Basic triode vacuum tube circuit.

Brief Overview of Vacuum Tubes and Circuits

Above is a vacuum tube triode used for RF amplification in early television tuners. Marketed as a Nuvistor. To quote,

The Nuvistor vacuum tube was invented by RCA (Radio Corporation of America). It was announced in 1959 as a type of vacuum tube designed to compete with the then-emerging bipolar junction transistors.

Vacuum tubes are used in numerous applications, including RF and audio amplifiers, voltage rectifiers, and voltage regulators.

They operate by thermionic emission, where a metal (cathode) is heated, "boiling" off electrons in a vacuum. Without our resistance, free electrons fill the space.

A positively charged metal plate (anode) attracts the electron to move to the positive side of the power supply. This anode or plate supply voltage can be hundreds or 12 volts with late-designed "space charge" tubes for car radios.

With a plate and cathode alone, the vacuum operates as a rectifier due to charges moving in one direction, from cathode to anode (plate).

When a mesh electrode or "grid" is placed between the cathode and plate, a negative charge relative to the cathode controls the level of electron flow between the cathode and plate. This configuration is called a triode, and it operates as an amplifier.

By varying the voltage on the grid, you can either attract or repel electrons. A negative voltage on the grid repels electrons, reducing the current to the anode. A positive voltage on the grid attracts electrons, allowing more current to pass to the anode.

Small changes in the grid voltage can cause large changes in the anode current, which is the basis for amplification in vacuum tubes. This is because the grid controls a large number of electrons with relatively small voltage changes.

Early vacuum tubes used the heater (or filament), a hot tungsten wire, as a cathode. Later designs used a separate cathode isolated from the filament used only as a heater.

Heat management is critical. Heat is needed at the cathode to allow the emission of electrons. However, heat is also created at the plate (anode), particularly with beam power tubes. Examples include audio output tubes in guitar amplifiers and horizontal and vertical output tubes in vacuum tube televisions.

Vacuum tube applications didn't end overnight, as it took several years for solid-state to develop. In car radios, where 12-volt operation "space charge" vacuum tubes flourished, the audio power outputs were PNP germanium transistors. In contrast, the small signal radio frequency circuits were still "space charge" type vacuum tubes. Even those would be replaced by transistors when prices came down.

Types of Tubes:

Diode: Simplest form with just a cathode and anode, used for rectification.
Triode: Adds a grid for control, used for amplification.
Tetrode/Pentode: Additional grids for better performance (like reducing unwanted capacitance effects).

In vacuum tube televisions, the opposite took place. Low-power RF and audio circuits went to a solid state first, and high-power output tubes went last

Vacuum tube rectifiers gave way to selenium rectifiers, later replaced by silicon B+ rectifiers.

To summarize according to GROK:

There are various types of vacuum tubes like diodes (which have only a cathode and an anode), triodes (with a cathode, anode, and one grid), tetrodes (with an additional screen grid to reduce capacitance between anode and control grid), and pentodes (with yet another grid called the suppressor grid for better performance at high frequencies).

Vacuum tubes were crucial in early electronics for amplification, signal processing, and computing before being largely replaced by transistors due to the latter's smaller size, lower power consumption, and reliability. However, they're still used in some high-fidelity audio equipment, certain guitar amplifiers, and other specialized applications where their unique sonic characteristics are valued.

Electron flow in a Crook's tube under high voltage.

Electron flow from negative to positive in a Crook's tube turns a small pin wheel.

The electron was discovered in 1897 by J.J. Thomson during his experiments with cathode rays.

Vacuum tube history:
1883 Edison discovers thermionic emission.
1904 Fleming develops the first vacuum diode.
1906 De Forest invents the triode Audion tube.
1920s-40s Vacuum tubes dominate electronics industry.
1947 Transistor is invented, leading to tube decline.
2000s Revival of interest in audio and display applications.

Vacuum tube Black and White television chassis.
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Vacuum tube diode half wave power supply.
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Are vacuum tubes still manufactured for Musicians?

Yes, vacuum tube amplifiers are still made for musicians, though the industry has changed in recent years.

Production: Vacuum tubes are still made, but in smaller quantities than before. There are only a few factories left in the world that produce vacuum tubes, and none of them are in the United States.

Demand: Vacuum tubes are still used in high-end guitar amplifiers and stereo equipment. Audiophiles and audio engineers praise tube gear for its rich midrange and extended frequency response.

Supply chain: The tube industry has been affected by a number of factors, including the Russian invasion of Ukraine, export restrictions on Russian-made tubes, and a fire at a tube manufacturer in China. The closure of Shuguang, the world's largest tube manufacturer, in 2019 also contributed to a shortage.

Manufacturers:Some companies that produce vacuum tube amplifiers include Audio Hungary, Audio Research, and Western Electric. Western Electric, a factory near the Tennessee-Georgia border, plans to resume production of guitar amp tubes.

Designers: Some Japanese designers, like Ken Shindo, are known for their vacuum tube amplifiers.

Here's a summary based on general knowledge and trends up to 2024:

Niche Markets: Vacuum tubes have found a sustained niche in several areas:

Audio Equipment: Audiophiles and musicians often prefer the sound of tube amplifiers for guitars and high-fidelity audio systems due to the unique tonal qualities and harmonic distortion that tubes produce, which many consider warmer or more pleasing than solid-state alternatives.

High-End Electronics: Some high-end audio equipment manufacturers continue to use vacuum tubes for preamps and power amps where the characteristic sound of tubes is desired.

Radio Transmitters: Certain broadcasting stations, especially those using older equipment or requiring specific transmission qualities, might still use vacuum tubes.

Military and Industrial Applications: Vacuum tubes can be more resistant to electromagnetic pulses (EMPs) and radiation, making them useful in certain military applications or in space technology where reliability under extreme conditions is necessary.

Manufacturing:

Legacy Production: A few traditional factories, mainly in Russia, China, and Eastern Europe (like Slovakia), continue to produce vacuum tubes. These facilities often cater to both new demand and replacement parts for older equipment.

New Manufacturers: There has been a resurgence of interest leading to boutique manufacturers in various countries, including the USA, producing vacuum tubes for the enthusiasts' market. These might focus on quality, improved design, or specific sound characteristics.

Modern Innovations: While maintaining the traditional aspects of vacuum tube technology, some modern manufacturing might incorporate newer materials or slight modifications to improve longevity or performance.

Challenges and Trends:

Demand vs. Supply: The demand for tubes isn't what it was during their peak usage era, leading to fewer manufacturers. However, the niche demand ensures they're still in production.

Price: Due to the specialized nature of current production, vacuum tubes can be quite expensive compared to mass-produced semiconductor components.

Research and Development: There's ongoing research into making vacuum tubes more compatible with modern technology, perhaps reducing size or power requirements, though these are more experimental or for very specific applications.

6L6 vacuum tube power pentode.

What is the cost of a 6L6 vacuum tube?

The 6L6 is a popular power audio out tube. As of December 19, 2024 a "Brand New Current Matched Quad (4) Electro-Harmonix 6L6GC Vacuum Tubes" is advertised on Ebay for $170. Four 6L6s used and tested is $51.

Answer: Based on information from various sources, including online marketplaces and discussions on X, the cost of a 6L6 vacuum tube can vary significantly based on brand, condition (new old stock vs. new production), and specific type (e.g., 6L6GC, 6L6G, etc.). Here's a general overview:

NOS (New Old Stock) Tubes: These can be quite expensive due to their rarity and preferred sound characteristics by audiophiles and musicians. For instance, an NOS RCA Black Plate 6L6GC might go for around $349.99 per tube, as seen from specialized vacuum tube sellers.

Current Production Tubes: New production tubes are generally less expensive than NOS. For example, a Sovtek 6L6WXT+ might be priced significantly lower, around $29.99 to $39.99 per tube, depending on the seller and any additional features like low noise or microphonics.

Special Variants and Brands: Prices can fluctuate with tubes like the 7581A Philips ECG Sylvania, which might be priced around $39.99 per tube, indicating that specialized or branded versions could command different prices even among current productions.

Market Sentiment from X: While specific prices aren't directly quoted from X posts, there's an indication of shock over high prices, suggesting that some 6L6 tubes might be listed or discussed at prices that users find surprisingly high, like several hundred dollars.

General Market Observation: From both the search results and X posts, it's clear there's a wide range for pricing. Vintage or highly sought-after models can reach into the hundreds of dollars per tube, while modern productions from current manufacturers might be available for much less, appealing to a broader user base.

Remember, these prices are indicative and can vary based on where you're buying from, the condition of the tube, its rarity, and its audio characteristics. If you're looking to purchase, it would be wise to check multiple sources for the best deal or to ensure you're getting the specific type of 6L6 tube you need for your equipment or collection.

Vacuum Fluorescent Display Pin Connections

Vacuum Fluorescent Display

How does a vacuum fluorescent tube operate?

On Ebay "US Stock! 12 pcs IV-6 / IV6 / IV 6 Nixie Era VFD vacuum tubes NOS NEW" at $40.

A Vacuum Fluorescent Display (VFD) operates on principles that combine aspects of vacuum tubes, neon lights, and modern display technology.

A thin wire filament (cathode) is heated, which emits electrons through thermionic emission. This filament is usually coated with materials like barium, strontium, or calcium oxide to enhance electron emission at lower temperatures.

The anodes (one for each number segment plus decimal point for 8) are phosphor-coated segments or characters positioned in front of the filament. Each anode can be a segment of a digit, a complete digit, or even a pixel in more complex displays.

A mesh "grid" is placed between the filament and the anodes. This grid controls the flow of electrons from the filament to the anodes. By applying a positive or negative voltage to the grid, you can allow or block electrons from reaching the anodes. This also controls brightness.

A positive voltage is applied to the grid and the specific anodes that need to be lit. This positive potential attracts the negatively charged electrons, accelerating them towards the anodes.

When these electrons strike the phosphor coating on the anodes, the kinetic energy of the electrons excites the phosphor, causing it to emit light. Different phosphors can be used to produce different colors, although greenish-blue is most common in consumer electronics.

By selectively charging the grid and (the 8) anodes, you can control which segments light up, thus forming numbers, letters, or other symbols. For instance, to display the number "1," only the anodes corresponding to the segments needed for "1" would be energized.

In more complex displays or to save on connections, VFDs often use multiplexing. This means that not all segments are lit at once; instead, they are lit in rapid succession so fast that the human eye perceives them as continuously illuminated.

Advantages:

Brightness and Visibility: VFDs can be very bright and are easily visible in both dark and bright environments.

Color Variety: They can display various colors depending on the phosphor used.

Longevity: They tend to have a long life if not subjected to extreme conditions.

Disadvantages:

Power Consumption: They require more power compared to LCDs, especially for heating the filament.

Heat Generation: The filament heating produces heat, which might not be ideal for all applications.

Complexity: More complex to manufacture than some other display technologies like LCD or LED backlit displays.

VFDs were popular in the 1980s and 1990s for applications like VCRs, car dashboards, microwave ovens, and audio equipment but have largely been replaced by LCDs and OLEDs in modern devices due to their lower power consumption and versatility. However, they still find niche uses where their unique aesthetic or functional characteristics are desired.

12-volt high voltage generator creates streamer through xenon flash tube.
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The above material credit GROK the X AI with additions by Lewis Loflin.

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I've been a part-time adjunct professor at a local community college teaching electricity and electronics.

Today I do this for the shear love of electronics.

I have 45 years experience in electronics, from vacuum tubes to modern solid state and industrial controls. I tend to teach from a general science viewpoint.

Some sample projects. New 7/15/2024 on my electronics website.

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