Vacuum tubes, commonly referred to as valves, are designed to control, alter, create, or intensify electrical indicators by manipulating the traffic of electrons in a low pressure space. Once used in electronic devices, such as televisions, radios, and computers, vacuum tubes are become increasingly obsolete and are now only used in a few specialised devices. These devices include electric guitar amplifiers and armed forces applications. Many musicians prefer vacuum tubes over their more modern counterpart, the transistor, in applications such as stereo amplifiers because they produce less distortion. The armed forces still find vacuum tubes useful because tube electronics are not affected by the radio waves caused by atomic explosives. Vacuum tubes are available in various types and can be made of materials, such as glass, ceramics, or metal.
Each vacuum tube is made up of a glass envelope, which is the tube itself; two diodes; a plate, which acts as an anode; a filament, which acts as a cathode; and a grid. Choosing the right vacuum tube is important for getting the best tone from an amplifier. Both the type and brand of the tube can affect the amplifier's tone. When purchasing vacuum tubes buyers should familiarise themselves with the various types, consider their advantages and disadvantages, learn how to identify used tubes, and understand new old stock vacuum tubes. It may be possible to buy vacuum tubes at specialist audio retailers, but the best option is to shop at eBay.
Types of Vacuum Tubes
There are many different types of vacuum tubes. They are divided into four general categories: the diode, triode, tetrode, and pentode.
Diodes were invented by John Ambrose Fleming in 1904. These vacuum tubes are almost exclusively used for changing alternating current (AC) to direct current (DC). They consist of a plate made of two electrodes: a cathode and anode. Together, these electrodes allow the current to surge in a single direction.
In 1906, just two years after the creation of the diode, triodes were invented by Lee de Forest. These vacuum tubes are made up of three electrodes, as well as a cathode and an anode. Triodes also have a grid that consists of a screen electrode together with a cathode and anode. The grid varies from positive to negative, affecting the surge of electrons from anode to cathode. This manipulates the flow.
Tetrodes were created by Albert Wallace Hull in 1926 as a substitute for triodes, whose reliability was below average when used in early radios. Tetrodes are secondary grid devices that have four electrodes. They are designed to increase the intensification of energy at high rates of recurrence. One popular tetrode, the beam tetrode, utilises distinctive design methods and has shaped plates. As the plates form a beam, they centre the electrons. This helps them to concentrate on specific parts of the anode, making the device more efficient.
In 1928, Bernard D.H. Tellegen invented the pentode to compensate for problems related to the tetrode. This vacuum tube contains five electrodes. The inadequate energy of the tetrode prevented the electrons from reaching the cathode. So, as an alternative, the pentode was built as a suppressor grid, to drive back the electrons that had fled to the anode.
Advantages and Disadvantages of Vacuum Tubes
Vacuum tubes offer buyers many advantages, especially when compared to transistors which are designed to serve the same purpose. However, there are some disadvantages that buyers should bear in mind when making a decision. The table below lists the advantages and disadvantages of vacuum tubes when compared with their counterparts.
Advantages of Vacuum Tubes
Disadvantages of Vacuum Tubes
Superior sound quality
Highly linear with positive feedback
Smooth clipping is widely considered more musical
Tolerance for large overloads and voltage spikes
Wider dynamic range due to higher operating voltages and overload tolerance
Highly independent of temperature
Simpler circuit design
Tubes can be replaced easily
Bulky, therefore, less suitable for portable devices
Higher operating voltages typically required
High power consumption
Glass tubes are fragile, compared to metal transistors
Higher associated cost
Create heat, which wastes up to 70 percent of power
High failure rate
The amount of space that a single vacuum tube requires can easily accommodate multiple transistors on a circuit board. Also, transistors use power more efficiently. These factors mean that the transistor has widely replaced the vacuum tube in most applications, such as televisions, radios, and computers. However, in some instances, vacuum tubes are more efficient or have a characteristic that makes them essential for a specific use. These instances include the use in radio station transmitters, guitar amplifiers, and communication satellites.
New Old Stock Vacuum Tubes
New old stock vacuum tubes (NOS) were made in previous years, but have never been used. These vacuum tubes should show no signs of burns or discolouration, with the exception of armed forces tubes which may show burn-in markings. Armed forces tubes are burnt in to ensure that they function properly, as most defective tubes burn out within the first 48 hours of use. Burn marks look like dark spots on the silver part, or flashing, of the glass tube. Most NOS vacuum tubes feature a manufacturer's logo on the glass, along with the vacuum tube type and a date code.
The vacuum tube pins should all be straight. Bent pins may indicate signs of use. However, this should not deter a buyer from purchasing the vacuum tube, as pins are known for bending easily. It is a good idea to inspect the locator pin of the NOS tube. The locator pin is moulded to the base of the vacuum tube and has a distinct shape which allows it to be inserted into a socket. Buyers should avoid buying vacuum tubes with a broken locator pin as installing the tube in the wrong position can result in severe damage to a buyer's device. All NOS vacuum tubes should come in their original packaging.
Checking Used Vacuum Tubes
When searching for vacuum tubes, buyers should consider their condition. Burn marks are a good indication that the tubes have been used, except in the case of armed forces vacuum tubes as described above. Another indication that the tubes have been used is a halo of colours around a spot of flashing. Vacuum tubes with very little flashing, or any white parts of flashing, may have experienced a loss of the vacuum. Buyers should avoid tubes with white parts on the flashing. Discolouration around the base of that tube that makes it darker that the rest of the base is likely to have been caused by heat. This denotes many hours of use. Other indications include heat cracks in the base and scrapes on the tube base left from tube holders or butterfly clamps.
How to Buy a Vacuum Tube on eBay
Buying a vacuum tube on eBay is a good idea because they may not be widely available. The selection of vacuum tubes sold on eBay is larger than may be found elsewhere. On eBay you have opportunity to browse the vacuum tubes to find just the right one at the best price. To navigate to eBay's assortment of vacuum tubes, buyers should use the search bar located on the homepage. Typing "vacuum tubes" yields a list of available items. If buyers know precisely which brand of vacuum tube they need, then it is possible to search by brand, such as "Philips vacuum tube".
When buying a vacuum tube on eBay, buyers should read the item description carefully. Sellers should indicate clearly the condition of the vacuum tube as well as listing the tube's test results. It is always a good idea to review a seller's feedback rating before placing a bid. Feedback ratings can offer an insight into a seller's reliability. For complete confidence throughout the buying process, buyers should look for Top-rated sellers. Top-rated sellers are recognised by the eBay community for dealing fairly. These sellers receive consistent positive feedback ratings and are known for providing high levels of customer service.
Vacuum tubes are electronic devices that produce energy flows in a controlled environment. Their primary function is the conversion of AC power to DC power. Once prominently used in home electronics, vacuum tubes are now only used for select applications such as older styles of amplifier for guitars. Choosing the right type of vacuum tube is important: buyers must determine which type is compatible with their device. For example, each type of vacuum tube alters the sound of an amplifier. Buyer's should also bear in mind the condition of the vacuum tube. Many older amplifiers do not allow for buyers to change the type of the tube. So, buyers must search for older models, such as new old stock (NOS) tubes. These tubes were manufactured some time ago, but have never been used. Whether buying old stock or used vacuum tubes, it is important to inspect them carefully for signs of use. Burn marks, discolouration, and scrapes should serve as indications that they may have suffered wear. eBay sellers offer a selection of vacuum tubes.