Introduction to electron tubes

1. Filament voltage: V; 2. Filament current: mA; 3. Anode voltage: V; 4. Anode current: mA; 5. Grid voltage: V; 6. Grid current: mA; 7. Cathode access resistance: Ω; 8. Output power: W; 9. Transconductance: mA/v; 10. Internal resistance: kΩ.

Calculation of several commonly used values:

Amplification factor μ=anode voltage Uak/grid voltage Ugk
indicates the ratio of anode voltage to grid voltage while maintaining the anode current unchanged.

Transconductance S=anode current Ia/grid voltage Ugk
indicates how many units of anode current will change if the gate voltage changes by one unit (such as mV) while maintaining the anode voltage unchanged.

Internal resistance Ri=grid voltage Uak/anode current Ia
indicates how many units of anode voltage will change if the anode current changes by one unit (such as mA) while maintaining the gate voltage unchanged.

The above values ​​can also be expressed as Amplification factor μ = transconductance S multiplied by internal resistance Ri. Let's

talk about these first. If you think you can read on, I will talk about several common tube types and structural working principles next time.

This time, let's talk about the structure and working principle of the electron tube. Taking care of our old habits, the tube types and unit circuits involved in the future will all take domestic tubes as examples. At the end, I will briefly talk about the respective characteristics and replacements of some common domestic tubes and imported tubes based on my own experience.

Before discussing, we must first define the scope of discussion, that is, it is limited to vacuum tubes.

Whether it is a vacuum tube with two electrodes, three electrodes or more electrodes, they all have a common structure, which is composed of a glass (or metal, ceramic) shell that is evacuated to a near vacuum and a filament, cathode and anode encapsulated in the shell. The filament of a directly heated electron tube is the cathode, and multi-electrode tubes with more than three electrodes also have various grids.

Let's talk about diodes first:
Consider a heated metal plate. When its temperature reaches more than 800 degrees Celsius, the electrons will accelerate and escape from the metal plate's attraction to them and escape to the space outside the metal surface. If a forward voltage of tens to tens of thousands of volts is added to this space (the cathode ray tube mentioned above has a high voltage of 7000-27000 volts on the anode), these electrons will be attracted to the forward voltage pole, flow through the power supply and form a loop current.
The metal plate (cathode), heating source (filament), and forward voltage plate (anode) are encapsulated in an appropriate shell, that is, the glass (or metal, ceramic) package shell mentioned above, and then evacuated to a near vacuum, which is an electronic diode.
It should be noted that due to the manufacturing process, impurity adhesion, and the material itself, a trace amount of residual gas will remain in the tube, and the finished tubes are coated with a layer of getter inside the tube. The getter generally uses nitrogen-doped evaporative zirconium aluminum or zirconium vanadium materials. At present, except for special purposes (such as ultra-high frequency and high-voltage rectification, etc.), in order to facilitate use and increase consistency, two diodes, or two-pole triodes, or three-pole triodes and two-pole pentaodes, etc. are assembled in one tube shell, which is a composite tube.

Next, let's talk about the triode:
the structure of the diode determines its unidirectional conductive nature. When a pole with an appropriate voltage is added between the cathode and the anode, this voltage will change the surface potential of the cathode, thereby affecting the number of cathode hot electrons flying to the anode. This is the modulation pole, which is generally made of a spiral grid made of metal wire, so it is also called the grid. This is what Sijiqing's friend said about the valve function. From this, we can know that when the signal voltage to be amplified is added between the grid and the cathode, its change will inevitably cause the anode current to change accordingly. Since the anode voltage is much higher than the cathode, the slight voltage change between the grid and the cathode can also cause the anode to produce a corresponding voltage change of dozens to hundreds of times. This is the principle of triode amplification of voltage signals.


The above picture! Lao Fei sacrificed a tube and cut it open for you to see:

This is a general-purpose dual triode 6N1 used for high-frequency amplification. 1 is the getter; 2 is the combination of the filament cathode and the grid; 3 is the anode.


Now break the glass shell and pay attention to the change in the color of the getter. In other words, once the getter of the tube turns into this milky white, the tube is useless regardless of whether the glass shell is broken or not.


Look carefully!
1: Anode; 2: Grid, the white part in the grid is the insulating layer of the grid and cathode; 3 is the cathode, which is a flat metal tube, and the filament is wrapped inside.


Next, let's talk about multi-grid tubes:

Common multi-grid tubes include quadrupole, pentode and heptode. Let's talk about pentode and heptode first. The quadrupole is more special and more than half of the models in commercial power amplifiers currently use this thing, so I will talk about it later.

The structure of the pentode is similar to that of the triode, but the difference is that it has two more grids than the triode, namely the screen grid and the suppression grid.

In general applications, the DC voltage added to the screen grid is equal to that of the anode. Its function is to help the anode to attract the electrons passing through the grid and accelerate them to fly to the anode. Therefore, for the same volume of electron tubes, the anode current of the tube with the screen grid is larger than that of the triode without the screen grid. In addition, the screen grid also plays a shielding role, thus improving the stability of the circuit operation.

Before understanding the role of the suppression grid, let's talk about a phenomenon: secondary electrons. When the filament heats the cathode, the anode will also be heated. Therefore, when the electrons flying out of the cathode hit the anode, some electrons will be knocked out from the anode plate. These are secondary electrons. In practical applications, the suppression grid must be connected to the cathode (so some tubes have already connected it internally). The purpose of adding the suppression grid is to use the equipotential of the suppression grid and the cathode to suppress the secondary electrons and prevent them from falling into the screen grid. In this state, the secondary electrons will be attracted by the anode again and fly to the anode again.

The structure of the heptode is similar to that of the pentode, but it has five grids. It is generally used in the frequency conversion circuit of radio reception. It has little to do with the audio amplifier circuit, so it is not mentioned.

Let's talk about the tetrode. Actually, the pure tetrode only appeared as a demonstration tube in the history of the development of electron tubes and has not entered practical use. This is another topic. Let's not talk about it. Now let's talk about the thing mentioned above that is used in more than half of the models of commercial power amplifiers-beam tetrode.

Smash more. Damn those bosses who smash XO in the bar, let's smash MULLARD!

Look at the picture below first.

All beam tetrodes are power tubes. The requirement for power tubes is to generate as large an anode current as possible. The beam tetrode has some special arrangements in the structure of the electrode, so that it can generate a larger anode current than other power tubes while maintaining a similar volume to other power tubes.

From the picture, we can see several structural features of the beam tetrode:
1. The cathode is elliptical, which increases the effective emission area of ​​the cathode, thereby increasing the emission of thermal electrons.
2. Like the pentode, a screen grid is added between the suppression grid and the anode. The function has been mentioned before.
3. A pair of bow-shaped metal plates are added between the screen grid and the anode (this is the key point, pay attention to the following statement), which is the beam screen. The beam screen is connected to the cathode in the tube, that is, it has the same potential as the cathode. It forces the electron flow that has crossed the screen grid to be beamed toward the anode along the opening direction of the bow-shaped metal plate. Okay, let's review the definition of current in junior high school: the electron flow flowing through a unit cross-sectional area per unit time. Here, when the electron flow is beamed out, the density must increase, so the anode current is cleverly increased. This is the key to the beam tetrode being able to form a larger anode current than other power tubes while maintaining a similar volume to other power tubes.