A high-quality tube amplifier with 6P3P design

1. Circuit Principle

As shown in the figure below. This circuit has the characteristics of low distortion, low noise, wide frequency response, etc. It is one of the common excellent circuits in the current electronic tube power amplifier circuit. The power tube 6P3P adopts the standard connection method. The signal is input from the control grid (pin ⑤), and the screen grid (pin ④) is connected to the power supply. The characteristic of this connection method is high amplification efficiency. The negative voltage of the 6P3P grid is -19V, the screen voltage is 300V, and the screen current is 60mA. The output power is about 7.5W, which can meet the requirements of general home environment.

The power circuit uses traditional tube rectification and CLC filter to achieve harmony and balance in the sound of the whole machine. The tube rectification has the function of protecting the power tube during the warm-up process when the machine is turned on, which is particularly important when using expensive tubes. The CLC filter has good filtering effect and low internal resistance of the power supply, which is of great benefit to reducing noise and improving the dynamics of the whole machine.

The output transformer is an important part of the tube power amplifier circuit. If you don't have the conditions to make it yourself, you can buy a finished product. The output transformer core used in this machine is 32mmx65mm, with 3300 turns in the primary pole, divided into two layers. The wire diameter is φ0.18mm; the secondary has a total of 172 turns, divided into three layers, and the wire diameter is φ0.82mm. The air gap of the silicon steel sheet is 0.08mm, the working current is 70mA, and the power is 10W.

2. Assembly

The circuit of this machine is simple, and the components used are few. It can be welded by scaffolding, which is simple to manufacture and debug, and has a high success rate. When making it, you can first weld the power supply and the filament power supply part, and then weld the amplifier circuit after the power supply is normal. It should be noted that when the power supply is unloaded, the voltage is slightly higher, and the capacitor withstand voltage must meet the requirements.

3. Testing and debugging

First, check whether there are any quality problems in the circuit welding, such as cold soldering, leaking soldering, short circuit, open circuit, and whether the welding slag and wire ends are clean.

Before powering on, measure the resistance of the DC high-voltage power supply to the ground (at both ends of the high-voltage circuit). The value 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. 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 driver power supply to the ground. The value should be greater than the bleeder resistor.

Power-on measurement, do not plug in the power amplifier tube, and measure the DC voltage value of the anode of the power amplifier tube. The no-load value should be 1.2-1.4 times the effective value of the AC voltage. Measure the sub-high voltage, and the no-load DC voltage should be close to or equal to the anode voltage. Measure the grid bias voltage of the power amplifier tube, and the value should be close to the preset voltage value. At the same time, the negative grid voltage of each power amplifier tube should be adjusted to the maximum value (negative). Measure the voltage values ​​of the voltage amplifier stage and the driving stage, and 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 it to the speaker, disconnect the loop negative feedback circuit. Turn on the machine, connect the red probe of the DC voltmeter to the cathode, insert the black probe into the screw hole of the chassis, adjust the fixed grid bias adjustable resistor, and observe the voltage reading while adjusting. Be careful in this process, move slowly, and make sure to adjust the potentiometer in a small amplitude each time. Divide the voltage reading by the cathode resistance value to get the quiescent current of the tube.

IV. Precautions

1. When debugging the tube amplifier, do not use a dummy load and connect it to a speaker.

Because when using a dummy load, the positive feedback howling cannot be discovered in time, which will cause the anode current of the power amplifier tube to increase sharply in a very short time, causing the primary winding of the output transformer to overcurrent and burn out. At the same time, the anode of the power amplifier tube will turn red due to exceeding the maximum anode dissipation power. Do not take your hand off the power switch when turning on the machine to prevent sudden abnormalities and unnecessary human losses.

2. Since the cathode of the electron tube can only emit electrons after being heated, and the anode can only generate a current, 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 in the debugged tube amplifier. Once an abnormal phenomenon is found, turn off the power immediately to eliminate the fault.

3. The primary of the output transformer and the anode of the power amplifier tube must not be open-circuited, otherwise the screen grid current will increase, causing the screen grid to turn red and burn the tube. The secondary of the output transformer must not be open-circuited with the speaker, otherwise the resistance reflected to the primary will increase. When the anode current of the tube changes, a very 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 due to overcurrent and burn the power amplifier tube due to excessive load.

4. 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 in a very short time due to the absence of grid bias or the presence of positive voltage. The voltage withstand 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 positive current to increase and burn the power amplifier tube.

5. One-point grounding should be used when welding the circuit.

In short, in order to make an ideal tube amplifier, the maker needs to have a deep understanding of the principles of tube amplifiers, repeatedly adjust the working point of each stage and the inter-stage gain, replace different amplifier tubes for matching, and also need to conduct multiple comparisons and auditions to obtain the most ideal performance indicators and the best sound effects.