Keywords: 6n1, FD422, tube, power amplifier circuit diagram
First, the principle of FD422 amplifier circuit
See the drawings in the drawings. The audio signal is sent from the input terminal, and the gate of the input stage 6n1 tube is sent to the gate 6n1 tube after 100kΩ volume potentiometer, and R1 is a gate leakage resistance. 6N1 is a cholangi, a softening, suitable for SRPP circuits.
The SRPP circuit is characterized by high frequency amplification, high loss impedance, low output impedance, small distortion, wide frequency, large dynamic, high-frequency transient response, sound quality is soft. The SRPP circuit has the advantage of a common cathode amplification and cathode follower, which enables the luminous level to the power amplifier to achieve the best impedance match.
The power amplification stage is connected to a triode by the FD422 heated five-pole, which constitutes a single-ended ambigutrient amplifying circuit. The screen load impedance is 3.5kΩ, the screen voltage is 416V, the cathode voltage 27V, the screen current is 77mA, and the self-grating negative pressure is used. The power consumption of the power amplifier is p = ui = (416-27) × 0.077 = 30W. According to the maximum output efficiency of 35%, the maximum output power of the unit is P = 30 × 0.35 = 10.5W.
Second, the power part
The power supply is to ensure that the key points of this performance must be improved, in order to provide an abundant power to the circuit, the power transformer power is 250W. In order to reduce the internal resistance, the crystal diode rectification and the large capacity of electrolytic capacitor can be used. The machine is used in the Italian "Red Cardinal" 1000UF / 450V electrolytic capacitor, and if there is no large-capacity electrolytic capacitor, it can be used in parallel.
Third, the selection of components
Components are an important factor affecting the sound quality of the amplifier. Output transformers, volume potentiometers, resistors, coupling capacitors, cathode bypass capacitors, and power filter capacitors affect the sound quality, so it should be carefully selected. Don't think that the bile duct is cheap, it will be casually, and the horse will be casual.
Output transformers are critical, and the author uses a 50W finished product. Level coupling capacitors and cathode bypass capacitors are important components that affect tone quality, no matter what brand or second-hand metal, its withstand voltage is enough and does not have leakage, and the capacity is close, do not differ too much, it is best able to Pairing, conditional preferred oil immersive capacitance. The resistance power is divided into addition to 1W or more, the cathode can be wound by the glaze wire with a cathode.
The chassis can be designed according to its own components, size itself.
Fourth, production and debugging
This circuit design can reduce installation errors in the principle of "concise first". According to the first-fitting power supply part, the order of the last loader part is loaded. Each part is applied, carefully checks, detects, and then the next part is installed, so that the success rate is high. The components should be close to the connection, the star point is grounded, the solder joint is smooth, welding is firm, avoiding drainage. The working voltage of each point is marked in the figure for reference. After preliminary adjustment, the operating points of each tube are basically in normal state. If the familiar disc is played, the play sound must be good, but it is not very good, because the bile duct, transformers, blocking elements, wires, etc. are new, It also needs to be patient, you can listen to the side, after a period of time, the sound is different. If the low frequency extension is insufficient, an appropriate amount increase the bypass capacity of the output stage. In addition, the output stage gate leakage resistance value can be adjusted within a certain range according to the effect.
Author: Wei Yongjin
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