How to repair a tube amplifier? Analysis of the six common faults

1. The output power decreases and the sound becomes weak

1. Aging of the power tube. The screen current of the power tube can be measured. Use a 100ma DC meter, connect the negative pen to the screen electrode, and the positive pen to the output transformer. Turn on the high voltage to read the screen current from the meter. Under normal bias conditions, if the measured screen current is less than the normal value, it can be said that the power tube is aging. If the measured screen current is greater than the normal value, there may be several situations: a. The screen voltage of the power tube is too high, especially the screen grid voltage is too high; b. The quality of the power tube itself is problematic, the screen consumption itself is large, and the output power is bound to decrease. If the screen current cannot be measured, it means that the power tube is damaged.

2. Abnormal grid bias. In the power amplifier circuit with self-supplied grid bias, common grid bias faults include: a. No bias. The reasons for this situation include power tube failure, no screen current, no voltage drop across the cathode resistor, and breakdown of the cathode bypass capacitor. b. Low bias. The reason is aging of the power tube or low screen voltage. c. High bias. The reasons include increased screen voltage, especially increased screen current due to increased screen voltage, increased cathode resistance, leakage or breakdown of the grid cross-connected capacitor, and positive voltage on the grid. In addition, an open cathode resistor will also increase the bias. At this time, the screen current is very small and there is parasitic oscillation in the circuit.

3. The output transformer is partially short-circuited. This will cause the screen current to increase, causing the screen to turn red, the output to decrease, and the distortion to increase. If the primary is partially short-circuited, the output voltage will not decrease when it is unloaded. When the load is connected or the load is very light, as long as the gate excitation voltage reaches the rated value, the screen of all power tubes will turn red. This is a typical phenomenon. When checking whether the primary of the output transformer is partially short-circuited, the primary and secondary wiring of the output transformer can be disconnected from the circuit, and 220v AC power can be sent from the primary end. Use the AC gear of the multimeter to measure the voltage between the two primary ends and the center head of b+. Under normal circumstances, the voltages of the two ends are equal. When there is a partial short circuit, the voltage of one end is lower than that of the other end. If the fuse burns out immediately when the 220v AC power is connected, it means that the partial short circuit is very serious and the output transformer must be replaced.

Before checking whether there is a short circuit fault on the secondary of the output transformer, first check whether the high-frequency suppression circuit and negative feedback circuit components connected in parallel on the secondary are deteriorating, failing, or breaking down, and then check whether there is a breakdown short circuit between the secondary line and the iron core.

4. The driving stage excitation voltage (or power) is insufficient. If the gate excitation voltage (or power) of the power tube is insufficient, the rated power output cannot be achieved no matter how normal the power tube works.

5. When multiple tubes are connected in parallel for push-pull operation, the screen suppression resistor or grid suppression resistor of one or more tubes is open-circuited. At this time, not only is the distortion large, but the output power is also small.

6. The cathode bypass capacitor of the self-supplied gate bias fails and forms an open circuit, generating negative current feedback, which may affect the output power of some tube amplifiers.

2. The power amplifier stage cannot add high voltage

There are two situations when the high voltage cannot be added: one is that the fuse burns out immediately when the power is turned on, and the other is that the fuse suddenly burns out during the operation of the tube amplifier and cuts off the high voltage power supply. Disconnect the high voltage b+ of the center head of the amplifier's output transformer from the high voltage power supply, and then turn on the high voltage. If the fuse still burns out or the high voltage cannot be started at this time, the fault is not in the power amplifier circuit, but in the power supply circuit; if the high voltage can be started after disconnecting the high voltage b+ connection, then it can be confirmed that the fault is in the power amplifier stage.

If the high voltage power supply of the power amplifier stage cannot be added, the following aspects should be checked:

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

2. Check if the output transformer is short-circuited. The most common problem is that the primary or secondary coil is short-circuited.

3. Overload or short circuit. Overload or short circuit can increase the screen current and cause overload, burn out the fuse or fail to apply high voltage.

3. Parasitic Oscillation

When the amplifier has parasitic oscillation sounds such as "hissing" high-frequency oscillation and "puffing" low-frequency oscillation, the screen loss will increase, the screen will turn red, the output will decrease, and it will not work at all. There are several reasons for parasitic oscillation:

1. Negative feedback resistors and other components are deteriorating or damaged.

2. The bypass capacitor in parallel with the secondary of the output transformer is open-circuited or broken down, causing high-frequency oscillation.

3. Damage or deterioration of the screen and grid resistors of multi-tube parallel push-pull operation can easily cause oscillation. When replacing the grid resistor, do not use a wire-wound resistor because its inductance will cause oscillation.

4. Power tubes, especially high mutual conductance power tubes, and components in oscillation suppression circuits may have parameter changes after long-term use, which may also easily cause oscillations.

5. The power supply voltage is too high. If the power supply voltage is too high, the normal working state of the power tube will be destroyed and oscillation may also be caused.

4. The power tube screen turns red

When the amplifier is working normally, if you see the screen turn red in a bright environment, it is an abnormal phenomenon. The reasons for the screen turning red may be:

1. Excessive load causes excessive screen current. This phenomenon is quite common, mainly due to improper speaker impedance matching, short circuit of external line, or partial short circuit of primary coil of output transformer.

2. The negative gate bias decreases, or there is no negative gate bias, or a positive gate bias appears.

The reasons for the reduction of negative gate bias may be: failure or reduction of capacity of the negative bias power supply filter capacitor; the center slider of the voltage divider load potentiometer is adjusted too low; aging of the rectifier tube; local short circuit of the secondary of the bias power supply transformer; serious leakage of the cathode bypass capacitor of the self-supplied gate bias; slight leakage of the primary and secondary (or coupling capacitor) of the input transformer, etc.

The reasons for the lack of negative grid bias may be: the center tap of the input transformer is open; the bias power supply filter capacitor is short-circuited; the bias load resistor is damaged. The rectifier or bias power supply transformer is damaged; the self-supplied negative grid bias cathode bypass capacitor is broken; the grid resistor or the secondary of the input transformer is open; the tube socket is damaged, causing the grid pin to be separated from the tube socket.

3. The screen voltage or screen grid voltage of the subsequent power tube increases, which increases the screen current and makes the screen turn red.

The reasons for the increase in screen voltage 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 circuit is blocked, and the output voltage increases. c. The filter choke coil is partially short-circuited, the inductance is reduced, the voltage reduction is reduced, and the output voltage increases.

The screen grid voltage increases (referring to machines that use beam tetrodes and pentodes as power amplifiers), and the ability to absorb electrons increases, which increases the screen current and makes the screen red. Some of the reasons may be: a. The primary of the high-voltage power transformer is short-circuited locally, which increases the secondary high voltage and increases the rectifier output DC voltage. b. Improper adjustment of the secondary high-voltage potentiometer. c. The turns of the secondary high-voltage filter choke are short-circuited locally, which increases the output voltage. d. The discharge circuit is blocked, and the output voltage increases.

4. Ultrasonic or high-frequency parasitic oscillations cause the screen to turn red. These two types of parasitic vibrations are caused by the positive feedback of the total parasitic capacitance of the subsequent stage. An effective way to judge is that when the screen turns red, replace the load impedance with a resistor of about 1/20 of the amplifier output power, and the resistance value is equal to the output impedance. Turn on the machine without sending any signal. After a few minutes, if you feel the heat when you touch the resistor, then there is high-frequency parasitic oscillation.

5. The push-pull tube ages, destroying the push-pull balance and causing the screen to turn red. In a push-pull amplifier, especially in a parallel push-pull (such as a 150W amplifier that generally uses two KT-88 tubes in parallel), if one of the tubes ages, the internal resistance increases and the screen current decreases, the tube that has not aged is overloaded, the screen current increases, and the screen turns 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 causes the screen to turn red.

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

8. Some amplifiers are improperly designed. The screen voltage, screen grid voltage, filament voltage are too high, or the negative grid bias is too small, the static screen current is too large, and even in static state, the screen electrode will turn red.

5. Distortion

Distortion refers to the large difference between the output and input waveforms of the amplifier, 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 the driver-stage push-pull tubes is aging (or damaged), causing the gains of the two tubes to be different, or one side of the primary of the output transformer (or the secondary of the input transformer) is partially short-circuited or open-circuited; the change in the anti-vibration resistance of the plate and the grid will also destroy the push-pull balance and cause distortion.

2. Some amplifiers use RC coupling with the previous stage. When the coupling capacitor on one side changes value (capacity decreases, fails, leaks, etc.), distortion occurs. If the capacitor leaks, the negative grid bias of the next stage electron tube will decrease, or even become a positive voltage, generating grid current and causing distortion.

3. If the fixed negative grid bias is too high or too low, the working point of the electron tube will change, or the input signal is too large, etc., which can cause the electron tube to work in the nonlinear part and cause distortion.

4. The power tubes of small power amplifiers generally work in AB1 class (or A class) push-pull amplification. If the peak voltage of the input signal is greater than the negative gate bias, gate current will appear in the power tube. Since the internal resistance of the gate circuit in this type of working state is large, it is easy to cause distortion.

5. In amplifiers above medium power, power tubes generally work in AB2 (or B) push-pull amplification. If the output power of the driver stage is insufficient or the internal resistance is too large due to aging of the driver tube, distortion will occur. The driver stage should use a tube with small internal resistance and use a step-down transformer for phase inversion to obtain a stable output voltage.

6. The value of the screen load resistance, cathode resistance or screen grid resistance changes, causing the working point of the electron tube to change and work in the nonlinear region, causing distortion. The grid resistance is open-circuited, causing blocking distortion. At the same time, the load impedance is too light or too heavy, causing the output impedance of the electron tube to be mismatched, causing distortion or weak sound, etc.

7. If the power supply voltage is unstable or too high or too low, it will change the operating points of each level of the electron tube and cause distortion.

6. Communication sound

Generally speaking, since the voltage amplification factor of the subsequent stage is not large, the AC noise caused by a power amplifier stage failure is not very obvious, but there are several types of failures that can cause obvious AC noise.

1. The gate and cathode inside the power tube are short-circuited or leaking, the cathode and filament are short-circuited, and the filament power transformer is poorly grounded.

2. Fixed bias filtering is poor.

3. The leakage between the primary and secondary of the driving transformer, or the leakage of the gate cross-connected capacitor makes the gate positively charged, etc.

4. The whole machine is poorly grounded. Especially for scaffolding welding and tube amplifiers powered by AC power for filaments, the grounding requirements are very high. During the debugging process, various grounding points should be constantly tested to obtain the best signal-to-noise ratio and the resistance of the grounding point.