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1. The front stage adopts 6H8C low-frequency voltage amplifier tube to realize the popular SRPP connection and cathode follower circuit. The advantages are simple structure, good linearity, strong overload capacity, less circuit phase shift, high input impedance, low output impedance and coordination with 6Ï€3C Complete the pre-amplification and post-level promotion tasks (this route has been described in many articles and will not be repeated here).
2. To promote the stage and power amplifier stage, in order to give full play to the potential of 6Ï€3C and 845 tube high-fidelity sound quality, a single-ended Class A output circuit without negative feedback is used. The circuit diagram of the whole machine is shown in the attached drawing.
Second, the selection and production of components
1. The output transformer is customized from Sichuan Guanghan Kaili Electric Appliance Factory, of which the model (kse75A) output transformer has an input impedance of 5.5kΩ, an output of 0-4-8Ω, and a power of 50W. This product adopts a layered and segmented design. It is made by using Cable paper as interlayer insulation, withstands high voltage and has good frequency characteristics. It has been verified to have extraordinary expressive sound performance and good insulation performance.
2. Power Supply: According to the working characteristics of single-ended Class A amplifiers and the high technical requirements for power supply, the author uses three transformers for filament, high voltage and high voltage rectification to effectively prevent the channel crosstalk of the signal loop caused by sharing a group of power supplies. The 300W step-up power transformer adopts a disassembled product on a military communication machine and a self-winding filament transformer. A full-bridge rectifier circuit combined with gallstones is adopted. This method can not only meet the high-voltage delay power supply, but also effectively prevent high-voltage impact on the tube, improve the service life of the tube, and make the sound behave thick and stable and energetic. Energetic. Domestically used 0.1nF / 850V oil-immersed capacitors, FRAK0470μF / 400V electrolysis in Germany, RIFA 470μF / 400V electrolysis in Sweden and 470μF / 450V electrolytic capacitors in Japan Chemical Industry are used in different places, and form a Ⅱ shape filter circuit with choke This proves that the filtering effect is particularly good, and the outstanding feeling is that the background is extremely quiet). The connecting wire is made of "silver-coated copper" multi-strand copper wire with good conductivity. Finally, according to the circuit diagram lap welding, no longer elaborate.
3. Amplifier part: The design drawing of the base is handed over to the metal sheet factory for processing, and is made by stamping with 2mm thick steel plate. Install all connectors and volume potentiometers on the base. Install transformers and capacitors, and set ground. Choose a thicker two-core shielded wire (practice proves that the AC interference is particularly small) as the connection wire of the filament, and it is closely connected to the bottom board wiring, and the shielding layer is grounded. When starting the machine, first test whether the voltage of each filament is correct. Use a single-core silver-plated shielded cable and Qatar RCA gold-plated Socket, solder the signal cable, and trace it closely to the edge of the bottom plate. Reasonably choose the layout of resistance and capacitance. Initially adjust the gate negative pressure so that the gate negative pressure of 6Ï€3C is about -14V. The negative gate voltage of 845 is -90V. Finally connect the high voltage circuit.
It is worth mentioning that: the circuit diagram uses a very simple capacitive coupling method, because Cl, C2, C5, C6 play a role of inheritance, the author used a better quality Swedish RIFA420 series capacitors and "vitamin" silver film capacitors in parallel, filtering Capacitors C3 and C4 use FRAKO 220μF / 385V electricity. Content. The cathode resistors are all American DALE high-precision non-inductive axial lead fever resistors (other resistors are made of domestic military "big red guns"), and the bypass capacitors are Dutch "Philips" capacitors. This plays a vital role in adjusting the sound of the whole machine.
3. Inspection and debugging
1. Connect dummy load or speakers.
2. Start-up and repeated debugging: make the negative voltage of the gate of 845 be -90V, at this time the DC voltage across the cathode resistance is about 4V, according to I = V / R, I = 4v / 41Ω = O.1A, the anode voltage is + 740V. The negative voltage of the gate of 6π3C is -14V, the DC voltage across the cathode resistance is 8V, the same reason I = 8W180Ω = 43mA, the anode voltage is about + 200V (the voltage at each point should conform to the specific data marked by the circuit), so The static operating points Q of its 845 and 6P3P are in the Class A state to ensure Hi-Fj amplification.
3. Test adjustment: When using GB-98 vacuum millivoltmeter, when measuring no signal input, the voltage across the load is 0.7mV. If this standard is not met, you should check whether the wiring and filter circuit are correct. Measurement of output power: input music signal, put the potentiometer to the maximum state, use GB-9 type tube voltmeter to measure the voltage at both ends of the load (8Ω) is 12V, its rated output power is P = (I * V, get P = ( V ï¼ R) * V = 12 * 12 ï¼ 8 = 18W.
Adopt Marantz CD-14 type, B & WN803 speaker, time-space RSC-CD signal cable, MIT T2 two-line crossover speaker cable, and audition with the enthusiasts: the first song "Cai Qin Old Song" No. 3, the piano just started The sound gives a sense of stability like an iron shelf. The sound of the vibration of the strings and castanets is very clear before they are eliminated. It is as if there is a piano playing between the two speakers, followed by Cai Qin ’s magnetic The singing makes the machine delicately portrayed, and there is no charm in the plain.
Re-selecting "In That Faraway Place" in "Half Dreams", I only feel the setting sun in the west, there is a voice floating on the dark red grassland, gray, the invisible heard in the fog, an irreplaceable talk , Let us feel that the language is so pale ..., soft but not lack of passion, open and clear.
4. Features
1. Due to the use of higher-grade resistance-capacitance components (especially inter-stage coupling capacitors), the sound quality has been qualitatively changed, elegant and gentle, romantic and elegant. The rhyme is melodious.
2. The negative feedback circuit is removed, which greatly increases the details of music and its information. The low-frequency bounce is strong, easy and natural, sweet and rich.
3. The interchangeability of the electronic tube is good, the front stage adopts 6H8C, 6N8P, 6N9P, the driving tube adopts 6â…¡3C, 6P3P, EL34, 6P6P, KT88, and the power tube adopts 845. When using 211, the grid negative pressure must be reduced to about -30V, and the sound has its own advantages .
(4) Cheap and good quality, high cost performance. It is suitable for self-made peers with hands-on ability.
One. Design Idea of ​​845 Single-ended Class A Tube Amplifier