How to Improve the Sound Quality and Service Life of Vacuum Tubes

The invention of the electron tube is as dramatic as the discovery of penicillin and tires: there were several uncleaned lab dishes near the windows in the laboratory, and some mold accidentally floated in from the window and landed on the dishes. Scientists were surprised to find that some mold that fell into the lab dishes could inhibit the spread and growth of bad bacteria. After experimental analysis, this mold became one of the effective and widely used antibiotics; the same scene also happened in an experiment studying rubber, when sulfur in a glass was accidentally broken and poured into melted rubber liquid. After solidification, the rubber became a hard and tough material. The electron tube is certainly not made of a few metal plates sealed in a vacuum glass bottle for experiment for no reason. It has a story with the inventor Edison.
The direction of the current and the flow of electrons are exactly opposite

Before that, let me ask a small question: Is the direction of "current" in circuit analysis the same as the direction of "electron" flow in reality? The answer is no, the direction of current and electron flow are exactly opposite. Scientists in the past were unable to observe the direction of electron flow, so they unified the statement and set one pole of the battery as the positive pole, its voltage was positive, and the current flowed from the positive pole to the negative pole to form a closed loop. Because everyone unified the statement and practice, there were no conflicts for many years, until modern scientists had more sophisticated equipment, and after observation, they overturned the previous statement: "It turns out that electrons flow out from the negative end of the battery"! (In other words, electrons flow out from the negative end of the speaker of the amplifier and flow back from the positive end of the speaker)

As a user, you don't need to care about which one is true, just follow the conclusions of scientists. This is because after Edison invented the light bulb, he found that the filament of the light bulb he produced always burned out from the positive end, so he further experimented by adding a small metal plate to the light bulb. After lighting the light, he connected the metal plate to the meter, applied positive and negative voltages respectively, and observed the current.

For the scientists at that time, it was impossible for metal plates in a vacuum and not connected to generate current no matter how they were connected. But something strange happened. Edison discovered that a certain substance (actually electrons) would pass through the metal plates and "jump" from the negative pole of the battery to the positive pole. This discovery certainly aroused greater experimental motivation, and this phenomenon was called the "Edison effect." This was also the first time that scientists questioned the direction of current flow and the phenomenon of free electrons flowing in space.

The reason why metals can conduct electricity is that they have more free electrons, which facilitates the mutual flow of electrons. Therefore, electronic materials must be made of materials with good conductivity. Electrons also have another characteristic: negatively charged electrons are easily attracted by positive voltages, so-called like charges repel each other and opposite charges attract each other. From the Edison effect, we know that when metal substances are heated, free electrons active around protons are prone to become free. High temperature leads to increased electron activity. At this time, if there is a strong positive voltage in space, the free electrons will flow in space. Based on these knowledge that were already known at the time, Fleming (J.A. Fleming) manufactured the first diode electron tube in 1904, and De Forest Lee improved the diode and manufactured the first triode in 1907. Since the diode was successfully developed, the application of electron tubes began to be realized, and the development of electron tubes has been advancing by leaps and bounds since then. (See Figure 1 for details)

The triode is the most basic electron tube

The electron tube is also called "vacuum tube", which means that the inside of the glass bottle is evacuated to facilitate the flow of free electrons and effectively reduce the oxidation loss of the filament. Diode, triode, and pentode literally represent the number of basic "poles" inside the electron tube. The electron tube has three most basic poles. The first is the "cathode" (represented by K): the cathode is of course negative, it is the place where the electron flow is released, it can be a metal plate or the filament itself. When the filament heats the metal plate, the electrons will be freed and scattered in the small vacuum glass bottle. The second pole is the "plate" (represented by P), which is basically the outermost metal plate of the electron tube. The outermost dark gray or black metal plate of the electron tube is usually the plate. The plate is connected to a positive voltage, which is responsible for attracting the electrons emitted from the cathode (using the principle of opposites attracting each other) as the end point of the electron's free travel. The third pole is the "gate" (G). From the structural point of view, it is like a circle of thin coils, just like a fence, fixed between the cathode and the screen. The electron flow must pass through the gate to the screen. By passing voltage between the gates, the flow of electrons can be controlled. It acts like a faucet, with the function of flow and blocking.

The engine needs fuel to run, and the electron tube uses electricity as its working power. Among the electrodes of the electron tube, the most important one should be the cathode, which is responsible for releasing electrons, which is the basis of all work.

The earliest electron tubes had a simple construction principle and used the filament directly as the cathode. In other words, when the filament was lit, the temperature of the filament increased, and electrons were released from the filament and went straight to the screen through the grid. This type of electron tube is called a "direct-heated electron tube". 300B is a tube of this type. Compared with other modern pentode electron tubes, 300B has a simple structure and low output power.

Filaments can be made of different materials. Since the direct-heated triode directly uses the filament as the cathode, the characteristics of the filament directly affect the performance of the direct-heated electron tube. Basically, the filament of the electron tube can be mainly divided into three types of materials. The first type is of course the high-temperature resistant tungsten wire. The high-purity tungsten wire is drawn into a thin filament and wound around the innermost layer of the electron tube. After the power is turned on, the temperature can be increased. However, the tungsten wire must be heated to more than 2,000 degrees before the electrons can be dispersed. Therefore, when the electron tube made of tungsten filament is ignited, it will emit a brilliant brightness and the temperature is frighteningly high. Don't be surprised, it's not that the electron tube is going to burn out, but that it is like this! However, it takes a lot of electricity to light up the tungsten filament. The advantage is that the tungsten filament is very durable and is generally used in electron tubes with higher power or longer life. In some cases, the life of this vacuum tube can reach tens of thousands of hours. It can be used as a light bulb at home. It is both durable and decorative, killing two birds with one stone! Another type of filament uses thoriated tungsten alloy, which only needs to be heated to more than 1,000 degrees to work, which is more energy-efficient. The most commonly used filament should be alkaline earth oxide, which is made by coating the outside of the filament with a thick layer of alkaline earth oxide, which looks like a white-gray substance. It only needs to be heated to about 70 degrees (appearing to be about dark red) to obtain sufficient electrons, so the operating temperature is the lowest and the most energy-efficient. Generally speaking, it only needs to be supplied with a DC of about 6.3V to work normally.

Direct-heated tubes certainly have their inherent advantages, but they have a fatal disadvantage, which is that the cathode is easily changed due to the temperature change of the filament. When the filament voltage changes, or when the filament is supplied with AC power, the cathode is in an unstable state. Therefore, some people advocate that direct-heated tubes should be powered by DC, while others emphasize that AC power must be used to avoid damaging the cathode. This debate has long been a controversial topic in the audio industry.

The stability of the indirectly heated electron tube is higher

In order to solve the filament problem of directly heated electron tubes, electron tube designers decided to separate the filament from the cathode and put a metal sleeve next to the filament to allow the filament to directly heat the metal plate, and electrons are emitted from the metal plate. This heating method is called a "directly heated electron tube."

In this way, the electron tube seems to be much more stable. Since the volume and heat storage capacity of the metal sleeve are much larger than those of the traditional filament, even if the temperature of the filament changes temporarily, or even stops heating for a few seconds, the temperature change of the metal plate is limited. This is the main reason why some electron tubes can still sing for more than ten seconds after they are turned off. Since the cathode and the filament are independent, the cathode plate must be heated indirectly by the filament, so the filament is changed to tungsten filament material again for durability, and a layer of white magnet is coated on the outer layer of the tungsten filament, which is insulated on the one hand and has a shaping effect on the other hand. Since the indirect heating effect is poor, thorium, barium or other substances that are conducive to electron dispersion will be coated on the cathode metal plate. Therefore, the metal plate of the electron tube always looks gray and black, unlike the normal metal plate. Also, because the production and assembly must rely on manual labor, there will always be many small scratches on the metal plate, so users do not have to worry about accidents when purchasing electron tubes.

What are the differences between direct-heated and indirect-heated tubes? For general users, they don't need to care about the difference between direct-heated and indirect-heated tubes. However, for designers, indirect-heated tubes usually have a larger filament current due to indirect heating, and the indirect-heated structure must heat the cathode metal plate, so there is a slow warming period after starting up. If it is the front stage, a delay design must be made to prevent the startup pulse from damaging the back stage.

According to the development process, the earliest electron tubes were of course directly heated. The diode was developed first. The function of the diode is like the current diode transistor, which has the functions of rectification and internal detection in the radio. The diode can also become a voltage regulator after proper design. Since the working principle of the electron tube is very simple, many scientists joined the research and development work after the first electron tube was successfully manufactured. The first triode was successfully manufactured by an American scientist in 1907, which opened the advent of the radio era, bid farewell to the phonograph, and entered the amplifier era.

Working principle of electron tube

Now, let's take a step further and look at the working principle of the simplest electron tube.

After disassembling an electron tube, it is drawn in the attached figure. From the figure, we can see that when the filament is lit, the temperature of the filament gradually rises. Although it is in a vacuum state, the temperature of the filament is transmitted to the cathode metal plate in the form of radiant heat. When the temperature of the cathode metal plate reaches the temperature at which electrons are freed, the electrons will fly out of the metal plate. At this time, the electrons are negatively charged. When a positive voltage is added to the screen, the electrons will be attracted and fly toward the screen metal plate, passing through the grid to form an electron flow. The grid is like a switch. When the grid is not charged, the electron flow will stably pass through the grid to reach the screen. When a positive voltage is added to the grid, it has an attractive effect on the electrons, which can enhance the speed and power of the electron flow. On the contrary, when a negative voltage is added to the grid, the electrons must take a detour to reach the screen due to the principle of like charges repelling each other. If the structure of the grid is large, the electron flow may be completely blocked.

The gate can be used to easily control the flow of electrons. The input signal is connected to the gate, and a proper bias is added. A resistor is connected to the gate to amplify the signal. Like transistors, electron tubes have a variety of amplification forms (in fact, the amplification form of transistors is an extension of the application of electron tubes). By combining different electronic materials such as resistors, inductors, transformers, and capacitors, a variety of electronic products can be created.

If you observe the inside of the tube wall of the electron tube, you can see a thin film similar to mercury adhering to the glass wall. This is a design that extends the life of the electron tube. Except for a very small number of low-voltage electron tubes (not referring to the low working voltage, but referring to the low-pressure gas inside the electron tube), most electron tubes must be evacuated to work properly. The pins of the electron tube are metal pins. Although they are encapsulated with glass, there is still a chance of leakage between the glass and the metal pins. The metal vapor deposition (i.e., degassing agent) in the glass tube will react with the gas. Its purpose is to absorb the gas to maintain the vacuum inside the electron tube. After this thin layer of metal is oxidized, it will turn white, indicating that the electron tube has leaked. Therefore, if the electron tube is broken, this layer of vapor deposition will also turn white. Therefore, when buying old electron tubes, you should also pay attention to the condition of the vapor deposition. It is better to be like mercury. If it starts to turn pale and peel off, it means that the electron tube has entered old age.