermionic Valves on eBay

eBay make it possible for people to find rare items that they otherwise would not be able to get. If the product isn’t there, you can check back daily because sellers list new things constantly. Thermionic valves can be found by typing the name into the eBay search bar. Enter "Thermionic Valves", then click the Search button to view the results. If there are not a lot of listings, try searching by the other names like "Vacuum Tube" or "Electron Tubes". Not all sellers call the valves by the same name, especially if they are located in another region.

If shopping for vintage valves, be sure to take caution when ordering. Never purchase one that has not been tested and only order from sellers who accept returns if it ends up not working. Also beware of NOS claims. If the product is truly NOS, it comes with the original box with no wear and tear. The seller’s description should not say anything about it being used except to be tested to make sure it functions.


Thermionic valves have deep roots in history and have contributed greatly to the advancement of technology throughout the 19th and 20th centuries. They nearly single handedly made it possible to develop wireless technology, radio, computers, and televisions. They may be complex little devices but their value and capabilities should never be underestimated. Music connoisseurs today still recognise their importance and seek to find the older, original thermionic valves that were quality made and still produce some of the best sounds despite the fact that technology has advanced since then. Finding them can be a challenge but if people use the information provided then they can save themselves a lot of hassle and frustration by simply shopping on eBay. The website makes it possible for anyone to find different versions of the rare valves when they otherwise would not be able to.

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