6N2 + 6P3P cheap single-ended tube amplifier The author has produced a post-tube power amplifier. The bile tube used is the cheap and easy-to-purchase 6N2 + 6P3P. The audition effect is good. Write the production process and steps to share with enthusiasts (see the circuit).
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Description for 1.2L Food Choppers
300W/350W
with dobule blades
1 speed easy to control
Carton box: 60.5*41*26.5cm 6pcs/ctn
20'GP: 2646pcs 40'HQ: 6294pcs
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1. Circuit form The front stage voltage amplifier adopts a common cathode amplifier circuit + cathode output device, and the latter stage adopts a single-ended Class A circuit. The beam-emitting diode 6P3P is connected to a triode. In theory, the triode connection method is obviously better than the standard connection method and the super linear connection method. The only shortcoming is that the conversion efficiency of the anode is low and the output power is small; each bile duct in the circuit is in a class A working state The screen current change is small and stable. Both front and back stages adopt self-sufficient grid bias with simple circuit and stable and reliable operation. The whole machine has no negative feedback of this stage and large loop.
Second, the production process The base was transformed from the Canadian-made UNIKA engineering satellite receiver purchased for 20 yuan from the second-hand market. The transformer is installed horizontally. The method of opening the square hole of the transformer is to drill a few small holes side by side with a fine drill at the four corners of the design position, then saw it out with a hacksaw blade, flatten the four corners with a flat file, and then smooth it with fine sandpaper. glitch. Although this method is "stupid", the holes are very precise.
2. Circuit resistors, capacitors, etc. do not pursue expensive expensive products, and use ordinary goods accumulated over the years. The carbon film and metal film resistors are mixed, but the corresponding positions in the two channels use the same type of parameters, the power is above 1W, and the individual positions use 3W. The only coupling capacitor in the circuit is a 0.82μF polypropylene film capacitor, and the filter capacitor is all “black diamondâ€. The power transformer and output transformer are the finished products from Hebei Yongnian mail order.
Following the principle of "conciseness first", the power supply is a crystal full-bridge rectifier. Although the tube rectifier has a more daring taste, the current supply speed is not good, and the feet are slightly soft during large dynamic playback. The filter circuit uses several large-capacity capacitors and a high-power resistor to form a CRC filter network.
Select and place the required resistance and capacitance of the two channels separately, and measure and pair them with a multimeter one by one to ensure that the corresponding position parameters of the two channels are consistent.
The circuit of the whole machine adopts scaffolding welding, and a copper wire with a diameter of 1.5mm is set as the ground bus between the two power tube bases, and the grounding components of the front and rear stages are connected to this line. The signal input base to the potentiometer and then to the 6N2 grid all use high-quality double-core shielded wire and one end is grounded, and the tube filament is powered by AC. The connection of the resistance components in the circuit should be as short as possible. The AC power cord and the filament wire should be tightly twisted with high-quality wires and attached to the bottom board to reduce the interference of the AC magnetic field and cannot be connected to the signal channel of the amplifier circuit. The wiring is parallel, not close to the gate components of the input stage. The center of the filament power supply should be reliably grounded.
Third, the debugging steps The debugging of the tube power amplifier circuit is a complicated and meticulous work. After the entire circuit is connected, do not rush to turn on the power first. You should carefully check the circuit for any wrong or missing components and circuits. Lead, and use a multimeter to measure the resistance value of each key point in the circuit to ground one by one.
1. Measurement before power-on 1) The ground resistance of the AC inlet circuit should be infinite, and the resistance of the primary coil of the power transformer should be displayed when the power switch is placed in the "ON" position.
2) Measure the resistance to ground of the high-voltage rectifier circuit. The value should be close to or equal to the discharge resistance.
3) Measure the resistance of the two-channel front-end power circuit to ground. The measured value should be close to or equal to the resistance of the bleeder resistor.
4) Measure the resistance to ground of each symmetrical point in the two channels, and the measurement result should be very close or equal.
2. Measurement after power-on After the power-on measurement has no abnormality, connect a speaker with the same nominal impedance as the output transformer to conduct power-on measurement.
1) Measurement without insertion of the tube, without plugging in the electronic tube, connect the power supply of the whole machine, measure the rectified DC high voltage, the positive electrode of the decoupling capacitor of the previous stage power supply, and the no-load ground voltage of the tube base of each tube with a multimeter. The design values ​​are basically consistent.
2) Intubation measurement When the voltage of each part is measured without intubation, insert the electronic tube after turning off for a few seconds, turn on the power switch, do not leave the power switch at this time, and closely monitor the machine for excessive heat, fire, or risk Where the smoke is, see if the filaments of the tubes are lit normally. If there is any abnormal phenomenon, it should be shut down in time to eliminate the fault. If it is normal, you can use a multimeter to measure the DC high voltage. If the high voltage drop after loading is too large, it proves that the power margin of the power transformer is too small or the internal resistance of the power supply line is too large. Measure the voltage value of the anode and cathode of each tube to ground. The screen voltage should be within the range of the tube parameters. After measuring the cathode voltage, the screen current can be converted using Ohm's law (I = U / R), and the screen power consumption of the tube can be converted according to the formula P = UI. This value cannot exceed the maximum screen power consumption of the tube Dissipated power! Then measure the voltage value of the two-channel symmetrical point to ground, the measured value should be the same. At this time, there should be a weak circuit noise in the speaker. If a large amount of hum occurs, it is due to poor filtering, decoupling, or inappropriate grounding. The source of the fault should be found and eliminated.
At this point, the sound quality performance of this unit is basically satisfactory after accessing the audio source for listening.
In order to ensure that each tube works in the best condition and improves the sound quality, the working points of the front and rear tubes must also be adjusted.
3. Adjust the static working point of the tube.
1) Pre-stage adjustment The working point of the pre-stage amplifier tube is taken in the middle of the grid voltage-screen current characteristic curve. The grid negative pressure should be half of the maximum grid negative pressure of the amplifier tube. The working current is adjusted to 40% of the maximum screen current of the amplifier tube. % -50% is the best. This machine adjusts the grid negative pressure of the input stage to -1.17V, the screen current is about 2.1mA, 6N2 performs well, and the hearing is satisfactory.
When adjusting, short-circuit the signal input to ground, change the resistance value of the load resistance of the screen of the amplifier tube or the resistance of the cathode, you can change the negative gate voltage and the size of the screen current, the relationship is: a. The screen resistance remains unchanged, the cathode resistance ↑ —cathode voltage ↑ —gate voltage ↓ —screen current ↓, and vice versa. B: The cathode resistance is unchanged, the screen resistance ↑ —cathode voltage ↑ —gate voltage ↓ —screen current ↓, and vice versa. In this level of adjustment, the screen current and grid negative pressure of the bile duct will be affected by both the screen resistance and the cathode resistance.
2) After the adjustment, the working point of the Class A power amplifier tube is on the straight line part of the grid voltage-screen current characteristic curve. The amplitude of the grid's input signal should not exceed the negative pressure range of the power amplifier tube. Exceeding it will increase distortion. The characteristic of Class A power amplifier is that the working current remains unchanged when strong and weak signals are input. During inspection, the ammeter can be connected in series in the screen loop of the power amplifier tube. When the grid has a large amplitude signal input, if the screen current of the power amplifier tube decreases, it means that the gate negative voltage is too high. If the screen current increases , Indicating that the gate negative pressure is too low. The size of the screen current should be appropriate, the current is large, and the hearing is better, but the thermal noise will increase accordingly. The current is small, the noise is also small, and it is beneficial to prolong the life of the bile duct, under normal circumstances. It is appropriate to adjust the current of the power amplifier tube to 60% -70% of the maximum screen current. This machine fixes the negative gate voltage of 6P3P at -26V, the corresponding screen current is about 47mA, and the static screen power consumption is about 17W.
Because the screen load of the power amplifier tube is the primary coil of the output transformer, only the value of the cathode resistance can be changed during adjustment to change the size of the screen current (the screen current value can also be adjusted within a small range by changing the resistance value of the curtain grid), The relationship is: cathode resistance ↑ —cathode voltage ↑ —gate voltage ↓ —screen current ↓, and vice versa.
When adjusting the static working point of the bile ducts at all levels, the cathode resistance can be removed first, and a potentiometer with a suitable resistance (generally 500Ω-2kΩ) and a slightly higher power potentiometer (preferably wire wound) is welded to the cathode At the position of the resistance, first put the potentiometer in the middle position, connect to the voltmeter, and turn the potentiometer left and right in small steps until the voltmeter shows the proper voltage value. Remove the potentiometer and measure the resistance with a multimeter. Use resistors with the same resistance instead. You can also adjust while listening to the sound until you adjust to the best sound quality, the lowest distortion, and the most daring.
After the production and debugging of the whole machine is completed, a pair of small speakers made by car speakers are connected, and the "Sweetness" by Deng Lijun is auditioned. The singing is sweet and beautiful. Put in the male voice test disc "Moonlight Forest", I feel that the treble is clear, the midrange is transparent, full and full, and the bass is mellow and flexible. Then use the guzheng, violin and other string music test machine, the clear and bright tone and the wide range are restored to the fullest.
This machine is very suitable for playing in the study room; bedroom and other small places. I believe that with the extension of the time of the machine, the quality of the playback will be further improved. 
