The use of common sense in the tube (top)

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

DFN1006 TP-RCLAMP3321P ESD Diode Low Capacitance

Filter 18.432M

As a key hub device in the "amplifier" and various tube equipment, the quality and working condition of the tube will directly affect the sound quality of the "amplifier" and the working performance of the equipment. It is important to choose and use the tube reasonably and correctly. Here are some of the most basic and useful knowledge and skills that must be mastered.

Key point 1: When selecting an electronic tube, first determine the appropriate product type according to the characteristics and requirements of the specific application circuit. For example, for a general amplifying circuit, the pre-stage requires a higher voltage gain, and a high-amplification voltage amplifying transistor or a pentode should be used. The triode has less noise, but the gain is lower than the pentode; although the pentode has a high gain, the distortion is greater than the triode. Therefore, the first stage of the amplifier is usually a pentode, and the rear stage is usually a triode. For example, in the foremost stage where the working signal is extremely small, if an additional signal is required to automatically compress or expand the circuit, a remote cut-off pentode can be used. Otherwise, in order to avoid waveform distortion, especially in the rear front stage where the working signal is large, a sharp cut-off pentode should be used. For another example, when the electron tube is operated in a high frequency circuit, a high frequency electron tube whose working frequency meets the requirements and the interelectrode capacitance is small should be selected. Secondly, it should be ensured that the parameters of the selected tube meet the requirements of the application circuit, especially the limit parameters must have sufficient margin. For example, the tube used for the power amplifier stage should be selected according to the output power requirements. In the power amplifier tube, the distortion of the triode is small, and the internal resistance is also small; and the beam quadrupole has the advantages of high power sensitivity and small pushing power. This is why the general medium and small power amplifier stages use beam quadrupoles. For another example, when the power amplifier stage is a push-pull circuit, two power amplifier tubes (ie, "pairing tubes") having the same or very close characteristics should be selected. In general, the consistency of the triode is better and easier to select. However, due to the special arrangement between the grating wires, the beam quadrupole causes a deviation in characteristics, so although the models are identical, there are often large differences due to static and dynamic operating characteristics.

Point 2: The tubes are connected to the working circuit through special tube sockets. Common sockets are made of insulating materials such as ceramics or bakelite, and have sockets for inserting the pins of the tube and soldering pieces for the welding wires. The tube holder is also designed to match various tubes, but the number of jacks is generally the same as the number of pins of the corresponding tube. However, some tubes have only four legs, but the design also applies to the eight-legged socket. Figure 1 shows the physical picture of a common domestic small-sized electronic tube socket. The center of the small 7-legged and 9-legged tube seat is equipped with a grounding isolation seat, and the center of the 8-pin socket has a positioning hole (with a small gap). The round hole) coincides with the tube with the center "key" (ie, the positioning pin), and the insertion position is ensured when the tube is inserted.

The arrangement of the socket sockets is the same as that of the tube pins, that is, in the clockwise direction on the opposite side of the tube seat (the soldering piece faces upwards), and the left side of the socket (small GZC-7 or GZC-9 type tube socket) The first jack starts at 1, 2, 3, ..., or from the center of the hole (GZ-2C 8-pin socket), the first jack on the left, the number 1, 2, 3... . If the number is from the front of the tube, counterclockwise, starting from the first jack on the right side of the largest interval of the jack (small GZC-7 or GZC-9 type), 1, 2, 3... Or from the center hole (GZ-2C type 8-pin socket), the small hole on the right side of the first jack starts counting 1, 2, 3... When selecting the pipe socket, pay attention to the use of high-quality ceramic products. Inferior bakelite products and adhesive tape products are best not used, because the screen voltage of various tubes is up to several hundred volts. If the quality of the tube seat is not good, it will easily cause leakage. It produces noise or hum. At the same time, the connector of the socket must have good elasticity. It is not ideal if it is too loose or too tight. Especially when the small tube is too tight, it will easily cause the electronic pin to bend. In severe cases, the tube may be cracked or the pin may be broken.

Commonly used preamplifier tubes are mostly thumb or peanut tubes, such as 6N1, 6N2, 6N11, 12AX7, 12AU7, 6C1, 6J1, etc. GZC type porcelain 7-pin or 9-leg tube holders should generally be used; ordinary power amplifier tubes are generally bottles Glass tube (ST tube) or tubular glass tube (GT tube), such as 6P3P, 6L6, 6CA7, EL34, KT88, 6550, etc., generally use GZ-2C porcelain 8-pin socket; other power amplifier tubes such as 2A3, 300B, 211, 805, 807, 811, 845, etc., use medium-sized porcelain 4 feet and porcelain 5-legged sockets.

Point 3: "In order to extend the service life of the tube, it is necessary to achieve "five don't" in practical applications:

First, the filament voltage should not exceed ±10% of the rated value, and is preferably limited to ±5%. If the filament voltage is not supplied as specified, the life of the tube will be significantly shortened. For example, for a tube with a nominal voltage of 6.3V, the actual working voltage is preferably controlled within 5.9 to 6.6V, and must not be lower than or exceed 5.7 to 6.9V. Too low will cause the cathode to be "poisoned", and too high will cause the cathode to overheat, which will shorten the service life of the tube.

Second, the value of any tube should not exceed its maximum rating, and the parameters that reach the limit cannot be more than one. Even if it exceeds the limit for a short time, it will affect the service life. For example, when the screen loss power and the curtain grid loss power exceed their maximum allowable values, the former causes the board to be red hot and the cathode is damaged, thereby significantly shortening the life of the tube; when the latter is exceeded, the tube life is also shortened due to overheating.

Third, don't forget and violate the electrode power-up sequence. For example, the beam quadrupole or five-pole power tube must not be added with the screen grid voltage before the screen voltage is applied. Otherwise, the curtain grid current will be large and the tube will be overheated and red, and the light will shorten the life of the tube. Causes damage to the pipe. For another example, it is recommended to set the start-up cathode preheating, that is, after the cathode is energized and heated to a sufficient temperature, the gate negative voltage is first applied, and then a high-voltage power source such as a curtain grid or a screen electrode is added to prevent the cathode from being damaged and prolonging the service life.

Fourth, do not install power tubes and rectifiers with large heat generation in a non-vertical manner to avoid heat dissipation and life. However, the installation position of ordinary small electronic tubes can be free from this limitation, regardless of vertical, horizontal or inverted. It is worth noting that a few direct-heating tubes, such as 2A3, 5Y3GT, and 5U4G, must be installed vertically due to the filament structure.

Fifth, do not let the tube be subjected to severe vibration, and do not use it in an environment that is too humid and hot. Severe vibration will cause the position of each electrode in the tube to change, causing a change in electrical parameters, resulting in a decrease in the performance of the tube, thereby affecting normal operation; in severe cases, the electrodes will collide, causing the tube to be scrapped. When the ambient temperature rises and the shell is overheated, it will cause electrolysis of the electron tube glass and damage the internal getter, causing premature tube damage. It can be seen that when installing the power amplifier tube and the rectifier tube, it is necessary to take good ventilation measures to facilitate heat dissipation.

Point 4: Pay attention to the reasonable selection of the external resistance and capacitance components of the tube to ensure the stable and reliable operation of the tube. For example, the DC resistor connected to the control gate circuit of the power amplifier tube must not exceed the maximum value given in the manufacturer's manual or the tube characteristics manual to prevent the control gate from emitting electrons and generating a so-called reverse gate current (also called The gate reverse current) causes the output to decrease, the distortion to increase, and the unstable operation. Generally, the power resistor has a gate resistor of 500 kΩ or less at a self-bias voltage, and a gate resistor at a fixed bias voltage is usually 47 to 100 kΩ. For another example, when a rectifying electron tube adopts a capacitor input filter circuit, the larger the capacitance of the filter capacitor is, the larger the peak current of the screen is. When the capacitance of the filter input capacitor is too large (5Y3GT type tube is larger than 40μF), the screen power supply impedance value must be increased to more than the value specified in the characteristic manual (50Ω for the 5Y3GT tube) to limit the peak current of the screen to no more than Rating.