Analysis of Common Faults of Tube-amplifier Power Amplifiers

                                  Analysis of Common Faults of Tube-amplifier 

First: The output power becomes smaller and the sound becomes weak

1. Power tube aging

The screen current of the power tube can be measured. Use a 100mA DC ammeter, connect the negative test lead to the screen pole, and the positive test lead to the output transformer. Under normal bias conditions, if the measured screen current is less than the normal value, it can indicate that the power tube is aging.

2. The grid bias is abnormal

In self-sufficient grid bias power amplifier circuits, common grid bias faults are:

A. There is no bias voltage. There are several reasons why the power tube fails and there is no screen current, there is no voltage drop across the cathode resistor, and the cathode bypass capacitor is broken down.

B. The bias voltage is small, because the power tube is aging or the screen voltage is low.

C. The bias voltage is high, due to the increase of the screen voltage, especially the increase of the screen grid voltage, the increase of the screen current, the increase of the resistance of the cathode resistance, the leakage or breakdown of the grid cross-link capacitor, which causes a positive voltage on the grid, etc.

In addition, the open circuit of the 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. Partial short circuit of transformer out of elm

When there is a partial short circuit, the voltage at one terminal is lower than the voltage at the other terminal. Before checking whether there is a short-circuit fault on the secondary side of the output transformer, first check whether the high-frequency suppression circuit and negative feedback circuit components connected in parallel on the secondary side have deterioration, failure and breakdown, and then check the connection between the secondary line and the iron core. Whether there is a breakdown short circuit.

Second: the high voltage of the power amplifier stage cannot be added

There are two situations when the high voltage cannot be added: one is that the fuse blows immediately when the power is turned on, and the other is that the amplifier suddenly blows the fuse during work and cuts off the high voltage power supply. Start checking from the following aspects:

1. Observe or test whether the electrodes inside the power tube are connected.

2. Check whether the output transformer is broken down or short circuited. It is common that the primary or secondary coils are broken down and shorted.

3. The load is too heavy or the load is short-circuited. Overload or short circuit can cause the screen current to increase and overload, blow the fuse or fail to apply high voltage.

Third: Parasitic oscillation

When the amplifier has parasitic oscillations such as "hissing" high-frequency oscillations and "fluttering, fluttering" low-frequency oscillations, the reasons are as follows:

1. Components such as negative feedback resistors have deteriorated or been damaged.

2. The open circuit or breakdown of the bypass capacitor connected in parallel to the secondary side of the output transformer causes high-frequency oscillation.

3. The screen and grid resistance of multi-tube parallel push-pull work are damaged or deteriorated, which is also easy to cause oscillation. To replace the gate resistor, never use a wirewound resistor, as its inductance will cause oscillations.

4. The parameters of the power tube, especially the high mutual conduction power tube and the components in the oscillation suppression circuit will change after a long time of use, and it is also easy to generate 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, which can also cause oscillation.

Fourth: The power tube screen is extremely red

When the amplifier is working normally, if the screen is red in a bright environment, it is abnormal. The causes of the redness of the screen may be:

1. The load is too heavy and the screen current is too large

This phenomenon is relatively common, mainly due to improper speaker impedance matching, or a short circuit in the external line, or a partial short circuit in the primary coil of the output transformer.

2. Negative grid bias decreases, or there is no negative grid bias, or positive grid bias occurs

Possible reasons: failure of the filter capacitor of the negative bias power supply; the center slider of the voltage divider load potentiometer is adjusted too low; aging of the rectifier tube: open circuit of the center tap of the input transformer; short circuit of the filter capacitor of the bias power supply; open circuit of the gate resistor or the secondary of the input transformer ; The tube base is damaged, so that the gate pin is separated from the tube base, etc.

3. The screen pressure or curtain grid pressure of the rear stage power tube increases

Increase the screen current and make the screen extremely red.

Reasons for increased screen pressure may be:

A. The primary coil of the high-voltage power transformer is partially short-circuited, which increases the AC voltage of the secondary high-voltage coil; the output DC voltage increases after rectification;

B. The discharge resistor is disconnected and the output voltage rises;

C. The filter choke coil is partially short-circuited, the inductance is reduced, the step-down is reduced, and the output voltage is increased.

4. Ultra-audio or high-frequency parasitic oscillations cause the screen to turn red

These two parasitic oscillations are caused by the positive feedback of the total parasitic capacitance of the subsequent stage. The effective judgment method is that when the screen is red, replace the load impedance with a resistor with an amplifier output power of V20, and the resistance value is equal to the output impedance. No signal is sent in when the power is turned on. If you touch the resistor with your hands and feel heat after a few minutes, then there is high-frequency parasitic oscillation.

5. The aging of the push-pull tube destroys the push-pull balance and causes redness of the screen

In push-pull power amplifiers, especially in parallel push-pull (for example, in a 150W loudspeaker, two KT-88 tubes are generally connected in parallel), one side of the tubes is aging, the internal resistance increases, and the screen current decreases, so there is no burden on the aging tubes. Heavy, the screen current increases, and the screen becomes 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 this side, and causes the screen to turn red.

7. The input signal is too large, so that the output current and voltage exceed the rated value, causing the screen to turn red.

8. Some amplifiers are not designed properly. Due to the high screen voltage, screen voltage, and filament application pressure, or the negative grid bias voltage is too small, the static screen current is too large, and even when it is static, the screen will turn red.

Fifth: Distortion

The so-called distortion means that the difference between the output of the amplifier and the input waveform is too large, and the sound amplified by the amplifier is different from the original input sound. The main reasons are analyzed as follows:

1. One of the push-pull power tubes or push-pull tubes is aging (or damaged); the anti-vibration resistance of the screen and grid changes, which will also destroy the push-pull balance and cause distortion.

2. Some push-pull amplifiers are coupled with the front stage by resistance-capacitance. When the value of the coupling capacitor on one side changes (capacity changes, failure, leakage, etc.), distortion occurs.

3. The power tubes of small power amplifiers generally work in Class 1 (or Class A) push-pull amplification. If the peak value of the input signal voltage is greater than the negative grid bias voltage, the power tube will have a grid current. The internal resistance is large, so it is easy to cause distortion.

4. The value of the load resistance of the screen electrode, the cathode resistance or the grid grid resistance changes, so that the working point of the electron tube changes, and it works in the non-linear region, causing distortion. The gate resistor is open circuited, 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.