First, the output power becomes smaller, the sound becomes weak and weak.
1? Power tube aging. The screen flow of the power tube can be measured. With a 100mA DC meter, the negative meter is connected to the screen, the positive meter is connected to the output transformer, and the high voltage can be used to read the screen flow from the meter. Under normal bias conditions, if the measured screen current is less than the normal value, the power tube aging can be explained. If the measured screen current is greater than the normal value, there may be several cases: A, the power tube screen pressure is too high, especially the curtain grid pressure is too high; B, the power tube itself has a quality problem, its own screen consumption is large, the output The power is bound to decrease. If the screen flow is not detected, the power tube is damaged.
2? The gate bias is not normal. In the self-supplied gate biased power amplifier circuit, the common gate bias faults are: A, no bias, the cause of this situation is power tube failure without screen flow, no voltage drop across the cathode resistor, cathode bypass capacitor It is broken down and so on. B. The bias voltage is small because the power tube is aging or the screen voltage is low. C, the bias voltage is high, the reason is that the screen pressure is increased, especially the curtain grid pressure is increased, the screen current is increased, the cathode resistance is increased, the gate junction capacitor is leaking or breakdown, and the positive voltage is added to the gate. Kind. In addition, the open circuit of the cathode resistor also increases the bias voltage, at which time the screen current is small and the line has parasitic oscillations.
3? The output transformer is partially shorted. This will cause the screen to increase, causing the screen to turn red, the output to decrease, and the distortion to increase. If it is a primary local short circuit, the output voltage will not decrease when it is no-load. When the load or load is very light, as long as the gate excitation voltage reaches the rated value, the power tube will be red on all screens. This is a Typical phenomenon. When checking whether the output transformer primary is partially short-circuited, the primary and secondary wirings of the output transformer can be disconnected from the circuit. 220V mains is fed from the primary side, and the voltages of the two primary ends and the B+ center head are measured by the universal meter AC gear. When normal, the voltage at the two terminals is equal. When there is a partial short circuit, the voltage at one line is lower than the voltage at the other line. If the fuse is burned as soon as the 220V mains is connected, the partial short circuit is very serious and the output transformer must be replaced.
Before checking the output transformer secondary for short-circuit faults, first check the high-frequency suppression circuit and the negative feedback circuit components in parallel on the secondary side for deterioration, failure and breakdown, and then check between the secondary line and the core. There is no breakdown short circuit. Welcome to Home Theater Network
4? The boosting stage excitation voltage (or power) is insufficient. The power tube gate excitation voltage (or power) is not enough, no matter how the power tube works normally, there is still no rated power output.
5? Multi-tube parallel push-pull operation, in which one or several of the screen suppression resistors or the gate suppression resistors are open, at this time not only the distortion is large, but also the output power is small.
6? The self-supplied gate biased cathode bypass capacitor fails to form an open circuit, generating negative current feedback, which may affect the output power for some amplifiers.
Second, the power amplifier level high voltage can not be added
There are two situations in which the high voltage cannot be added: first, when the power is turned on, the fuse is immediately blown, and second, the amplifier suddenly blows the fuse during the working process and cuts off the high voltage power supply. Disconnect the high-voltage B+ of the output transformer center of the amplifier from the high-voltage power supply, and then turn on the high voltage. If the fuse is still blown at this time or the high voltage cannot be started, the fault is not in the power amplifier circuit, but in the power supply circuit; if the high voltage B+ is disconnected After the connection, the high voltage can be started, then it is certain that the fault is at the power amplification stage.
The high-voltage power supply of the power amplifier stage should not be checked from the following aspects:
1? Observe or test whether the electrodes are connected inside the power tube.
2? Detect if the output transformer breaks through the short circuit. It is common to have a short circuit between the primary or secondary coils.
3? The load is too heavy or the load is shorted. Excessive load or short circuit can cause the screen to increase and overload, blow the fuse or add high voltage.
Third, parasitic oscillation
When the amplifier has a high-frequency oscillation such as "嘶啦嘶" and a low-frequency oscillation such as "bashing and fluttering", the screen consumption is increased, the screen is red, the output is reduced, and the output is not working. There are several reasons for parasitic oscillations:
1? Negative feedback resistors and other components are deteriorated or damaged.
2? The bypass capacitor of the output transformer secondary parallel circuit opens or breaks down to cause high frequency oscillation.
3? Multi-tube parallel push-pull operation of the screen, grid resistance damage or deterioration is also likely to cause oscillation. To replace the gate resistor, the wirewound resistor should never be used because its inductance will cause oscillation.
4? The power tube, especially the high-conductance power tube and the components in the suppression oscillation circuit, change the parameters after a long time of use, and are also prone to oscillation.
5? The power supply voltage is too high. Because the power supply voltage is too high, the normal operation of the power tube can also cause oscillation.
Fourth, the power tube screen is extremely red
When the amplifier is working normally, it is not normal if the screen is reddish in a brighter environment. The cause of the screen redness may be:
1? The load is too heavy and the screen is too large. This phenomenon is more common, mainly due to improper matching of the speaker impedance, or short circuit on 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 reduction may be: negative bias power supply filter capacitor failure or capacity reduction; voltage divider load potentiometer center slider is too low; rectifier aging; bias power transformer secondary partial short circuit; self-sufficient gate bias The cathode bypass capacitor of the voltage is seriously drained; the primary and secondary (or coupling capacitors) of the input transformer are slightly leaky.
The reason for no negative gate bias may be: input transformer center tap open; bias power supply filter capacitor short circuit; bias load resistance is damaged. The rectifier or bias power transformer is damaged; the self-contained negative gate bias cathode bypass capacitor breakdown; the gate resistor or the input transformer secondary open circuit; the stem is damaged, and the gate pin is separated from the stem.
3? The screen pressure or the curtain grid pressure of the power tube of the latter stage is increased, so that the screen flow is increased and the screen is red.
The reason for the increase of the screen voltage may be: A, the local coil of the high voltage power transformer is partially short-circuited, so that the AC voltage of the secondary high-voltage coil is increased; the output DC voltage is increased after rectification; B, the discharge circuit is blocked, and the output voltage is increased. C. The filter choke coil is partially short-circuited, the inductance is reduced, the step-down is reduced, and the output voltage is increased.
The curtain grid voltage rises (referring to the machine that uses the beam quadrupole and the pentode to make the power amplifier stage), the ability to absorb electrons is enhanced, the screen flow is increased, and the screen is red. Some of the reasons may be: A, the primary partial short circuit of the high voltage power transformer, so that the secondary high voltage rises, and the rectified output DC voltage increases. B. The secondary high voltage potentiometer is improperly adjusted. C. The secondary high voltage filter chokes a local short circuit between turns, causing the output voltage to rise. D. The discharge circuit is blocked and the output voltage is increased.
4? Super audio or high frequency parasitic oscillations, causing the screen to become red. These two parasitic oscillations are caused by the positive feedback of the total parasitic capacitance of the latter stage. An effective method of judging is to change the load impedance to a resistance of about 1/20 of the output power of the amplifier when the screen is red, and the resistance is equal to the output impedance. When the power is turned on, the signal is not sent. After a few minutes, if the hand touches the resistor and feels hot, there is a 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 KT-88 tube every two parallel), one of the tubes is aging, the internal resistance increases the screen flow, and the aging tube is not burdened. Heavy, the screen flow increases, and the screen is red.
6? A short-circuit on one side of the primary coil of the output transformer destroys the push-pull balance, causing the screen flow on the side to increase and the screen to become red.
7? The input signal is too large, so that the output current and voltage exceed the rated value, causing the screen to become red.
8? Some amplifiers are not designed properly. Due to the screen pressure, the curtain grid pressure, the filament voltage is too high, or the negative gate bias is too small, the static screen flow is too large, even when static, the screen will be red.
Five, distortion
The so-called distortion means that the output of the amplifier is too different from the input waveform, and 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 gain of the two tubes is different, or the output transformer primary (or the secondary of the input transformer) is partially short-circuited or open; the screen and gate The extreme value of the anti-vibration resistor also destroys the push-pull balance and causes distortion.
2? Some amplifier push-pull and the front stage are coupled with a RC, and the distortion occurs when the coupling capacitor on one side becomes variable (capacity becomes small, failure, leakage, etc.). If the capacitor leaks, it will also make the negative gate bias of the next-stage tube smaller, or even become a positive voltage, creating a gate current, causing distortion.
3? Fixed negative gate bias is too high or too low, so that the working point of the tube changes, or the input signal is too large, so that the tube can work in the nonlinear part, causing distortion.
4? The power amplifier of the small power amplifier generally works in the AB1 class (or class A) push-pull amplification. If the input signal voltage peak value is greater than the negative gate bias voltage, the power tube will have a gate current due to the operation of the gate. The resistance is large, so it is easy to cause distortion.
5? In the amplifier above the medium power, the power tube generally works in the AB2 class (or class B) push-pull amplification. If the output power of the push stage is insufficient or the internal resistance is too large due to the aging of the push tube, it will cause distortion. To push the stage, use an electron tube with a small internal resistance and reverse phase with a step-down transformer to obtain a stable output voltage.
6? Screen load resistance, cathode resistance or screen grid resistance change, the operating point of the tube changes, working in the nonlinear region, causing distortion. The gate resistor is broken, causing blocking distortion. At the same time, the load impedance is too light or too heavy, causing the output impedance of the tube to be mismatched to cause distortion or sound. Welcome to Home Theater Network
7? The power supply voltage is unstable or too high or too low, which will change the working point of the tubes at all levels and cause distortion.
Sixth, the sound of communication
Generally speaking, since the voltage amplification factor of the latter stage is not large, the hum which is caused by the power amplifier level fault is not very obvious, but there are several faults but a significant hum can be seen.
1? The inner side of the power tube is short-circuited or leaked, and the cathode and the filament are short-circuited, and the filament power transformer is poorly grounded.
2? Poor fixed bias filtering.
3? Push the leakage between the primary and secondary of the transformer, or the leakage of the gate junction capacitor to make the gate positively charged.
4? The whole machine is poorly grounded. In particular, the amps for the welding of the scaffold and the AC power supply for the filaments have high requirements for grounding. During the commissioning process, the grounding points should be tried continuously to obtain the best signal-to-noise ratio. The smaller the resistance of the grounding point, the better.
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