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1 Measurement before power-on
The resistance of the DC high-voltage power supply to ground (both ends of the high-voltage circuit) should be close to or equal to the resistance of the discharge resistor. Measure the resistance between the AC incoming circuit and ground. The value should be infinite. Measure whether there is an open circuit (resistance is infinite) or short circuit (resistance is about zero), the normal value should be close to the DC resistance of the load. Measure the resistance of the power supply of the voltage amplification stage and the driving stage to the ground, the value should be greater than the discharge resistance.
2 Measurement after power-on
After the power is not inserted into the power amplifier tube, measure the DC voltage value supplied to the anode of the power amplifier level. The no-load value should be 1.2 to 1.4 times the effective value of the AC voltage. To measure the secondary high voltage, the no-load DC voltage should be close to or equal to the anode voltage (the voltage regulator circuit should be equal to the output value of the regulator). Measure the grid bias voltage of the power amplifier tube (using a fixed bias voltage), the value should be close to the predetermined voltage value. At the same time, the grid negative pressure of each power amplifier tube should be adjusted to the maximum value (negative). Measure the voltage value of the supply voltage amplification stage and the driving stage. The anode voltage of each stage should be close to or equal to the set working voltage value.
Adjust the static current of the power amplifier tube, plug in the power amplifier tube and connect the speaker. Open the loop negative feedback circuit. Turn on the power and connect the DC voltmeter to the cathode of the power amplifier tube (insert the black test pen in the screw hole of the chassis and connect the red test pen to the cathode), adjust the fixed grid bias adjustable resistance, and observe the voltage reading while adjusting. During this process, you must be careful and move slowly, and the amplitude of the potentiometer adjustment must be small each time. Divide the reading of the voltmeter by the value of the cathode resistance, which is the static current of the tube.
It is especially important to note that no dummy load should be used when debugging the tube amplifier (change the traditional concept of using dummy load in the transistor circuit), and the speaker should be connected. Because when using a false load, the positive feedback howling will make the strong ultrasonic frequency oscillation not found in time, and it will cause the anode current of the power amplifier tube to increase sharply in a short time, causing the output transformer primary winding to burn out due to overcurrent. At the same time, the power amplifier tube also causes the anode to become red due to exceeding the maximum anode dissipation power. Do not leave the power switch when starting the machine to prevent sudden abnormal situations, which may lead to unnecessary human losses. Because the cathode of the electron tube is heated, it can emit electrons. Only anode current will be generated. Therefore, there is a transition time of a few seconds from the warm-up state to the normal working state. During this time, observe with your eyes and listen with your ears to observe the changes of the tuned amplifier. Once you find abnormal phenomena, you should immediately turn off the power to eliminate the fault.
The primary of the output transformer and the anode of the power amplifier tube must not be open, otherwise the curtain grid current will increase and the curtain grid will redden and burn the electron tube. The secondary of the output transformer must not be open to the speaker, otherwise the resistance reflected to the primary will increase, and when the anode current of the tube changes, an extremely high induced voltage will break down the insulating layer and burn the output transformer. The secondary of the output transformer must not be short-circuited for a long time, otherwise the anode of the power amplifier tube will be red and burned due to excessive load.
The fixed grid bias circuit must not be open circuit, short circuit or other abnormal conditions, otherwise the anode will be red and burned in a short time due to no grid bias voltage or positive voltage of the power amplifier tube. The withstand voltage of the cathode bypass capacitor of the power amplifier tube of the self-sufficient gate bias circuit must be large and highly reliable. Otherwise, once the breakdown short circuit causes the grid cathode to have the same potential, the positive current will increase and burn the power amplifier tube.
If you can strictly follow the above method. It is very helpful for protecting output transformer and power amplifier tube.
The factor that has a greater influence on the life of the bile duct is the working state of the common cathode. If the cathode is not preheated or the preheat is not sufficient, high voltage electricity is applied to the screen and the curtain grid, which will cause the bile duct to emit electrons in the cold cathode field. , Often emit electrons in the cold cathode electric field will accelerate the bile duct cathode poisoning and premature aging. If no effective control method is adopted, the service life of bile duct will be shortened. In order to delay the aging of the bile duct cathode and protect the filament from the impact of large currents at startup, a protection circuit should be installed to protect the bile duct. When starting, regulate the high-voltage delay start and the soft start of the filament. To avoid damage to the bile duct when the power supply is impacted when starting up, the control circuit is used to protect the bile duct and extend the service life of the bile duct.
In short, to make an ideal tube amplifier, the producer needs to understand the principle of the tube amplifier, repeatedly adjust the working point of each stage and the inter-stage gain, replace different amplifier tubes to match, and need to go through multiple comparison auditions to find the most ideal Performance index and the best sound effect.
Precautions for tube commissioning
Check whether there are quality problems in circuit welding and whether there is any improper welding process. Whether the ground wire and the wiring are reasonable is an important factor to improve the success rate of debugging the bile machine and improve the quality of the bile machine.