Tube power amplifier adjustment
The circuit of the tube power amplifier (tube amplifier) ​​is simpler than the transistor machine, easy to make successfully, and has better music replay effects, especially in the expression of emotions, so the tube amplifier is very popular with enthusiasts after the resurgence. The most important characteristic of the tube amplifier is the bile taste. Does the tube you welded also have a warm, mellow, smooth, sweet taste? If not, the sound floor is similar to the transistor machine, or harder than the transistor machine. If the dry, or self-made bile preamplifier and buffer are connected to the playback system, and the sound of the playback system does not change as the media says, it should measure the working point of each tube. Whether it is working in the best condition, otherwise it will be adjusted carefully and carefully. Only when the electronic tubes work in the best working state can the charm of the circuit and each bile duct be exerted to achieve a satisfactory playback effect.
In addition to the poor performance of the timbre, the amplifier with unadjusted working point also has the phenomenon of low volume and distortion. Although the sound quality of an amplifier is affected by many factors, it is ultimately determined by the level of production. When making equipment, audiophiles generally choose excellent circuits based on the bile ducts and components accumulated in their hands, or weld them according to the map of the famous machine. The specifications and values ​​of the components are not much different from the requirements on the circuit diagram. The arrangement of the components, the length of the wiring, the quality of the welding, or other differences, such as the level of the B + voltage, will affect the performance of the playback, so the soldered tube machine is not necessarily a strong bile. of. It doesn't matter if there is no guts, as long as the bile ducts at all levels of the amplifier work in the best state through proper and reasonable adjustment and verification, the requirements for playback can be achieved.
In addition to reducing the noise to an acceptable level and changing the brand or capacity of the input and output coupling capacitors to change the sound, the most important thing is to adjust the screen pressure, screen flow and grid negative pressure to make the bile duct Work at a suitable working point to make the sound system play a good sound, and this is precisely what some articles talk about less or take a very simple two-sentence description, or else it is "no adjustment is needed" Can work. If the bile duct does not enter the working state, then change the brand-name capacitor, the bile taste will not come out.
When adjusting the bile machine, the data provided in the tube manual should be used as the basis of the circuit. When there is no tube manual, the parameter values ​​given in the circuit diagram or additional bile duct information should be respected. The working point of the triode is determined by the screen pressure and the grid negative pressure. After the screen pressure is determined, the grid negative pressure can be adjusted to adjust the working point. After the screen pressure of the beam tube or the pentode rises to a certain level, the grid pressure changes It will have a greater impact on the working point, so you can adjust the curtain pressure and grid negative pressure to select the working point.
Some articles have been introduced to reduce the noise of the bile machine and replace the coupling capacitor to adjust the timbre. This article will not repeat it. Here we will talk about the method of adjusting the working point of the bile duct.
1. Gate negative pressure circuit
When adjusting the working point of the bile duct, the gate negative pressure is often involved, so first talk about the gate negative pressure circuit. The electron tube is a voltage control element. The three main electrodes (filament, grid and screen electrode) are to be supplied with appropriate voltage. The filament is called the armor electricity, the grid is called the electricity electricity, and the screen electrode is called the electricity electricity. The gate voltage is generally a negative voltage connected, which is traditionally called "gate negative voltage" or "gate bias voltage". In order to stabilize the bile duct, the negative grid voltage must be supplied with direct current. According to different working types of bile ducts, there are two methods for supplying negative pressure of the grid: one is to use the voltage drop generated by the screen flow of the tube (or screen flow + curtain grid flow) through the cathode resistance to make the grid obtain negative pressure. It is called self-sufficient grid negative pressure, and is generally used in Class A amplifier circuits with stable screen flow. The other is to set a negative pressure rectifier circuit in the power supply section to supply the gate negative pressure, which is called fixed gate negative pressure. It is mainly used for Class A or Class B or Class B power amplifiers with large changes in screen current. Using self-contained grid negative pressure, the bile duct is safer. When fixed grid negative pressure is used, when the negative pressure rectifier circuit fails and the bile duct loses the grid negative pressure, the screen current will rise too high and burn out the bile duct, so there is no self-contained grid Negative pressure works reliably.
The process of self-contained grid negative pressure is as follows: Figure 1 shows the flow of current in the circuit. When the tube is working, the screen and curtain grid absorb electrons, and the current flows from the negative pole of the power supply through the cathode resistance RK, screen, and output. The current of the transformer primary coil and the curtain grid together go to the positive pole of the high voltage to become a load circuit. When the current flows through RK, RK produces a voltage drop. The voltage across RK is negative at one end of the ground wire and at the cathode. One end is positive. In this way, there is a potential difference between RK and the ground, and the gate resistor R1 connects the gate and ground, so there is also a potential difference between the gate and the cathode. Due to the different grid negative pressure required by different electron tubes, the resistance of the cathode resistance is also different, for example, the cathode resistance of 6V6 is 300Ω, and the cathode resistance of 6L6 is 170Ω. The resistance of the cathode resistance can be obtained by Ohm's law: cathode resistance = grid negative pressure / amplifier tube current (screen current + curtain grid current). When a signal is input to the grid, the screen current is immediately controlled and fluctuates. The current on the cathode resistance is also fluctuating, and the resulting potential difference is also fluctuating. The phase of the voltage fluctuation on the cathode resistance happens to be opposite to the input signal, so it is weakened. In order to reduce the input signal, this situation is usually called current negative feedback of the current stage. This effect reduces the amplification gain of the current stage. The component causing the voltage fluctuation on the cathode is an audio frequency AC component, so a large-capacity electrolytic capacitor is generally connected in parallel with the cathode resistor to bypass the AC component, and the DC voltage of the cathode resistor is relatively stable.
There is also a way to generate grid negative pressure, called contact grid negative pressure. The generation process is shown in Figure 2. This grid negative pressure is generated by the electron tube itself. When electrons run from the cathode to the screen, they pass through the grid. When there is no negative pressure on the grid, the electrons are not rejected by the grid, then they will hit the grid from time to time on the way to the screen, and the electrons that hit the grid will be returned by the grid resistance R Cathode, the direction of electron flow is from the grid to the cathode, so a voltage drop occurs when electrons flow through R, the grid is the negative terminal, and the cathode is the positive terminal, because there are few electrons touching the grid, resulting in a current of less than 1μA Although the resistance of R is very large, it is calculated at 10MΩ, but the generated voltage is only about 1V. This kind of grid negative pressure supply method is rarely seen, and can only be used in the small-signal amplifier circuit at the input end. The input signal is less than 1V. If the output of the pickup is only a few mV, this gate negative voltage circuit is very suitable.
2. Adjustment of voltage amplification stage
The voltage amplification stage is responsible for the main amplification task of the whole machine, and there must be no distortion, so it is required to work in the Class A state. In the Class A state, its operating point is in the middle of the linear segment of the grid voltage-screen flow characteristic curve. At this time, the grid negative pressure is half of the maximum grid negative pressure of the amplifier tube, and the operating current should be 30 It is suitable between% and 60%, and should not be too small.
The adjustment method is very simple, as long as the resistance of the cathode resistance is adjusted, first connect the ammeter (the maximum range is slightly larger than the maximum screen current of the tube, such as 6SN7 screen current is 8mA, available 10mA ammeter) in the cathode circuit, as shown in the figure As shown in the cathode circuit of 3a V1, the positive electrode of the ammeter is connected to the cathode resistance, and the negative electrode is connected to the chassis. If the cathode resistance does not have a bypass capacitor, in order to avoid the current meter and wiring from affecting the working state of this stage, it is best to parallel one at both ends of the ammeter 100μ / 50V electrolytic capacitor, dotted line CA in the figure. If the cathode resistance RK has a bypass capacitor, the connection of the ammeter is shown in Figure 3b. You can also string the ammeter into the screen circuit. Then change the resistance of RK or the screen voltage of V1 to make the working point of V1 reach the best state. You can also use the method of measuring the voltage across the cathode resistance RK, and then use Ohm's law (A = V / R) to calculate the current.
Different amplifier tubes require different working currents. For example, 6SN7 can be adjusted to 3 ~ 4mA, the bile duct screen current increases, the sound is warm and rich, but the noise will also increase. Noise is an important indicator of the voltage amplification stage, noise cannot Large, so be sure to give consideration to both noise and timbre when adjusting. Specific to a certain amplifier, how much the screen current is adjusted to, it is also possible to find a working point with the best timbre by listening while listening.
When the resistance of the screen load resistor R2 is used relatively high, the distortion is small, but at this time, the rectified output must have a higher voltage. If conditions permit, RK and R2 can be used to form several groups for listening , Find a group of combinations with low noise, mellow, full and good transparency.
The negative gate voltage should be greater than the swing amplitude of the input signal voltage. If 6SN7 is used for voltage amplification, the input signal comes from the CD player, and the output voltage of the CD player is 0 ~ 2V, then the negative gate voltage of 6SN7 should be adjusted to above -3V. For example, the gate negative pressure of 12AX7 and 6N3 tubes is designed to be -2V. If the input signal voltage is high, a signal attenuation resistor can be set at the input terminal, as shown in Figure 4, to properly reduce the input signal voltage and maintain distortion-free amplification.
12AX7 is a musical bile duct. Generally, I like to use it to make a pre-amplifier to make the music sense of the whole system better. Please pay attention when adjusting the working point, because the screen flow of 12AX7 is very low, the maximum is only 1? 2mA.
3. Adjustment of the inverted phase
The purpose of adjusting the inverting stage is to make the upper and lower output signals of the output terminal symmetrical and equal to reduce distortion.
Figure 5 is a screen-inverted load-loaded inverter circuit. This circuit is recognized as a good sound circuit. There are quite a few famous machines at home and abroad that use this circuit. The screen of V in the circuit is opposite to the output voltage of the cathode, and the current The audio currents through R2 and RK are equal, so as long as R2 and RK are equal, the output voltages of the screen and cathode are equal, so the output signals with opposite phase and equal amplitude are obtained. Therefore, the general circuit diagram requires these two resistors to be The same value is used in pairs, but in fact, because the output impedance is not the same, the output voltage on the load is not equal, so the load with the same resistance value is not necessarily the best state, so a slightly different resistance value should be used , When there is no instrument to measure, you can judge whether there is obvious distortion by audition. When the magazine held the Grand Prix in 1997, the resistance of the RK in the circuit used was 43k, which was slightly larger than R2 (36k), and a symmetrical output could be obtained to reduce distortion.
Figure 6 is a cathode-coupled inverter circuit, also known as a long-tail inverter circuit. The frequency characteristic of this circuit is very flat. It is also an inverter circuit used by many famous machines. It generally requires two screen load resistors (R1, R2). To be the same, if the measured amplitude difference between the upper and lower output voltages is large, or the amplifier is distorted, after adjusting the operating point of each tube, the distortion cannot be completely eliminated, you can try to increase the resistance of RK by 5% ~ Around 10%, the distortion may be smaller.
4. Adjustment of power amplifier stage
Figure 3a is a class A power amplifier stage. The working point of the power amplifier tube is the linear part of the grid voltage and screen current characteristic curve. The swing of the grid's input signal 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, the work is stable and the distortion is low, and this feature can be used to check whether the working point of the power amplifier level is appropriate. When checking, put the ammeter in the screen loop of the power amplifier tube, see Figure 3a. When the grid has a signal input, if the screen current of the power amplifier tube increases, it means that the grid negative pressure is too low. If the screen current decreases, It shows that the negative voltage of the grid is too high, and it must be adjusted to the minimum change of screen current. The size of the screen flow should be appropriate. When the screen flow is large, the sound quality is good and the distortion is smaller. The screen flow is small, which is beneficial to the life of the bile duct and can be adjusted according to needs.
When adjusting, pay attention not to exceed the maximum screen consumption of the power amplifier tube. In the working state of Class A, the screen pressure of the power amplifier tube × screen flow is equal to its static screen consumption, and the screen will become red after exceeding it. Amplifier tubes generally require that the bile ducts use no more than one limit value parameter, and cannot exceed the limit parameter. The screen flow is generally adjusted to 70% to 80% of the maximum screen flow.
The adjustment method is to adjust the resistance of the cathode resistor R5. The resistance of R5 is determined according to the sum of the grid negative pressure of the amplifier tube, the screen current and the curtain grid current. The screen current of 6V6 in Figure 3a can be adjusted to about 30mA ( The maximum screen current is 45mA), the cathode voltage is 10V, and the screen voltage is 280 ～ 300V. When the screen pressure is high (above 300V), the change of the screen voltage has a greater influence on the screen current, the screen voltage and the negative pressure of the screen can be adjusted appropriately to select the working point, and the conditions can be used to stabilize the screen voltage The circuit makes the power amplifier tube work more stable.
The adjustment of the push-pull amplifier stage is to balance the two push-pull power amplifier tubes, and the grid negative pressure and the screen current of the two power amplifier tubes should be equal. Taking FIG. 7 as an example, when the grid negative pressure is not equal, adjust the grid negative piezoelectric potentiometer RP, when the screen current is different, increase the cathode resistance of the power amplifier tube with large screen current or add another resistor, such as RK in Figure 7, if the screen current difference is large, it means that the power amplifier tube is not matched and should be replaced. A power amplifier tube. On some circuit diagrams, the cathode of the power amplifier tube is connected to a 10Ω resistor. It is to check the working state of the power amplifier tube. As long as the voltage drop of this resistor is measured during adjustment, the increase or decrease of the screen current can be known.
When adjusting the screen current, you should also pay attention to the change of the B + voltage. If the screen current is large, the B + voltage decreases a lot, it means that the power supply has insufficient margin or the power supply has a large internal resistance, the filter resistance is large, and the choke The wire diameter is small or the inductance is large, you can reduce the resistance of the filter resistor or connect the B + wiring of the screen electrode of the power amplifier tube to the input terminal of the filter circuit. Although the ripple of B + is large at this time, the The hum has little effect and can still be at an acceptable level.
V. Adjustment of negative feedback
After the line has negative feedback, it will reduce harmonic distortion, but it will affect the transient performance deterioration, so the amount of negative feedback should not be too large, generally about 6dB, the adjustment method is to change the value of the negative feedback resistance, as shown in the figure R6 in 3a, Ra in FIG. 7, the amount of feedback is determined according to the playback effects such as sound field, positioning, sweetness of human voice, music sense, etc., subject to ear listening satisfaction. If the negative feedback circuit is turned on, the amplifier will scream. This is because the polarity of the feedback is reversed. As long as the negative feedback cable is connected to the other end of the output transformer, this end can be changed to ground. Some negative feedback loops are connected in parallel with a small capacitor. If the value of this capacitor is not selected properly, it may cause distortion or self-excitation. Therefore, when this phenomenon is found, it is simply removed.
After the adjustment of the above method, each tube has entered the best working state, and then playing familiar records, the playback effect will be different, and the courage will increase a lot.
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