Adjustment and sound calibration of the hottest tu

2022-08-15
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Adjustment and sound calibration of electron tube amplifier

an electron tube amplifier is assembled, the sound test is normal, and it is only a part of the workload. To think of a good sound, there is still a lot of meticulous work to do, that is, debugging and sound calibration, because only after careful and reasonable adjustment and verification, can all levels of amplifier tubes work at the best working point, and then after sound calibration, make the sound of the amplifier mellow, Only when the music is rich, dynamic and sharp, and the frequency is wide, can the music be detailed, clear, and pleasant. The sound calibration work needs more energy and takes a long time, even several months to complete, so we should have perseverance and patience. Now let's talk about the debugging and sound correction methods of the electron tube amplifier

when welding, enthusiasts usually choose excellent lines or lines according to the name of the machine according to the existing components in their hands, and carry out welding according to the diagram. Although the specifications and values of components are not different from the requirements on the circuit diagram, and some components are even higher grade, but the arrangement of components, the length of wiring, the quality of welding, or other differences, such as the level of b+ voltage, the size of current, etc, Will affect the playback effect, so the welding machine is not necessarily beautiful when it is turned on. It needs careful debugging to make each amplifier work in a good working state, so as to give full play to the charm of each bile duct and line and achieve a satisfactory playback effect

the adjustment and sound correction of courage include: reducing noise and communication sound to an acceptable level; Adjust the screen pressure, screen current and grid negative pressure of the electron tube to make the electron tube work at a better working point; Replace the capacity and brand of the interstage coupling capacitor, the capacity and brand of the b+ filter capacitor, and even the brand of the small signal line, resistor and electronic tube in the machine, so that the playback system can play 3. The main technical parameters of the carton detection equipment: (can be customized according to customer requirements) make a good sound

there have been many articles on the elimination methods of communication sound in the past, and this article will not repeat it. If there is a "hissing, hissing" sound after the volume potentiometer is turned up, it indicates that the circuit is excited, which is the cross connection induction caused by the unreasonable arrangement and wiring of components. You can dial some wires or components to hear whether there is a response. You should find them one by one, and then change the position to eliminate the induction. When the volume is turned on by the positioner, there is no noise in the playback system, but when it is turned to a certain position, there is a sudden noise. If it is turned on again after this position, the noise will disappear, which is caused by the unreasonable arrangement of components in the input part. The elimination method is to rearrange the components of the input part and change the routing

The operating point of the

triode is determined by the screen voltage and grid negative pressure. After the screen voltage is determined, the negative pressure of the grid can be adjusted to adjust the working point. After the screen pressure of the pentode rises to a certain extent, the change of the curtain grid pressure will have a great impact on the working point. Therefore, the curtain grid pressure can be adjusted, and the post-processing of molded products such as length sawing and surface sanding can also be carried out with ordinary woodworking machines like wood; The working point can be selected by using recycled plastics and waste natural fibers to realize waste utilization and grid negative pressure. When the capacity of the power supply is large and the internal resistance is low, adjust the screen current, and the b+ voltage will generally not change. If the margin of the power supply is not large, the b+ voltage will decrease greatly when the screen current is adjusted large

I. grid negative pressure circuit

the grid of the electron tube is generally connected to negative pressure, which is customarily called "grid negative pressure" or "grid bias". There are two ways to supply grid negative pressure: one is to use the voltage drop generated by the screen flow (or screen flow plus curtain grid flow) of the electron tube flowing through the cathode resistance to make the grid obtain negative pressure, which is called self-contained grid negative pressure. It is generally used in class a amplifier circuits with stable screen flow. The other is to set a set of negative pressure rectification circuit in the power supply part (the power supply comes from the separate winding of the transformer or is extracted from the negative end of the b+ power supply) to supply the grid negative pressure, which is called the fixed grid negative pressure, and is mainly used for class a and class B or class B power amplification stages with large changes in the screen current

there is another way to generate grid negative pressure, which is called contact grid negative pressure, that is, the cathode of the electron tube is directly grounded without cathode resistance. When the electrons rush from the cathode to the plate, the electrons on the grid will return to the cathode from the grid resistance, causing a voltage drop on the grid resistance, but the generated voltage is only about 1V. Therefore, this grid negative pressure supply can only be used in the amplification stage where the input signal is less than 1V. If the pickup volume output is only a few MV, this kind of gate negative pressure circuit root is suitable

II. Adjustment of voltage amplification stage

the voltage amplification stage generally works in class a state, and its working point is in the middle of the linear segment of the grid voltage screen current characteristic curve. At this time, the grid negative pressure is half of the maximum grid negative pressure of the amplification tube, and the working current should be 40% - 60% of the screen current, which should not be too small

the adjustment method is shown in Figure 1, as long as the resistance value of cathode resistance R3 is adjusted. The ammeter can also be connected in series into the plate circuit. Then change the screen voltage of R2 to make the working point of V1 reach the best state. The voltage at both ends of cathode resistance R2 can also be measured, and then the current can be calculated by Ohm's law

different amplification tubes require different working currents. When the bile duct screen flow is large, the sound is warm and rich, but the noise will also increase

when the resistance value of the plate load resistance R2 is large, the distortion is small, the gain is high, and the dynamic is large, but at this time, there must be a relatively high b+ voltage. Otherwise, the pressure drop of R3 is also large, which will reduce the dynamic range and make the treble worse. R3 is also related to the resistance of the grid resistance of the next level. The maximum resistance of R3 is not more than 1/2 of R4. The resistance of R3 also involves the frequency response width of the amplifier. If possible, R2 and R3 can be formed into several groups with different resistance values to try to find a group of combinations with low noise, high dynamic and good sound quality

The gate negative pressure of

v1 should be greater than the swing amplitude of the input signal voltage. If 6SN7 is used for voltage amplification, and the signal output voltage of the input signal from the CD player or DVD, CD, DVD is 0~2v, then the gate negative pressure of 6SN7 is generally adjusted to the range of -1.8~2v, and there will be no obvious distortion. For V1, if 12AX7 or 6n3 is used, the grid negative pressure of this tube is designed to be -2v. When the input signal voltage is high, in order to reduce distortion, a signal attenuation voltage dividing resistance can be set at the input end, as shown in Figure 2, so that the input signal can be appropriately reduced. The grid negative pressure of 12AX7 or 6n3 can be adjusted to -1.7~-1.8v to maintain true amplification

when you want to know whether the grid has bias voltage, use a voltmeter with high internal resistance to measure the grid voltage to the ground

when the gain of the amplifier is sufficient, the resistance value of R3 in Figure 2 can be appropriately increased to reduce the input sensitivity of V1. At this time, the noise of the amplifier can be reduced. If the quality of R1, R2 and R3 is good enough, at this time, the noise level of the input stage only determines the inherent noise level of the amplifier tube v. If V is a low-noise speaker, the signal-to-noise ratio of the whole machine is very high

III. adjustment of the inverter stage

the purpose of adjusting the inverter stage is to make the upper and lower output signals of the output terminal symmetrical and equal, so as to reduce distortion

Figure 3 shows the screen negative split load phase inversion circuit. With this circuit, the phase of the output voltage of V's plate is opposite to that of the cathode, and the audio current flowing through R2 and rk is equal. Therefore, as long as R2 and rk are equal, the output voltage of the plate and cathode are equal, so two output signals with opposite phases and equal amplitudes are obtained. In fact, because the output impedance is not the same, the output voltage on the load is not equal, so r2=rk is not necessarily the best state, Therefore, slightly different resistance values should be used. When there is no measuring instrument, you can judge whether there is obvious distortion by audition. Sometimes the resistance value of rk is 43K Ω, which is slightly larger than R2 (36K Ω) to obtain a symmetrical output with less distortion

Figure 4 shows the cathode coupled inverter circuit, also known as the long tail inverter circuit. The frequency characteristic of this circuit is very flat. Generally, the two plate load resistors (R1 and R2) are also required to be equal. If there is a large difference between the upper and lower output voltage amplitudes, or the amplifier is distorted, when checking the bias circuit of each tube for abnormalities, try increasing the resistance of RK by about 5%~10% to increase the negative feedback, and the distortion may be smaller. Some also add a wire wound potentiometer to the screen circuit of one tube to adjust the screen voltage of the two tubes to make the output voltage amplitude equal

IV. adjustment of power amplification stage

V2 in Figure 1 is a class a power amplifier. The working point of the power amplifier tube is at the midpoint of the straight-line part of the grid voltage and screen current characteristic curve. The swing of the input signal of the grid does not exceed the negative pressure range value, and distortion will occur when it exceeds. The characteristic of class a power amplifier is that the working current remains unchanged when a strong signal or a weak signal is input, and the operation is stable with low distortion. Using this characteristic, we can check whether the working point of the power amplifier level is appropriate. During the inspection, connect the ammeter in series in the screen circuit of the power amplifier tube. When the grid has signal input, if the screen current of the power amplifier tube increases, the negative pressure of the grid is too low. If the screen current decreases, it indicates that the negative pressure of the grid is too high, and it must be adjusted until the screen current changes to the minimum. The size of the screen flow should be appropriate. When the screen flow is large, the sound quality is good, the distortion is small, and when the screen flow is small, it is beneficial to the life of the bile duct

when adjusting, pay attention not to exceed the maximum screen consumption of the power amplifier tube. In class a working state, the product of screen voltage and screen current is equal to its static screen consumption, and the screen will turn red after exceeding it. Generally, the screen current should be adjusted to 70% - 80% of the maximum screen current

the adjustment method is to adjust the resistance value of cathode resistance R5. In Figure 1, the screen current of 6v6 can be adjusted to about 30mA (the maximum screen current is 45mA), the cathode voltage is 10 volts, and the screen voltage is 280~300 volts. When the screen voltage is high (above 300V), the change of the curtain grid voltage has a great impact on the working point. The curtain grid voltage and the negative pressure of the grid can be adjusted appropriately to select the working point. If possible, the curtain grid voltage can be stabilized by using a voltage stabilizing circuit or wy3p, which can make the power amplifier work more stably

after the adjustment, if the volume is turned up and the sound is fuzzy, it is the leakage of the cross capacitor C5, which offsets the negative pressure of the grid

the adjustment of the push-pull amplification stage is to make the grid negative pressure and screen current of the two power amplifier tubes equal. Take Figure 5 as an example. When the grid negative pressure is not equal, adjust the grid negative pressure potentiometer W. when the screen current is different, increase the cathode resistance of the power amplifier tube with large screen current. If the difference between the screen currents is large, it indicates that the power amplifier tubes V1 and V2 are not matched, and it should be adjusted again by changing a power amplifier tube. On some circuit diagrams, the cathode of the power amplifier tube is connected with a 10 Ω resistor, which is used to check the working state of the power amplifier tube. When adjusting, as long as the voltage drop of this resistor is measured, the value of the screen current can be known

when adjusting the screen current, you should also pay attention to the change of b+ voltage. When the screen current is large, the b+ voltage decreases a lot, which means that the margin of the power supply is insufficient or the internal resistance of the power supply is large, the resistance value of the filter resistance is large, the wire diameter of the choke is thin or the inductance is large, which can reduce the resistance value of the filter resistance, or change the b+ wiring of the de power amplifier plate to the input end of the filter circuit. At this time, although the ripple of the b+ voltage is large, However, because the output stage is push-pull amplification, the primary two tube plate load coil of the output transformer can counteract the AC sound, so it has little impact on the AC sound level of the whole machine. If the b+ voltage swings by tens of volts with the overtone of the music during the sound test, it is also caused by the above reasons. It is necessary to increase the capacity of the power supply and reduce the internal resistance of the power supply. Try to play the sound after adjustment. When the volume potentiometer is wide open and a "squeak" sound is emitted in the sound phase,

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