How to Buy Vacuum Tubes on eBay

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How to Buy Vacuum Tubes on eBay

Vacuum tubes are electrical devices that allow electrical current to be passed through and magnified. They are no longer used for most modern day electronics, as they have been replaced by different types of transistors. However, vacuum tubes are still used in some hi-fi audio systems and high-powered amplifiers. Although they are a very basic instrument, they were originally utilised for lots of different electronics. Users replacing a vacuum tube in an amplifier or hi-fi audio system should check the labelling of the old vacuum tube to find important details and information.

Before buying a vacuum tube replacement, buyers should understand how it works. They should be aware of the tube's basic structure, as well as some important aspects of its labelling. Labelling of vacuum tubes is sometimes confusing, so it is a good idea to determine the device's vacuum tube requirements.

Vacuum tubes are not difficult to find or to purchase, but specific information about the amplifier's electrical requirements is needed to determine the proper type. Vacuum tubes can be found in electronic stores, audio retailers, and many music specialty shops. They are also available at a variety of websites online. For instance, eBay has a wide selection of every type of vacuum tube imaginable available for purchase. When shopping for vacuum tubes on eBay, some considerations buyers must keep in mind are the vacuum tube's components, its labelling, the quality of manufacture, its price, and whether or not the tube is compatible with the buyer's device.

Vacuum Tube Labelling

The labelling on vacuum tubes tends to be a long string of letters and numbers that, at first, can appear very confusing. The characters on the label indicate the filament voltage, the number of internal parts in the vacuum tube, and its model number. The first letter on the label indicates the voltage of its heat supply or filament, while the second letter indicates the model type and the number of internal electrodes.

The first table lists the possible first letters found on the vacuum tube label and the corresponding type of heater supply.

First Letter

Heater Supply




200mA AC/DC


Battery Supply (1.2 to 1.4V)


6.3V AC/DC



Knowing the tube type is important, as it depicts the power levels available for both the tube and the device. Some vacuum tubes fit into the socket but are not necessarily compatible with the equipment. The table below lists the possible second letters found on the vacuum tube label, and the corresponding tube type.


Tube Type


Single Diode


Double Diode




Triode Output Tube






Hexode or Heptode




Pentode Output

Classifications of vacuum tubes vary by country, and this labelling method is commonly used in the United Kingdom. When searching for a specific type of vacuum tube, look for tubes that match the amplifier, hi-fi audio system of the device in question. Different tubes work more efficiently with different devices, and this should be considered before making a purchase. The literature accompanying devices such as radios, amplifiers, and hi-fi audio systems should recommend one or more specific vacuum tubes that are suitable for that device.

Parts of a Vacuum Tube

Most vacuum tubes need between 200 and 400 volts to function properly, while transistors need much lower voltages to operate. Many audiophiles insist that vacuum tubes produce a better quality audio than transistors, which is why vacuum tubes remain readily available, even though they are an aging technology. The basic structure of a vacuum tube includes an outer glass casing, a cathode, an anode, and one or more "grids". The size and materials used to create these components vary with the power and size of the vacuum tube. To do any maintenance on a device, it is helpful to recognise the different parts of a vacuum tube and to understand the purpose each serves.

An electrode is a device that can transmit and amplify electrical currents. The number of electrodes in a vacuum tube differs, depending on the type of vacuum tube it is. The more electrodes in a tube, the greater the electrical amplification achieved. A diode (or two electrode tube) is the simplest form of vacuum tube. It consists of one hot cathode and one anode, held together within a small evacuated glass tube. When the cathode is heated, electrons from the cathode become increasingly active; they then leave the cathode and enter the space around it. This process leaves a positive charge that is opposite and equal to the charge of an electron. Because different charges attract, there is a force that pulls the electrons back. The other electrode in a diode is the anode. When the anode has a higher potential than the cathode, the electrons are attracted to it when they leave the cathode.


Hot cathodes are electrodes that transmit electrical signals as they heat up. The cathode interacts with the anode to pass electrical signals in a vacuum tube. The earliest cathodes used a method called direct heating, and were heated by passing a current through a wire. Directly heating a cathode is not efficient because it requires a connection to the heat supply, which also needs to run the cathode. In other words, two sources are needed to accomplish one function. Because of this, a slight humming is sometimes picked up on the signal due to interference from the heater. On the other hand, an indirectly heated cathode features a heater that is disconnected from the cathode; heat is radiated to the cathode from a short distance away. This prevents interference from the heat source, but it also means that indirect cathode vacuum tubes take longer to warm up.


The anode interacts with the cathode and any other electrodes. This is why valves are typically shaped into a tube, so that the anode can pick up the maximum number of electrons leaving the cathode and create the current flow. Smaller valves, which are often used in radio receivers, use anodes made of nickel or nickel-plated steel. For higher powered applications, the anode must be able to dissipate more heat, and other materials are required. Some common materials found in an anode include carbon, molybdenum and zirconium. The types of materials used depend on the size of the vacuum tube.


The grid is another type of electrode that allows flowing current to pass through. Grids typically are mesh-shaped, and are made of nickel or a nickel alloy. If a grid overheats, the vacuum tube may fail. For this reason, is it important to avoid overheating of the grid. Grids are sometimes carbonised, and some have added fins to increase their surface area, which helps them to cool down.

Price of Vacuum Tubes

The price of vacuum tubes can vary greatly. Sometimes an older model vacuum tube is more expensive than a current one. The reason for this is that many older models have been discontinued, so there are fewer of them available. Brands such as Dyson and Miele are usually more expensive but dominate the vacuum industry and offer the best quality.


Vacuum tubes use electrodes to amplify and conduct electrical signals. They were invented in the early 1900s, and over the decades, vacuum tubes have powered many types of electronics. They were a giant step on the path to our modern, digital life. Recently, they have largely been replaced by transistors, and are now used mainly in amplifiers and hi-fi audio systems.

When purchasing a vacuum tube it is important to know a bit about its basic structure and parts. Buyers should also understand how each component works, and how to read the label on a vacuum tube. Knowing how to read the labels makes it much easier for shoppers to select the right equipment. Labelling conventions are different from country to country, so buyers should be familiar with the country's labelling system when purchasing vacuum tubes from abroad.

Vacuum tubes are sometimes called "thermionic valves". The inner components of a vacuum tube include the anode, the cathode, one or more grids, and an external glass casing.

eBay has a wide selection of vacuum tubes (or thermionic valves) of every type and in prices to satisfy any budget.

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