Notes on Tube Amplifier Debugging

Check whether there are any quality problems in the circuit welding and whether there are any improper welding processes. Whether the ground wire and the wiring are reasonable is an important factor in improving the success rate of debugging the tube amplifier and improving the quality of the tube amplifier.

1 Measurement before power
on The resistance of the DC high-voltage power supply to the ground (at both ends of the high-voltage circuit) should be close to or equal to the resistance of the bleeder resistor. Measure the resistance between the AC power input circuit and the ground, and the value should be infinite. Measure whether the output is open (infinite resistance) or short (about zero resistance). The normal value should be close to the DC resistance of the load. Measure the resistance of the voltage amplifier and the driving power supply to the ground, and the value should be greater than the bleeder resistor.

2 Measurement after power on
After powering on without plugging in the power amplifier tube, measure the DC voltage value supplied to the anode of the power amplifier stage. The no-load value should be 1.2 to 1.4 times the effective value of the AC voltage. Measure the sub-high voltage 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 supplied to the power amplifier tube (using a fixed bias voltage), and the value should be close to the predetermined voltage value. At the same time, the negative voltage of the grid of each power amplifier tube should be adjusted to the maximum value (negative). Measure the voltage values ​​of the supply voltage amplifier 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 quiescent current of the power amplifier tube, plug in the power amplifier tube and connect the speaker. Disconnect the loop negative feedback circuit. Power on the machine, connect the DC voltmeter to the cathode of the power amplifier tube (insert the black test lead into the screw hole of the chassis and the red test lead to the cathode), adjust the fixed grid bias adjustable resistor, and observe the voltage reading while adjusting. Be careful in this process, move slowly, and adjust the amplitude of the potentiometer each time. Divide the reading of the voltmeter by the cathode resistance value, which is the quiescent current of the tube. It
is particularly important to note that when debugging the tube amplifier, a dummy load must not be used (change the traditional concept of using a dummy load in a transistor circuit), and a speaker should be connected. Because when using a dummy load, the positive feedback howling will make the strong ultrasonic frequency oscillation not be discovered in time, which will cause the anode current of the power amplifier tube to increase sharply in a very short time, resulting in overcurrent and burning of the primary winding of the output transformer. At the same time, the anode of the power amplifier tube will turn red due to exceeding the maximum anode dissipation power. Do not leave the power switch when turning on the machine to prevent sudden abnormalities and unnecessary human losses. Since the cathode of the electron tube can only emit electrons after heating, the anode will have positive current. Therefore, there is a transition time of several seconds from the preheating state to the normal working state. During this time, use your eyes and ears to observe the changes of the debugged tube amplifier. Once an abnormal phenomenon is found, turn off the power supply immediately to troubleshoot.
The primary of the output transformer and the anode of the power amplifier tube must not be open, otherwise the screen grid current will increase, causing the screen grid to turn red and burn the electron tube. The secondary of the output transformer must not be open with the speaker, otherwise the resistance reflected to the primary will increase. When the anode current of the electron tube changes, an extremely high induced voltage will be generated to break through the insulation 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 turn red and burn out due to excessive load.
The fixed grid bias circuit must not be open, short-circuited or have other abnormal conditions, otherwise the anode of the power amplifier tube will turn red and burn out in a very short time due to the lack of grid bias or the presence of positive voltage. The withstand voltage value of the cathode bypass capacitor of the power amplifier tube
in the self-supplied grid bias circuit must be large and reliable. Otherwise, once the short circuit is broken down, the grid and cathode will have the same potential, causing the anode current to increase and burn out the power amplifier tube. If the above method can be strictly followed, it will be very helpful to protect the output transformer and the power amplifier tube.
The factor that has a greater impact on the life of the tube is the working state of the common cathode. If the high voltage is applied to the screen and the screen grid when the cathode is not preheated or the preheating is insufficient, the tube will emit electrons in the cold cathode field. Frequent electron emission in the cold cathode electric field will accelerate the cathode poisoning and premature aging of the tube. If effective control methods are not adopted, the service life of the tube will be shortened. In order to delay the aging of the cathode of the tube and protect the filament from the impact of high current when starting up, a protection circuit should be installed to protect the tube. When starting up, adjust the high voltage delay start and the soft start of the filament. Avoid damage to the tube caused by power surges when starting up, and use the control circuit to protect the tube and extend the service life of the tube.
In short, to make an ideal tube amplifier, the maker needs to have a deep understanding of the principle of the tube amplifier, repeatedly adjust the working point of each stage and the inter-stage gain, replace different amplifier tubes for matching, and need to compare and audition many times to obtain the most ideal performance indicators and the best sound effects.