Design of 845 Single-ended Class A Tube Amplifier

1. Design idea of ​​845 single-ended Class A tube amplifier

1. The pre-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 it cooperates with 6π3C to complete the pre-amplification and post-stage driving tasks (this circuit has been explained in many articles, so I won’t repeat it here).

2. The driving stage and power amplifier stage use a single-ended Class A output circuit without negative feedback to give full play to the potential of high-fidelity sound quality of 6π3C and 845 electron tubes. The circuit diagram of the whole machine is shown in the attached figure.

2. Selection and production of components

1. The output transformer is custom-made from Sichuan Guanghan Kaili Electric Factory. The output transformer model (kse75A) 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, is finely made, and uses cable paper as interlayer insulation. It is resistant to high voltage and has good frequency characteristics. It has been verified by use and has extraordinary sound expression and good insulation performance.

2. Power supply part: According to the working characteristics of single-ended Class A power amplifiers and the high requirements for power supply technology, the author uses three transformers for filament, high voltage, and high voltage rectification to effectively prevent the channel crosstalk of the signal circuit caused by sharing a set of power supplies. The 300W step-up power transformer uses a disassembled product from a military communication machine and a self-wound filament transformer. The full-bridge rectifier circuit combined with gallstones is used. This approach can not only meet the high-voltage delayed power supply, but also effectively prevent high voltage from impacting the electron tube, and increase the service life of the electron tube, and can also make the sound performance thick, stable and energetic, with a strong gall flavor and abundant power. In different places, domestic 0.1nF/850V oil-immersed capacitors, German FRAK0470μF/400V electrolytic capacitors, Swedish RIFA470μF/400V electrolytic capacitors and Japanese chemical 470μF/450V electrolytic capacitors are used, and they form a II-type filter circuit with a choke coil (practice has proved that this has a particularly good filtering effect, and the prominent feeling is that the background is extremely quiet). The connecting wire uses "silver-clad copper" multi-strand copper wire with relatively good conductivity. Finally, it is welded according to the circuit diagram and no further explanation is given.

3. Power amplifier part: The design drawings of the machine base are handed over to the hardware iron sheet factory for processing, and it is made by stamping 2mm thick steel plates. Install all the connectors and volume potentiometers on the machine base. Install transformers and capacitors, and set ground wires. Choose thicker double-core shielded wires (practice has proved that this has a particularly small AC interference sound) as the connecting wires of the filaments, and run them close to the bottom plate, and the shielding layer is grounded. First test whether the voltage of each filament is correct when the machine is turned on. Use single-core silver-plated shielded wire and Kadashi RCA gold-plated socket, weld the signal wire, and run it close to the edge of the bottom plate. Choose and arrange resistors and capacitors reasonably. Initially adjust the gate negative voltage to make the gate negative voltage of 6π3C about -14V. The gate negative voltage of 845 is -90V. Finally, connect the high-voltage circuit.

It is worth mentioning that: the circuit diagram uses a very simple capacitor coupling method. Because Cl, C2, C5, and C6 play a role in connecting the past and the future, the author uses relatively high-quality Swedish RIFA420 series capacitors and "Vitamin" silver film capacitors in parallel, and filter capacitors C3 and C4 use FRAKO 220μF/385V capacitors. The cathode resistors are all American DALE high-precision non-inductive axial pin fever resistors (other resistors use domestic military "big red cannons"), and the bypass capacitors use Dutch "Philips" capacitors. This plays a vital role in regulating the sound of the whole machine.

III. Inspection and debugging

1. Connect a dummy load or speaker.

2. Turn on and debug repeatedly: make the negative voltage of the gate of 845 -90V, at this time the DC voltage across the cathode resistor is about 4V, according to I=V/R, I=4v/41Ω=0.1A, the anode voltage is +740V. Make the negative voltage of the gate of 6π3C -14V, the DC voltage across the cathode resistor is 8V, similarly I=8W180Ω=43mA, the anode voltage is about +200V (the voltage at each point should be consistent with the specific data marked by the circuit as much as possible), so that the static working point Q of 845 and 6P3P are both in Class A state to ensure Hi-Fj amplification.

3. Test adjustment: Use GB-98 vacuum millivoltmeter to measure the voltage across the load when there is no signal input, which is 0.7mV. If this standard is not met, check whether the wiring and filter circuit are correct. Output power measurement: input music signal, set the potentiometer to the maximum state, use GB-9 tube voltmeter to measure the voltage across the load (8Ω) to be 12V, and its rated output power is based on P=I*V, P=(V/R)*V=12*12/8=18W.

Adopt Marantz CD-14, B＆WN803 speakers, Super Time RSC-CD signal line, MIT T2 bi-wire speaker line, and listen with audiophiles: The first song is "Old Songs of Tsai Chin", the third song. The piano sound at the beginning gives people a sense of iron frame-like stability. The sound of the strings and castanets vibrating before disappearing is very clear, just like a piano playing between the two speakers, and then Tsai Chin's magnetic singing voice makes the machine delicately portrayed, and there is a special charm in the plainness.

Then choose "In That Faraway Place" in "Half Dream", and you can only feel the setting sun, and a voice floating on the dark red grassland, gray and misty, what you hear but cannot see, an irreplaceable confession, which makes us feel that language is so pale..., soft but not lacking passion, open and well-structured.
IV. Features

1. Due to the use of high-grade resistors and capacitors (especially inter-stage coupling capacitors), the timbre has undergone a qualitative change, elegant and warm, romantic and elegant. The tail rhyme is melodious.

2. The negative feedback circuit is removed, which greatly increases the music details and information volume, and the low-frequency bounce is strong, relaxed and natural, sweet and rich.

3. The interchangeability of the electron tube is good, the pre-stage uses 6H8C, 6N8P, 6N9P, the driving tube uses 6Ⅱ3C, 6P3P, EL34, 6P6P, KT88, the power tube uses 845, and when using 211, the grid negative voltage must be reduced to about -30V, and the timbre has its own merits.

(IV) Cheap and good quality, high cost performance. Very suitable for hands-on colleagues to make it by themselves.