FD422 tube power amplifier

2. Power supply part

The power supply is the key to ensuring the performance of this machine. Heavy materials must be used to provide sufficient power to the circuit. The power of the power transformer of this machine needs to be more than 250W. In order to reduce the internal resistance of the power supply, crystal diode rectification and large-capacity electrolytic capacitor filtering can be used. This machine uses Italian "Cardinal" 1000uF/450V electrolytic capacitors. If you don't have large-capacity electrolytic capacitors on hand, you can also use multiple ones in parallel.

3. 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 supply filter capacitors all affect sound quality and should be selected carefully. Don't think that bile ducts are cheap and you're careless about other components.

The output transformer is the key to the whole machine. The author uses a 50w finished product. Interstage coupling capacitors and cathode bypass capacitors are important components that affect the sound quality. No matter what brand or second-hand disassembled parts you choose, their withstand voltage must be sufficient and there must be no leakage. The capacities must be close and not too different. It is best to For matching, oil-immersed capacitors are preferred if possible. Unless otherwise indicated, the resistor power can be above 1w, and the cathode should be glazed wire-wound resistor of 30w.

The chassis can be designed by itself according to the volume and size of its components.

4. Production and debugging

This circuit design is based on the principle of "simplicity first", which can reduce installation errors. Follow the order of installing the power part first, then the output part, and finally the input part. After each part is installed, carefully check, test, and adjust it before installing the next part. This will increase the success rate. The components should be connected nearby, the star point should be grounded, the solder joints should be smooth, and the soldering should be firm to avoid missing soldering. The operating voltage at each point is marked on the diagram for reference. After preliminary adjustments, the working points of each tube are basically in a normal state. If you play a familiar disc, the sound playback effect will be good, but it is not very beautiful because the bile ducts, transformers, resistance-capacitance components, wires, etc. are all new. You also need to be patient to burn in the machine. You can burn it in while listening. After a period of time, the sound will be different. If you feel that the low-frequency extension is not enough, you can increase the bypass capacitance of the output stage appropriately. In addition, the value of the gate-to-drain resistance of the output stage can be adjusted within a certain range according to the effect.