Biliary duct 2A3C Class A amplifier

Biliary duct 2A3C Class A amplifier

According to the information, 2A3C is an improved version of the original power amplifier 2A3, using a large gourd-shaped high-strength quartz glass shell and high-quality white porcelain tube sockets (the tube pins are gold-plated); the vacuum degree is nearly 100 times higher than the original, and the original double-screen structure is changed to a single-screen structure, which increases the screen area and mechanical strength. In addition, the filament and grid are also integrated to greatly reduce the microphonic effect. The electrical performance of the tube has also been greatly improved. For specific parameters, see the attached table.

The power amplifier circuit that I used 2A3C to connect is shown in the figure. The circuit is a three-stage triode amplifier circuit. Its advantages are that the odd harmonic distortion is very small, the reproduced music is more intense, and the listening experience is better.

This circuit uses 1/2 6N3 as the pre-amplifier with a gain of about 29dB. The driver stage uses 6N10 parallel tube amplification to reduce internal resistance. The gain is about 20dB. The output power of this circuit is 5W.

2A3C uses a fixed gate bias circuit. By adjusting the resistance of W1 (or W2), the operating point of 2A3C can be changed and the static screen current can be controlled.

The audio output transformer is made as follows: the core tongue width is 26mm, the stack thickness is 40mm. The primary coil is wound with Φ0.19mm oxygen-free copper wire for 3600 turns. The secondary is wound with Φ0.8mm wire for 224 turns (8Ω), or 159 turns (4Ω). The primary is divided into three sections, and the secondary is wound in two sections sandwiched between the primary. The core is inserted in the same direction, and the air gap is filled with 0.1mm paper.

The power transformer has a power of 200VA. The core tongue is 35mm wide and 50mm thick. The primary is wound with Φ0.72mm wire for 512 turns, and the secondary high voltage is wound with Φ0.35mm wire for 766 turns. The gate voltage power supply is wound with Φ0.13mm wire for 160 turns, the 2.5V filament winding is wound with Φ.16mm wire for 6 turns, and the 6.3V coil is wound with Φ0.72mm wire in parallel for two 8 turns. The head end of one group is connected to the tail end of the other group as the center tap. The 5V filament coil is wound with Φ1.25mm wire for 12 turns.

Debugging method:

After checking, only plug in the rectifier tube and power it on. The voltage of each filament and the high voltage of the screen should basically meet the requirements. Adjust W1 (or W2) to make the negative grid voltage -65V. Turn off the power, and plug in each tube after the screen voltage disappears. Turn on the power, and measure the screen voltage and cathode voltage of each tube. Basically meet the requirements. Adjust W1 (W2) so that the voltage at the upper end of the 10Ω cathode resistor of 2A3C is 0.55V, and the screen current is 55mA at this time. The working point of the pre-stage and the driving stage can adjust the cathode resistor to match the voltage value in the circuit diagram. Adjust W3 to minimize the AC noise of the speaker.

After 24 hours of burn-in, the excellent performance of the machine gradually emerged. The sound quality is comparable to that of 300B. Although the output power is slightly smaller, the price is much cheaper than 300B.