Thermionic Valves Buying Guide

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Thermionic Valves Buying Guide

The thermionic valve, or vacuum tube, is a vital component found in tube amplifiers and other audio equipment. It is a small device that controls electronic current. This technology is considered vintage and has largely been replaced by solid-state transistors, but there are still musicians and audiophiles who favour the warm sounds produced by valve-based audio equipment.

If a thermionic valve is required to repair an older, vintage radio, or a tube amplifier, one can search secondhand shops or charity shops, but searching venues like these is not an efficient way to find a replacement thermionic valve. Audiophiles often turn to the Internet when looking for thermionic valves, and more specifically eBay. Looking for the right type of thermionic valve is a process that involves knowing when it is time to replace an old or worn out thermionic valve, the different types of thermionic valves available, and how to assess the condition of the thermionic valve offered for sale.

What Is a Thermionic Valve?

A thermionic valve, or tube, is a vacuum in a sealed container. This container is generally thin, transparent glass made in a cylinder shape. The simplest thermionic valve is the diode, which is basically an incandescent light bulb with an extra electrode inside it.

Thermionic valve technology was first discovered by Frederick Guthrie in 1873 and further developed by Thomas Edison. By the early 20th century a diode tube was used to detect radio signals, but the development of the triode tube in the early 1920s led to the development of electronic amplifiers, the first coast to coast telephone lines, and a revolution in radio technology.

Types of Thermionic Valves

In the 1920s, the 6L6 vacuum  tube, as it was called, was modified for radio applications by placing a top cap on the tube. The resulting vacuum tube was dubbed the 807 and was widely used in the Second World War for military transmitters, and continued to be used after the war. These two thermionic valves were the most successful vacuum tubes ever manufactured. The chart below highlights the different types of thermionic valves, their individual features, and the applications that use thermionic valves.

Type of Thermionic Valve




Low resistance to current flow, two electrodes

Radio demodulation, power conversion, over-voltage conversion, temperature measurements


Electronic amplification, three electrodes

High-end professional audio, microphone pre-amplifiers, electric guitar amplifiers, RF amplifiers and transmitters


Small oscillating current, four electrodes

Frequency multiplication, and negative resistance oscillators


High current output, five electrodes

High-end and professional audio equipment


Radio reception, seven electrodes

RF/IR amplification, frequency mixer

The tubes used before the War had special plastic or metal bases that attached to a glass envelope to hold the pins, but after the War, advances in miniaturisation and manufacturing processes made it possible for the pins to be mounted directly into the glass envelope, making the tubes smaller and reducing production costs.

The Rise of Solid State Technology

The advent of solid state technology changed the way electronic devices functioned. In devices with solid state technology, no tube is present because the current travels through solid materials rather than a thermionic valve. This technology was first presented in the 1930s, however, it was not until transistors were introduced in the 1940s that solid state technology really took off.

Today, most devices use the solid state technology, because solid state devices can be made smaller. The solid state audio technology is more efficient and cheaper than tube-based audio equipment. However, far from being defunct, there is still a sizable market for thermionic valves. Audiophiles and especially guitarists argue that tube amplifiers produce the best sound. Tube amplifiers and vintage radio do not reproduce sound more accurately compared to solid state technology, rather a tube amplifier contributes a warmer tone to the overall sound.

Replacing a Thermionic Valve

A guitarist knows its time to replace a tube in their tube amplifier when they hear an audible hissing or crackling sound. There may also be a perceptible lack of power. If the bass can no longer be adjusted by using the equaliser, then the problem is clearly the tube.

Once tube is identified as the problem, buying a new tube is imperative. It is important for the buyer to do a little research to determine the correct replacement tube. Each tube should have identifying information, making the task of locating a replacement thermionic valve a lot easier. Before rushing off to the local vintage electronics store, search eBay. This is because many electronics shops may no longer carry tubes. eBay has a vast selection of replacement thermionic tubes to choose from, and as an eBay member, a buyer has access to whole community of likeminded musicians and audiophiles eager to help out a fellow enthusiast find the correct thermionic valve.

Choosing the Correct Valve

Some modern amplifiers allow different types of tubes to be swapped out so that the user can choose the type of valve that imparts the best tone to the sound. Every type of thermionic valve has a character and tone of its own. Before choosing a particular tube, a buyer should read up on the various types of thermionic valves and determine which one is best meets their audio needs. A musician, should try a few different valves, because they can often be switched out to create different sounds.


This tube provides the 'big Fender amp tone'. The 6L6GC is generally used in the American-made amplifiers. It is favoured by musicians who love a sound with firm lows and pronounced highs. Two of these thermionic valves generate about 40 to 50 watts in a class AB amplifier, while four of these valves put out up to 100 watts. This tube is compatible with almost every amplifier.


For a smaller amplifier, this thermionic valve is perfect. This tube is used almost exclusively in American-made amplifiers from the 1950s, 1960s, and 1970s. It produces about half the output of the 6L6GC tube, allowing a musician to create a sound with pronounced distortion, if that is the sound they are going for.


For a big, British 'crunch' tone, the Marshall tube is capable of operating at higher voltages and producing more output than the 6L6GC. This thermionic valve is used for high-gain amplifiers.


The EL84 is sometimes called the 'baby EL34' and is another classic British output tube. This tube is used in AC15 and AC30 amps, as well as Class A circuits. Two of these thermionic valves in a cathode-based amplifier put out up between 15 and 18 watts, and four tubes will double that output. Many modern amps that mimic the Class A tone use this type of valve.


This tube is a direct substitute for the 6L6GC thermionic valve, but has a sound all of its own. Of European origin, the sound is bolder, firmer and rounder than the 6L6, and can put out more volume.


In the mid-1960s to mid-1980s, this tube was used in Marshall amplifiers to replace the EL34 valves that were in short supply at the time. This thermionic valve has a sound very different from the EL34, and sounds more like a bigger and louder version of the 6L6. This tube is most commonly used as a big output tube in large bass amps.

Buying Considerations for Thermionic Valves

Most of the thermionic valves offered are what are referred to as NOS valves. These valves are old stock, or valves that were never used, but just sat on shelves in their original packaging, for years, sometimes decades. If the buyer cannot inspect the valves in person, they should ask for photos of the valves listed for sale. When inspecting the photos, the buyer should check to see if there is evidence that the valves were used. For instance, used valves have burn marks that look like dark coloured spots on the silver base of the valve. Used thermionic valves have scratches on their plastic bases caused by contact with the tube clamps that hold them in place. Also, with the thermionic valves, the base could be discoloured. Light-coloured bases generally darken due to use, and there are often cracks in the the glass. These cracks are the result of the valve getting really hot. All these are are signs that the tubes have been used.

How to Buy a Thermionic Valve on eBay

When you are ready to start searching for the right thermionic valve, simply enter keywords into the search bar that describe the thermionic valves you are looking for. You will find this search bar on any page on eBay. If you enter a generic search phrase like 'thermionic valves' the search engine will populate a results page with current listings for thermionic valves.

You are able to narrow the search results by selecting various search filters and sorting the listings based on the the brand and the condition of the valve. Read the item description very carefully, and as stated earlier, it is very important to examine photos of the valves to determine whether or not the valves have been used in the past. Look for signs of wear and tear, such as cracks and discolouration. If you have any questions for the seller, you can contact the seller by clicking on their username, going into their profile, and clicking on the contact link.


Audio equipment that utilises thermionic valve technology is favoured by vintage audio enthusiasts and musicians because of the warm, mellow sounds that can't be reproduced by solid-state equipment. Because there is still a market for valve-based audio equipment, collectors, antique radio enthusiasts, and musicians can still source replacement thermionic valves. Eventually all valves need to be replaced. If a thermionic valve is required for a tube amp or vintage radio, the buyer should first research all of the types available and what voltage they require to operate properly. As vacuum tubes can be switched out to create different sounds, many buyers look into a collecting a variety of thermionic valves. When shopping for a thermionic valve, it is important that the buyer determine if the tube is new, NOS, or used. If one owns a device that needs a replacement thermionic valve, the best place to search for the right tube is on eBay.

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