Tube amplifier adjustment

Tube amplifier adjustment

The circuit of the tube amplifier (tube amp) is simpler than that of the transistor amp, and it is easier to make successfully. It also has a better music playback effect, especially in terms of emotional expression. Therefore, the tube amp is very popular among audiophiles after its revival. The most important feature of the tube amp is the tube taste. Does the tube amp you weld also have a warm, mellow, smooth and sweet tube taste? If not, the sound base is similar to that of the transistor amp, or harder and drier than the transistor amp, or the homemade tube pre-stage and buffer are connected to the playback system, and the change in the sound of the playback system is not as "immediate" as the media said, then you should measure the working point of each tube to see if it is working in the best state, otherwise you need to make careful and careful adjustments. Only when each tube works in the best working state can the charm of the circuit and each tube be brought into play to achieve a satisfactory playback effect.

A tube amplifier with an unadjusted working point will not only have poor sound quality, but also low volume and distortion. Although there are many factors that affect the sound quality of an amplifier, it is ultimately determined by the level of production. When making equipment, audiophiles usually choose excellent circuits or follow the circuit diagrams of famous machines based on the tubes and components they have accumulated. Although the specifications and values ​​of the components are not much different from the requirements on the circuit diagram, the ranking of the components, the length of the wiring, the quality of welding, or other differences, such as the level of B+ voltage, will affect the performance of the playback. Therefore, the welded tube amplifier may not have a strong tube flavor. It doesn't matter if there is no tube flavor. As long as the tubes at all levels of the amplifier are adjusted and calibrated appropriately and reasonably to work in the best state, the requirements for playback can be met.

The most important thing in adjusting the tube amplifier is to adjust the screen voltage, screen current and grid negative voltage, so that the tube works at the appropriate working point and the sound system can produce good sound. This is what is rarely mentioned in some articles or described in two very simple sentences, or it can work without any adjustment. If the tube does not work, the tube taste will not come out even if you change the capacitor of a famous brand.

When adjusting the tube amplifier, the data provided in the tube manual should be used as the basis for the circuit. If there is no tube manual, the parameter values ​​given in the circuit diagram or the additional tube data should be respected. The working point of the triode is determined by the screen voltage and the grid negative voltage. After the screen voltage is determined, the grid negative voltage can be adjusted to adjust the working point. When the screen voltage of the beam tube or pentode increases to a certain level, the transformation of the screen grid voltage will have a greater impact on the working point. Therefore, the screen grid voltage and the grid negative voltage can be adjusted to select the working point.

Some articles have introduced the methods of reducing tube amplifier noise and adjusting the tone by replacing coupling capacitors. This article will not repeat them. Here I will talk about my experience in adjusting the working point of the tube duct.

1. Negative gate voltage circuit

When adjusting the working point of the tube, the grid negative voltage is often involved, so first of all, the grid negative voltage circuit is discussed. The electron tube is a voltage-controlled element, and the three main electrodes (filament, grid and screen) need to be supplied with appropriate voltages. The one supplied to the filament is called A voltage, the one supplied to the grid is called C voltage, and the one supplied to the screen is called B voltage. The grid voltage is generally connected to a negative voltage, which is customarily called "grid negative voltage" or "grid bias voltage". In order to make the tube work stably, the grid negative voltage must be supplied with direct current. According to the working category of the tube, there are two ways to supply the grid negative voltage: one is to use the voltage drop generated by the electron tube screen current (or screen current + screen grid current) flowing through the cathode resistor to make the grid obtain negative voltage, which is called self-sufficient grid negative voltage, and is generally used in Class A amplifier circuits with relatively stable screen current. The other is to set a set of negative voltage rectifier circuits in the power supply part to supply the grid negative voltage, which is called fixed grid negative voltage, and is mainly used for Class A, B2 or Class B power amplifier stages with large changes in screen current. Using self-sufficient grid negative pressure, the tube is relatively safe. When using fixed grid negative pressure, when the negative pressure rectifier circuit fails and the tube loses the grid negative pressure, the screen current will rise too high and burn the tube. Therefore, it is not as reliable as the self-sufficient grid negative pressure.

The process of generating self-sufficient grid negative voltage is as follows: Figure 1 shows the flow process of current in the circuit. When the electron tube is working, the screen and the screen grid absorb electrons. The current flows from the negative pole of the high voltage of the power supply through the cathode resistor RK, the screen, the primary coil of the output transformer and the current of the screen grid to the positive pole of the high voltage, forming a load loop. When the current flows through RK, RK generates a voltage drop. The voltage at both ends of RK is negative at one end of the ground wire and positive at one end of the cathode. In this way, there is a potential difference generated by RK between the cathode and the ground wire. The grid resistor R1 connects the grid and the ground wire, so there is a potential difference generated by RK between the grid and the cathode. Since different electron tubes require different grid negative voltages, the resistance value of the cathode resistor is also different. For example, the cathode resistance of 6V6 is 300Ω, while the cathode resistance of 6L6 is 170Ω. The resistance value of the cathode resistor can be obtained by Ohm's law: cathode resistance = grid negative voltage/amplifier tube current (screen current + screen grid current). When the gate inputs a signal, the screen current is immediately controlled and fluctuates, and the current on the cathode resistor also fluctuates, and the generated potential difference also fluctuates. The phase of the voltage fluctuation on the cathode resistor happens to be opposite to the input signal, thus weakening the input signal. This situation is usually called negative feedback of the current at this stage, and this effect reduces the amplification gain of this stage. The component that causes the voltage fluctuation on the cathode is the audio AC component, so generally a large-capacity electrolytic capacitor is connected in parallel to the cathode resistor to bypass the AC component, and the DC voltage of the cathode resistor is relatively stable.

There is another way to generate negative gate voltage, called contact negative gate voltage. The process of generation is shown in Figure 2. This negative gate voltage is generated by the electron tube itself. When electrons run from the cathode to the screen, they pass through the gate. If there is no negative voltage on the gate, the electrons will not be rejected when passing through the gate. On the way to the screen, they will occasionally touch the gate. The electrons that touch the gate will return to the cathode through the gate resistor R. The direction of electron flow is from the gate to the cathode, so when electrons flow through R, a voltage drop is generated. The gate is the negative end and the cathode is the positive end. Because there are few electrons touching the gate, the current caused is less than 1μA. Although the resistance of R is very large, calculated as 10MΩ, the voltage generated is only about 1V. This way of supplying negative gate voltage is rarely seen and can only be used in small signal amplifier circuits at the input end. For amplifier stages with input signals less than 1V, such as a pickup output of only a few mV, this negative gate voltage circuit is very suitable.

2. Adjustment of voltage amplifier stage

The voltage amplifier stage is responsible for the main amplification task of the whole machine and cannot have distortion, so it is required to work in Class A state. In Class A state, its working point is in the middle of the linear section of the gate voltage-screen current characteristic curve. At this time, the gate negative voltage is half of the maximum gate negative voltage of the amplifier tube, and the working current should be between 30% and 60% of the maximum screen current of the amplifier tube, and should not be too small.

The adjustment method is very simple. Just adjust the resistance of the cathode resistor. First, connect the ammeter (the maximum range is slightly larger than the maximum screen current of the tube. For example, if the screen current of 6SN7 is 8mA, a 10mA ammeter can be used) in series in the cathode circuit, as shown in the cathode circuit of V1 in Figure 3a. The positive pole of the ammeter is connected to the cathode resistor, and the negative pole is connected to the chassis. If the cathode resistor has no bypass capacitor, in order to avoid the ammeter and wiring from affecting the working state of this stage, it is best to connect a 100μ/50V electrolytic capacitor in parallel at both ends of the ammeter, the dotted line CA in the figure. If the cathode resistor RK has a bypass capacitor, the connection method of the ammeter is shown in Figure 3b, and the ammeter can also be connected in series in the screen circuit. Then change the resistance value 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 resistor 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 tube screen current increases, and the sound is warmer and richer, but the noise will also increase. Noise is an important indicator of the voltage amplifier stage. The noise cannot be too large, so when adjusting, both noise and tone must be taken into account. For a specific tube amplifier, the screen current should be adjusted to a certain amount. You can also find a working point with the best tone by adjusting and listening.

When the value of the screen load resistor R2 is relatively high, the distortion is small, but the rectifier output must have a higher voltage at this time. If conditions permit, you can use RK and R2 with different resistance values ​​to form several groups for trial listening, and find a combination with low noise, mellow, full sound and good transparency. The
gate negative 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 a CD player, and the output voltage of the CD player is 0~2V, then the gate negative voltage of 6SN7 should be adjusted to above -3V. For example, the gate negative voltage of 12AX7 and 6N3 tubes is designed to be -2V. If the input signal voltage is high, a signal attenuation voltage divider resistor can be set at the input end, as shown in Figure 4, to appropriately reduce the input signal voltage and maintain distortion-free amplification.

12AX7 is a musical tube and is generally used to make preamplifiers to make the entire system more musical. Be careful when adjusting the working point, because the screen current of 12AX7 is very low, only 1-2mA at most.

3. Adjustment of the inverter stage

The purpose of adjusting the inverter stage is to make the upper and lower output signals at the output end symmetrical and equal to reduce distortion.

Figure 5 is a screen-cathode split load inverted circuit. This circuit is recognized as a good sound circuit. Many famous machines at home and abroad use this circuit. The screen and cathode output voltages of V in the circuit are opposite in phase, and the audio currents flowing through R2 and RK are equal. Therefore, as long as R2 and RK are equal, the output voltages of the screen and cathode are equal, so an output signal with opposite phases and equal amplitudes is obtained. Therefore, in general circuit diagrams, these two resistors are required to have the same value and be used in pairs. However, in fact, due to the different output impedances, the output voltages on the loads are not equal, so using a load with the same resistance value is not necessarily the best state. Therefore, slightly different resistance values ​​should be used. When there is no instrument measurement, it can be judged by listening to whether there is obvious distortion. When this magazine held the Grand Prix of Tube Amplifier Production in 1997, the resistance value of RK in the circuit used was 43k, slightly larger than R2 (36k), which can obtain symmetrical output and 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. Generally, the two screen load resistors (R1 and R2) are required to be the same. If the amplitude difference between the upper and lower output voltages is large, or the amplifier is distorted, and the distortion cannot be completely eliminated after adjusting the working points of each tube, you can try to increase the resistance of RK by about 5% to 10%, and 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 in the straight line part of the gate voltage and screen current characteristic curve. The swing of the input signal of the gate does not exceed the negative voltage range value. If it exceeds, distortion will occur. 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 and the distortion is low. This characteristic can be used to check whether the working point of the power amplifier stage is appropriate. During the inspection, the ammeter is connected in series in the screen loop of the power amplifier tube, as shown in Figure 3a. When there is a signal input to the gate, if the screen current of the power amplifier tube increases, it means that the negative voltage of the gate is too low. If the screen current decreases, it means that the negative voltage of the gate is too high. It must be adjusted until the screen current changes to the minimum. The size of the screen current should be appropriate. When the screen current is large, the sound quality is good and the distortion is small. When the screen current is small, it is beneficial to the life of the bile duct. It can be adjusted according to needs.

Be careful when adjusting not to exceed the maximum screen power consumption of the power amplifier tube. In Class A working state, the screen voltage × screen current of the power amplifier tube is equal to its static screen power consumption. If exceeded, the screen pole will turn red and the power amplifier tube will be burned out over time. It is generally required that no more than one parameter of the tube should reach the limit value, and it must not exceed the limit parameter. The screen current is generally adjusted to 70% to 80% of the maximum screen current.

The adjustment method is to adjust the resistance of cathode resistor R5. The resistance of R5 is determined by the sum of the negative grid voltage, screen current and screen grid current of the amplifier tube. 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 voltage is high (above 300V), the change of screen grid voltage has a greater impact on the screen current. The screen grid voltage and negative grid voltage can be appropriately adjusted to select the working point. If conditions permit, the screen grid voltage can be used with a voltage stabilizing circuit to make the power amplifier tube work more stably.

The adjustment of the push-pull amplifier stage is to balance the two push-pull power amplifier tubes, and the gate negative voltage and screen current of the two power amplifier tubes should be equal. Taking Figure 7 as an example, when the gate negative voltage is not equal, adjust the gate negative voltage potentiometer RP. When the screen current is different, increase the cathode resistance of the power amplifier tube with a large screen current or add another resistor in series, such as RK in Figure 7. If the screen current is greatly different, it means that the power amplifier tube is not paired and a power amplifier tube should be replaced. In some circuit diagrams, a 10Ω resistor is connected to the cathode of the power amplifier tube. It is used to check the working status of the power amplifier tube. When adjusting, just measure the voltage drop of this resistor to know the increase or decrease of the screen current.

When adjusting the screen current, you should also pay attention to the change of B+ voltage. If the B+ voltage drops a lot when the screen current is large, it means that the margin of the power supply part is insufficient or the internal resistance of the power supply is large, the filter resistor is large, the wire diameter of the choke is thin or the inductance is large. You can reduce the resistance of the filter resistor or connect the B+ connection to the screen of the power amplifier tube to the input end of the filter circuit. At this time, although the ripple of B+ is large, it has little effect on the AC noise of the whole machine and can still be at an acceptable level.

5. Adjustment of Negative Feedback

After the circuit has negative feedback, it will reduce harmonic distortion, but it will affect the transient performance. Therefore, the negative feedback should not be too large, generally about 6dB is appropriate. The adjustment method is to change the value of the negative feedback resistor, such as R6 in Figure 3a and Ra in Figure 7. The size of the feedback is determined by the playback effect such as sound field, positioning, sweetness of human voice, musicality, etc., and is based on ear satisfaction. If the amplifier makes a sound as soon as the negative feedback circuit is turned on, it means that the polarity of the feedback is reversed. Just connect the negative feedback connection line to the other end of the output transformer and ground this end. 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, if this phenomenon is found, simply remove it.

After the adjustments mentioned above, each electron tube has entered its best working condition. When you play familiar records again, the playback effect will definitely be different, and the tube flavor will be greatly enhanced.