Such tubes are no longer in production, having been fully replaced by semiconductors. This is a special kind of beam tetrode, with a pair of "beam plates" to constrain the electron beam to a narrow ribbon on either side of the cathode. Also, the control and screen grids have their wire turns aligned, much like the large ceramic tetrodes above. Unlike the ceramic tetrodes, the grids are at a critical distance from the cathode, producing a "virtual cathode" effect.
All this adds up to greater efficiency and lower distortion than a regular tetrode or pentode. The first popular beam tetrode was the RCA 6L6, introduced in Beam tetrodes still made today include the SV6L6GC and SVC; the former is most popular in guitar amplifiers, while the latter is the most common power tube in modern high-end audio amplifiers for the home. Today this design is seen only in glass tubes used in audio amplifiers, not in ceramic power tubes. An oxide-coated cathode can't heat itself, and it has to be hot to emit electrons.
So, a wire filament heater is inserted within the cathode. This heater has to be coated with an electrical insulation that won't burn up at the high temperatures, so it is coated with powdered aluminum oxide.
This is an occasional cause of failure in such tubes; the coating rubs off or cracks, so the heater can touch the cathode. This can prevent normal operation of the tube. And if the heater is running from AC power, it can put some of the AC signal into the amplifier's output, making it unusable in some applications.
Good-quality tubes have very rugged and reliable heater coatings. We want a good, hard vacuum inside a tube, or it will not work properly.
And we want that vacuum to last as long as possible. Sometimes, very small leaks can appear in a tube envelope often around the electrical connections in the bottom. Or, the tube may not have been fully "degassed" on the vacuum pump at the factory, so there may be some stray air inside. The "getter" is designed to remove some stray gas. The getter in most glass tubes is a small cup or holder, containing a bit of a metal that reacts with oxygen strongly and absorbs it.
In most modern glass tubes, the getter metal is barium, which oxidizes VERY easily when it is pure. When the tube is pumped out and sealed, the last step in processing is to "fire" the getter, producing a "getter flash" inside the tube envelope. That is the silvery patch you see on the inside of a glass tube. It is a guarantee that the tube has good vacuum.
If the seal on the tube fails, the getter flash will turn white because it turns into barium oxide. There have been rumors that dark spots on getters indicate a tube which is used.
Sometimes, the getter flash is not perfectly uniform, and a discolored or clear spot can occur. The tube is still good and will give full lifetime. Glass power tubes often do not have flashed getters. Instead, they use a metal getter device, usually coated with zirconium or titanium which has been purified to allow oxidation.
These getters work best when the tube is very hot, which is how such tubes are designed to be used. The Svetlana A and SV use such getters. The most powerful glass tubes have graphite plates. Graphite is heat-resistant in fact, it can operate with a dull red glow for a long time without failing. Graphite is not prone to secondary emission, as noted above.
And, the hot graphite plate will tend to react with, and absorb, any free oxygen in the tube. The Svetlana SV series and B use graphite plates coated with purified titanium, a combination which gives excellent gettering action. A graphite plate is much more expensive to make than a metal plate of the same size, so it is only used when maximum power capability is needed. Large ceramic tubes use zirconium getters. Since you can't see a "flash" with such tubes, the state of the tube's vacuum has to be determined by electrical means sometimes by metering the grid current.
A typical glass audio tube is made on an assembly line by people wielding tweezers and small electric spot-welders.
They assemble the plate, cathode, grids and other parts inside a set of mica or ceramic spacers, then crimp the whole assembly together. The electrical connections are then spot-welded to the tube's base wiring.
This work has to be done in fairly clean conditions, although not as extreme as the "clean rooms" used to make semiconductors. Smocks and caps are worn, and each workstation is equipped with a constant source of filtered airflow to keep dust away from the tube parts.
Once the finished assembly is attached to the base, the glass envelope can be slid over the assembly and flame-sealed to the base disc. A small glass exhaust tube is still attached, and enters the envelope.
The tube assembly is attached to a processing machine sometimes called a "sealex" machine, an old American brandname for this kind of device. The exhaust tubing goes to a multistage high-vacuum pump. The sealex has a rotating turntable with several tubes, all undergoing a different step in the process.
See more pictures of glass tube assembly and production. If you want to control a LOT of power, a fragile glass tube is more difficult to use. So, really big tubes today are made entirely of ceramic insulators and metal electrodes. Otherwise, they are much the same inside as small glass tubes--a hot cathode, a grid or grids, and a plate, with a vacuum in-between. In these big tubes, the plate is also part of the tube's outer envelope.
Since the plate carries the full tube current and has to dissipate a lot of heat, it is made with either a heat radiator through which lots of cooling air is blown, or it has a jacket through which water or some other liquid is pumped to cool it.
The air-cooled tubes are often used in radio transmitters, while the liquid-cooled tubes are used to make radio energy for heating things in heavy industrial equipment.
Such tubes are used as "RF induction heaters", to make all kinds of products--even other tubes. Ceramic tubes are made with different equipment than glass tubes, although the processes are similar. The exhaust tubing is soft metal rather than glass, and it is usually swaged shut with a hydraulic press. All the equipment for exhausting and conditioning the tube is much larger, since there is more volume to exhaust, and the large metal parts require more aggressive induction heating.
The ceramic parts are usually ring-shaped and have metal seals brazed to their edges; these are attached to their mating metal parts by welding or brazing. Many big radio stations continue to use big power tubes, especially for power levels above 10, watts and for frequencies above 50 MHz.
The reason is cost and efficiency--only at low frequencies are transistors more efficient and less expensive than tubes. Making a big solid-state transmitter requires wiring hundreds or thousands of power transistors in parallel in groups of 4 or 5 at a time, then mixing their power outputs together in a cascade of combiner transformers. Plus, they require large heat-sinks to keep them cool.
An equivalent tube transmitter can use only one tube, requires no combiner which wastes some power , and can be cooled with forced air or water, thus making it smaller than the solid-state transmitter.
This equation becomes even more pronounced at microwave frequencies. The space between the grid wires allows electrons room to pass through to the plate. The grid controls electron flow and is commonly called the control grid. Electron flow is controlled by changes in plate voltage. In the triode, the grid also affects electron flow. For example , a negative grid will repel many electrons back to the cathode.
This limits the number of electrons passing on to the plate. As the grid is made more and more negative, a point is reached where no electrons flow to the plate. This is the cutoff point of the tube. It is the negative voltage amount applied to the control grid that stops electron flow. The voltage applied to the control grid is called the bias voltage.
At cutoff, it is called the cutoff bias. Triode: voltage applied to the grid controls plate anode current. A triode with both plate and grid voltage is shown in Figure 5. Notice that the grid bias battery has its negative terminal connected to the grid. In electronic work, these voltages have specific names such as A, B, and C. The A voltage is for the heaters in the tube.
The B voltage is for the plate of the tube. The C voltage is for the grid of the tube. Figure 5. This triode circuit shows the connections for plate voltage and grid bias voltage. Figure 6 shows current through an electron tube as the grid bias is changed. The plate voltage is held at a constant value. The curve in this graph is plotted by measuring the value of current at each change of grid voltage. At a grid bias of negative two volts, the current is eight mA.
At negative six volts, the current drops to three mA. Figure 6. The change in plate current as a result of change in grid voltage. Without neutralization circuits, the triode is limited as an amplifier due to the shunting effect of electrode capacitance at high frequencies, Figure 7. To overcome this drawback, another grid is inserted in the triode. This grid is called the screen grid. It is placed between the control grid and the plate. The resulting four-element tube cathode, control grid, screen grid, and plate is called a tetrode, Figure 8.
Figure 7. Dotted lines show capacitance between the elements in a tube. Figure 8. Symbol for a tetrode. The screen grid is bypassed to ground, externally, through a capacitor. The grid is a good screen between the control grid and plate, and it stops the grid-plate capacitance CGP.
In looking at how a vacuum tube works it is also necessary to consider the effectiveness of the way in which electrons escape from the surface. The number of electrons emitted from the heated material per unit area is related to the absolute temperature as well as a constant 'b' that is a constant indicating the work an electron has to do to escape the surface. When working with temperatures of this order, it limits the materials that can be used on the cathodes of vacuum tubes.
Although, normally vacuum tubes are indirectly heated these days, this form of heating is less efficient than the directly heated option. As a result, some specialist tubes or valves that use tungsten or thoriated tungsten filaments sometimes use direct heating techniques. The electrons flowing between the cathode and the anode form a cloud of electrons and this is known as the "space charge".
The space charge tends to repel electrons leaving the cathode, forcing them back. However if the potential applied to the anode is sufficiently high then the space charge effect will be overcome, so that electrons will flow toward the anode. As the potential is increased on the anode, so the current increases. X-ray tubes are used to generate X-rays for medical or research purposes. When a high enough voltage is applied to the vacuum tube diode X-rays are be emitted, the higher the voltage the shorter the wavelength.
To deal with heating of the anode, caused by electrons hitting it, the disc-shaped anode rotates, so the electrons hit different parts of the anode during its rotation, improving cooling. In a monochromatic CRT a hot cathode or a filament acting as a cathode emits electrons. On their way to the anodes they pass through small hole in the Wehnelt cylinder, the cylinder acting as a control grid for the tube and helping to focus the electrons into a tight beam.
Later they are attracted and focused by several high voltage anodes. This part of the tube cathode, Wehnelt cylinder and the anodes is called an electron gun. After passing the anodes they pass the deflection plates and impact the fluorescent front of the tube, causing a bright spot to appear where the beam hits.
The deflection plates are used to scan the beam across the screen by attracting and repelling electrons in their direction, there are two pairs of them, one for the X-axis and one for the Y-axis. A small CRT made for oscilloscopes, you can clearly see from the left the Wehnelt cylinder, the circular anodes and the deflection plates in the shape of the letter Y. Traveling-wave tubes are used as RF power amplifiers on board communication satellites and other spacecraft due to their small size, low weight and efficiency at high frequencies.
Just like the CRT it has an electron gun in the back. The radio wave flowing through the helix interacts with the electron beam, slowing and speeding it up in different points, causing amplification.
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