How to Buy Vacuum Tubes

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How to Buy Vacuum Tubes

A vacuum tube, also known as a valve, is a glass tube that contains electrons in a vacuum that is used to amplify electronic signals. Invented by John Ambrose Fleming in 1904, these devices were common in all electronics until they were replaced by the transistor after its invention in 1947. Even early computers worked with vacuum tubes, but in the late 1940s they required over 10,000. These computers took up over 93 square metres of space. Transistors revolutionised the field of consumer electronics, making televisions and radios smaller and more affordable. However, vacuum tubes are still used in modern audio applications as they deliver superior sound quality. They can be found in guitar amplifiers, professional and high-end audio equipment, and in high power radio frequency applications.

Before shopping for vacuum tubes, learn what kinds exist and what is available for sale. In addition, an understanding of how to choose the right vacuum tube for a particular device simplifies the buying process. Vacuum tubes are available from specialist electronics suppliers and online from sites such as eBay.

How Do Vacuum Tubes Work?

Understanding which types of vacuum tube are available requires some background knowledge on how these devices work. All modern vacuum tubes are based on the same design. A heated cathode releases electrons into a vacuum. These electrons pass through a grid that controls the current, before striking the anode and being absorbed. Through this process a small AC signal voltage is amplified into a larger one. Most tubes use two different types of cathodes: thoriated filaments or oxide-coated filaments. The anode is simply a plate, sometimes made of graphite. The control grid is usually constructed from a piece of plated wire that has been wound around two soft metal posts.

Types of Vacuum Tube

The number of grids present determines the type of vacuum tube. The table below lists types of vacuum tube according to the number of plates or grids in the plate array.

Vacuum Tube Type

Description

Triode

Single grid

Commonly used in guitar amplifiers

"Low-mu" or low gain devices seen in high-end audio amplifiers

"High-mu" or high gain tubes used in radio transmitters and high-power audio amplifiers

Tetrode

Second grid added between the control grid and anode

Increases tube's gain

Pentode

Third grid added between the anode and second grid

Commonly used in guitar and high-end amplifiers

Pentagrid Converter

Five grids

Used as front-end frequency converter in radio receivers

No longer manufactured

In addition to the grid array, vacuum tubes come in different classes. These classes describe the way that the tubes work. The table below lists and describes vacuum tube classifications.

Vacuum Tube Classification

Description

Class A

Tube conducts the same amount of current at all times

Inefficient with electricity

Very low distortion

Class A1

Grid voltage is more negative than cathode voltage at all times

Yields greatest possible linearity

Class A2

Grid voltage is more positive than cathode voltage for part of all of waveform

Requires special driver circuit

Class AB

Only applies to push-pull amplifiers

When the grid in one tube is driven until it cuts off, the other tube takes over

More efficient than Class A

Higher distortion

Class B

Only applies to push-pull audio amplifiers

Like class AB, but tubes idle at or near zero

More efficient than Class A or AB

Higher distortion

Ultralinear

Uses only beam tetrodes or pentodes and special taps on output transformer

Taps connect to screen grids of tubes, so screens are driven with portion of output signal

Lower distortion

Usually seen in hi-fi amplifiers

As can be seen from the table above, some vacuum tubes are more energy efficient than others, while the amount of distortion produced also varies with the vacuum tubes used. Choosing the right vacuum tube has a pronounced effect in the sound quality produced by audio equipment.

How to Choose the Right Vacuum Tubes

The type of vacuum tubes chosen depends on the device in which they are used. They come in a range of sizes and voltages. The best way to check which tubes are required is to look at the tubes installed in the device. If the model numbers have rubbed off the vacuum tubes due to age, find the manual for that device, invest in a vacuum tube handbook, or research the device online through hobbyist forums and search engines. It is important to note that tubes do not work simply because they fit the available socket. Thousands of tubes feature an identical basing, but have vastly different specifications.

Some feel that different vacuum tube brands such as Dyson and Miele produced in an earlier era offer superior sound quality. If possible, users should experiment with different brands of vacuum tubes in order to create the sound desired from audio equipment. Although experts can give some advice with regard to vacuum tube brands, the best way to find the right one is to listen to the sound produced when using a number of different devices. However, variations in sound quality may be so slight that only true audiophiles would find this beneficial.

New, New Old Stock, and Used Vacuum Tubes

Vacuum tubes are generally available as new, new old stock (NOS), or used parts. New vacuum tubes have been manufactured recently and never used. NOS vacuum tubes are mostly used in vintage devices. These tubes are new, have never been used, but were created at an earlier date. Used parts have been installed and used before they are sold.

The only way to determine whether a vacuum tube works is to test it electronically. However, when buying new or NOS vacuum tubes, always inspect the tube itself for signs of wear. Ask the seller to remove the tube from the box for inspection. When buying online, ask for photos of the tube, not the box. In some cases, through no fault of the seller, used tubes were placed in new boxes. This was simply the result of careful repairmen who replaced vacuum tubes that were used, but placed the used tubes in the boxes of the replacement tubes. Check for lettering on the tubes that has been rubbed off due to handling to get an idea of the condition of the vacuum tubes.

How to Buy Vacuum Tubes on eBay

The first step in buying vacuum tubes on eBay is finding the vacuum tube that you need. To do this, simply type a search phrase, such as "RCA vacuum tube", into the search field, which is conveniently located on every page, to display the listings available. Refine your search by choosing the most appropriate category and the desired item specifications. Use the advanced search function if you would like to see more or fewer listings.

Learn About the Item and the Seller

Once you have found the vacuum tubes that you require, click on an individual listing to access the item listing page. Here you find detailed information about the listing, including the full item description and details about the item's condition, photos of the item, buying options, accepted payment methods, postage and packaging costs, and the seller's feedback information. If you would like more information about the listing, use the "Ask a question" link to contact the seller.

eBay's seller feedback feature allows you to see what other buyers have to say about a specific seller's products and customer service. Click on the number next to the seller's username to see this information. Detailed seller ratings provide additional information regarding aspects such as the reasonableness of postage and packaging costs and the accuracy of the item description in the listing. Once you have received the product from the seller and the transaction has been completed, remember to let other buyers know about your experiences by leaving feedback for the seller.

Conclusion

Vacuum tubes are used to amplify electronic signals and consist of glass tubes that contain vacuums filled with electrons. Although these devices have been largely replaced by transistors, vacuum tubes are still used in guitar amplifiers, high-end and professional audio equipment, and high-power radio frequency applications. Many audiophiles feel that the smoky sound delivered by systems that use vacuum tubes is superior to the sound yielded by systems that use transistors.

Vacuum tubes consist of heated cathodes that release electrons into a vacuum. These pass through a control grid before striking the anode, or plate, and being absorbed. The number of additional grids inside the device determines whether it is a triode, tetrode, or pentode vacuum tube. In addition, vacuum tubes are classified according to how they work, with variations in electrical efficiency and distortion levels across various classes.

Choosing the right vacuum tube depends entirely on the device that they are used for. It is best to research individual devices or to invest in a handbook that can be used to learn general guidelines before buying. Ample advice and information is also available online, through forums and specialist vintage audio hobbyist sites. Buyers can choose between new, new old stock (NOS), or used tubes. NOS tubes should not have been used at all. To ensure that the tubes are working, ask the seller whether they have been electrically tested. New, NOS, and used vacuum tubes are available from a number of eBay sellers.

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