Electron tube testing methods and parameters

Electron tube testing method

1. Appearance inspection

1. Observe the color of the top of the electron tube. The color of the top of a normal electron tube is silver or black. If the top has turned into milky white or light black, it means that the electron tube has leaked or aged.

2. Observe whether there is any debris in the tube. Gently shake or flick the glass shell of the electron tube with your fingers, and then turn it upside down a few times to carefully observe whether there are debris, white oxides, broken mica pieces and other debris inside. If there is any debris in the electron tube, it means that the tube has been vibrated strongly in the center, and there is a high possibility that the internal inter-electrode short circuit is present.

2. Use a multimeter to detect

1. Measure the filament voltage. Use a multimeter R×1 to measure the resistance of the two filament pins of the electron tube. The normal value is only a few ohms. If the measured resistance is infinite, it means that the filament of the electron tube is broken.

2. Detect whether the electron tube is aged. By using a multimeter to measure the emission capacity of the cathode of the electron tube, you can determine whether the electron tube is aged. During the test, the working voltage can be provided to the filament of the electron tube alone (no voltage is added to the other electrodes), preheat for about 2 minutes, use the multimeter R×100 gear, connect the red test lead to the cathode of the electron tube, and the black test lead to the grid (the 1.5V battery in the meter is equivalent to adding a positive bias grid voltage to the electron tube), and measure the resistance between the grid and the cathode. For a normal electron tube, the resistance between the grid and the cathode should be less than 3kΩ. If the resistance between the grid and the cathode of the electron tube is greater than 3 kΩ, it means that the electron tube has aged. The larger the resistance value, the more serious the aging of the electron tube.

What are the main parameters of electron tubes?

The main parameters of the electron tube include filament voltage, filament current, screen current, screen internal resistance, screen voltage, screen grid voltage, inter-electrode capacitance, amplification factor, conductivity, output power, etc.

(I) Filament voltage

The filament voltage VF refers to the rated working voltage of the electron tube filament. The filament voltages of electron tubes with different structures and specifications are also different. Usually, the filament voltage of the electron diode is 1.2V or 2.4V (double diode), the filament voltage of the electron tube with three or more electrodes is 6.3V, 12.6V (composite tube), and the filament voltages of some direct-heated electron tubes, low internal resistance tubes, beam tubes, etc. are also 2.5V, 5V, 6V, 7.5V, 10V, 26.5V and other specifications.

(II) Filament current

The filament current IF refers to the working current of the electron tube filament. The filament currents of electron tubes with different structures and specifications are also different. For example, for both beam tetrodes, the filament current of FU-7 is 0.9mA, while that of FU-13 is 5A.

(III) Screen internal resistance rP

Screen internal resistance refers to the ratio of the change in screen voltage VA to the corresponding change in screen current IA when the grid voltage VC remains unchanged.

(IV) Amplification factor μ

The amplification factor refers to the ratio of the effective values ​​of the two electric fields formed by the grid voltage VG and the screen voltage VA on the surface of the electron tube cathode k, or the ratio of the change in the grid voltage VG to the change in the corresponding screen voltage Va when the screen current Ia remains unchanged.

The amplification factor is used to reflect the amplification capacity of the electron tube. Usually, triode electron tubes with amplification factors greater than 40 are called high amplification factor tubes, triode electron tubes with amplification factors less than 40 and greater than 104 are called medium amplification factor tubes, and triodes with amplification factors less than 10 are called low amplification factor tubes.

(V) Conductivity S

Conductivity refers to the ratio of the change in gate voltage VG to the change in screen current Ia caused by the change in VG when the screen voltage VA is a constant.

Conductivity is used to measure the control ability of the gate voltage of the electron tube on the screen current.

(VI) Inter-electrode capacitance

Inter-electrode capacitance refers to the distributed capacitance between the electrodes of the electron tube.