The failure of the amplifier is generally nothing more than the following six categories.
1. The output power becomes smaller and the sound becomes weaker. 1? The power tube is aging. Can measure the screen flow of the power tube. With a 100mA DC meter, the negative meter pen is connected to the screen electrode, the positive meter pen is connected to the output transformer, and the screen current number can be read from the meter by turning on the high voltage. Under normal bias conditions, if the measured screen current is less than the normal value, it can indicate the aging of the power tube. If the measured screen current is greater than the normal value, there may be several situations: A. The screen pressure of the power tube is too high, especially the screen voltage of the curtain is too high; B. The quality of the power tube itself is defective, the screen consumption is large, and the output The power is bound to decrease. If the screen current cannot be measured, the power tube has been damaged.
2? The gate bias is abnormal. In the self-sufficient gate bias power amplifier circuit, the common gate bias faults are: A. No bias, the cause of this situation is power tube failure, no screen current, no voltage drop across the cathode resistance, cathode bypass capacitor Several types are broken down. B. The bias voltage is small, because the power tube is aging or the screen pressure is low. C. The bias voltage is high because of increased screen voltage, especially increased screen current, increased screen current, increased resistance of the cathode, leakage or breakdown of the gate cross-connect capacitor, and a positive voltage applied to the gate. Species. In addition, the open cathode resistance will also increase the bias voltage. At this time, the screen current is very small, and there is parasitic oscillation in the line.
3? The output transformer is partially short-circuited. It will cause the screen flow to increase, and the screen will become red, the output will decrease and the distortion will increase. If it is a primary partial short circuit, the output voltage will not decrease under no load. When the load is connected or the load is very light, as long as the grid excitation voltage reaches the rated value, all the power tubes will be red. This is a Typical phenomenon. When checking whether the output transformer primary is partially short-circuited, the primary and secondary wiring of the output transformer can be completely disconnected from the circuit, and the 220V mains power is sent from the primary end. The voltage of the two primary ends and the B + center head is measured with a multimeter AC block , Normally, the voltage of the two terminals is equal. When there is a partial short circuit, the voltage at one terminal is lower than the voltage at the other terminal. If the fuse is burned immediately after being connected to 220V mains power, it means that the local short circuit is very serious and the output transformer must be replaced.
Before checking the output transformer for short-circuit faults, first check whether the high-frequency suppression circuit and negative feedback circuit components connected in parallel on the secondary are degraded, failed, and broken, and then check between the secondary line and the iron core Whether there is a breakdown short circuit.
4? Insufficient driving voltage (or power) of the driving stage. The excitation voltage (or power) of the power tube grid is not enough, no matter how normal the working state of the power tube is, it still cannot have a rated power output.
5? Multi-tube parallel push-pull operation, one or several tubes of the screen suppression resistance or grid suppression resistance open circuit, not only large distortion at this time, but also low output power.
6? The self-sufficient gate biased cathode bypass capacitor fails to form an open circuit, which generates negative current feedback. For some amplifiers, it may affect the output power.
2. There are two situations in which the high voltage of the power amplifier stage cannot be added to the high voltage: the first is that the fuse blows immediately when the power is turned on, and the second is that the fuse suddenly blows the fuse during the work process and cuts off the high voltage power supply. Disconnect the high voltage B + of the center head of the output transformer of the amplifier from the high voltage power supply, and then turn on the high voltage. If the fuse is still blown or the high voltage cannot be started at this time, the fault is not in the power amplifier circuit, but in the power circuit; if the high voltage B + is disconnected After the connection, the high voltage can be started, then it can be sure that the fault is in the power amplification stage.
If the high-voltage power supply of the power amplifier stage cannot be added, it should be checked from the following aspects:
1? Observe or test whether the electrodes inside the power tube are connected.
2? Check whether the output transformer is short-circuited. It is common that the primary or secondary coils are short-circuited.
3? The load is too heavy or the load is short-circuited. Excessive load or short circuit can cause the screen current to increase and overload, blow the fuse or not apply high voltage.
3. When the parasitic oscillation amplifier has parasitic oscillation sounds such as "sizzling" high-frequency oscillations and "flapping" low-frequency oscillations, the screen consumption will increase at a lighter level, the screen will become red, and the output will decrease. jobs. There are several reasons for parasitic oscillations:
1? Components such as negative feedback resistors are deteriorated or damaged.
2 Open circuit or breakdown of the bypass capacitor in parallel with the secondary of the output transformer causes high frequency oscillation.
3? Multi-tube parallel push-pull working screen, grid resistance damage or deterioration is also likely to cause oscillation. Replacing the gate resistance, never use a wire-wound resistor because its inductance will cause oscillation.
4? Power tubes, especially high transconductance power tubes and components in the oscillation suppression circuit, whose parameters change after a long period of use, are also prone to oscillation.
5? The power supply voltage is too high. Because the power supply voltage is too high, the normal working state of the power tube is destroyed, and it can also cause oscillation.
4. When the power tube screen redness amplifier is working normally, if the screen redness is seen in a brighter environment, it is abnormal. The cause of the redness of the screen may be:
1? Excessive load causes excessive screen flow. This phenomenon is relatively common, mainly due to improper speaker impedance matching, or short circuit of the external line, or partial short circuit of the primary coil of the output transformer.
2? The negative gate bias is reduced, or there is no negative gate bias, or a positive gate bias occurs.
The reason for the negative gate bias voltage reduction may be: the failure of the negative bias power supply filter capacitor or the reduction in capacity; the center slider of the voltage-dividing load potentiometer is adjusted too low; the aging of the rectifier; the partial partial short circuit of the bias power transformer; High-voltage cathode bypass capacitors have serious leakage; the primary and secondary (or coupling capacitor) input transformers have slight leakage and other problems.
The reasons for no negative gate bias voltage may be: the center tap of the input transformer is open; the filter capacitor of the bias power supply is short-circuited; and the bias load resistance is damaged. The rectifier or the bias voltage transformer is damaged; the self-sufficient negative grid bias cathode bypass capacitor breaks down; the gate resistance or the input transformer secondary circuit is broken; the base is damaged, so that the gate pin is separated from the base.
3 The screen pressure or curtain grid pressure of the rear-stage power tube is increased, which increases the screen flow and makes the screen extremely red.
The reason for the increase in screen voltage may be: A. The primary coil of the high-voltage power transformer is partially short-circuited, causing the AC voltage of the secondary high-voltage coil to increase; the output DC voltage after rectification increases; B. The discharge discharge blocking circuit increases the output voltage. C. The filter choke coil is partially short-circuited, the inductance is reduced, the voltage drop is reduced, and the output voltage is increased.
The voltage of the curtain grid rises (refers to the machine that uses beam tetrode and pentode as the power amplification stage), the ability of absorbing electrons is enhanced, the screen current is increased, and the screen electrode is red. Several reasons may be: A. The primary partial short circuit of the high-voltage power transformer causes the secondary high voltage to increase, and the rectified output DC voltage increases. B. Improper adjustment of secondary high-voltage potentiometer. C. Local short circuit between turns of the secondary high-voltage filter choke increases the output voltage. D. The discharge discharge block circuit and the output voltage rises.
4? Ultra audio or high frequency parasitic oscillations, causing the screen to become extremely red. These two parasitic vibration oscillations are caused by the positive feedback of the total parasitic capacitance of the subsequent stage. An effective method of judgment is that when the screen is red, the load impedance is replaced by a resistance of about 1/20 of the output power of the amplifier, and the resistance value is equal to the output impedance. The signal is not sent after starting up. After a few minutes, if the hand touches the resistor and feels hot, then there is high-frequency parasitic oscillation.
5? Push-pull tube aging, destroying the push-pull balance, causing the screen to become red. In push-pull amplifiers, especially in parallel push-pull (such as 150W amplifiers generally use two KT-88 tubes in parallel), one of the tubes is aging, the internal resistance increases and the screen flow is reduced, and no aging tube is burdened Heavy, the screen flow increases and the screen is extremely red.
6? One side of the primary coil of the output transformer is partially short-circuited, which destroys the push-pull balance, increases the screen current on that side, and makes the screen extremely red.
7? The input signal is too large, making the output current and voltage exceed the rated value, causing the screen to become red.
8? Some amplifiers are not designed properly. Due to screen pressure, curtain grid voltage, filament voltage is too high, or negative grid bias voltage is too small, static screen flow is too large, even when static, it will make the screen extremely red.
5. Distortion Distortion refers to the difference between the output of the amplifier and the input waveform is too large, the sound amplified by the amplifier is different from the original input sound. The main reasons are as follows:
1? Push-pull power tube or push-stage push-pull tube has an aging (or damage), so that the two tubes have different gains, or the output transformer primary (or input transformer secondary) side is partially short-circuited or open circuit; screen and grid The change of the extreme anti-vibration resistance will also destroy the push-pull balance and cause distortion.
2? Some amplifiers' push-pull and preamp are coupled with resistance and capacitance, and distortion occurs when the coupling capacitor on one side changes in value (capacity becomes smaller, failure, leakage, etc.). If the capacitor leaks, it will also make the negative gate bias voltage of the next-stage electron tube smaller, or even become a positive voltage, resulting in grid current, causing distortion.
3? The fixed negative grid bias voltage is too high or too low, the working point of the tube changes, or the input signal is too large, etc., can make the tube work in the nonlinear part, causing distortion.
4? The power tubes of small power amplifiers generally work in the AB1 (or A) push-pull amplifier. If the peak value of the input signal voltage is greater than the negative gate bias, the gate current of the power tube will appear. The resistance is large, so it is easy to cause distortion.
5? In amplifiers above medium power, the power tube generally works in AB2 (or B) push-pull amplification. If the output power of the driving stage is insufficient or the internal resistance is too large due to the aging of the driving tube, it will cause distortion. In order to obtain a stable output voltage, the booster stage must use a tube with a small internal resistance and invert the phase with a step-down transformer.
6? The change of screen load resistance, cathode resistance or curtain grid resistance changes the working point of the electron tube and works in the nonlinear region, causing distortion. The gate resistance is broken, causing blocking distortion. At the same time, the load impedance is too light or too heavy, so that the output impedance of the tube does not match, causing distortion or light sound.
7? The power supply voltage is unstable or too high or too low, will change the working point of the electronic tubes at all levels, causing distortion.
6. AC sound Generally speaking, because the voltage amplification of the subsequent stage is not large, the AC sound caused by the failure of the power amplifier stage is not very obvious, but there are several kinds of failures that can produce obvious AC sound.
1? The internal and negative poles of the power tube are short-circuited or leaked, the cathode and the filament are short-circuited, and the filament power transformer is poorly grounded.
2? Poor fixed bias filter.
3. Promote leakage between the primary and secondary of the transformer, or leakage of the grid cross-connect capacitor to make the grid positively charged, etc.
4? Poor grounding of the whole machine. In particular, the duct welding and filament-powered amplifiers have high requirements for grounding. During the debugging process, each grounding point must be tried continuously to obtain the best signal-to-noise ratio. In addition, the resistance of the grounding point should be as small as possible.
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