Production of 6F3 single-ended small amp

STM32L151CCU6 microcontroller

Domestic switch needle KG-300K, needle head diameter 3.0mm, normally open switch needle

Photocoupler

This device is quite straightforward to assemble. Following the circuit diagram, ensure proper soldering and power it up. If there's an issue with oscillation or howling, splitting the output of the first stage into two lines should resolve it. Additionally, adjusting the 25kΩ feedback resistor (marked with *) may help optimize sound quality.

The entire system utilizes just two 6F3 tubes, one per channel. The triode section handles voltage amplification, while the pentode portion serves as the power output. The output transformer comes from a dismantled red light tube radio using a 6P1 output transformer, with an impedance of 5.5kΩ:8Ω. Given that the internal resistance of the 6F3 is 25kΩ and the 6P1 is 40kΩ, strictly speaking, the output of the 6P1 isn't ideal. A more suitable option would be a 4.5kΩ:8Ω transformer, which could enhance both output power and frequency response. Alternatively, connecting the 8Ω end of the 5.5kΩ:8Ω transformer to a 40Ω speaker effectively reduces the primary impedance of the transformer, achieving similar results.

The power transformer provides 50W, and the choke coil is homemade, featuring a 22mm x 26mm iron core wound with φ0.23mm enamel wire, totaling 4000 turns, resulting in an inductance of 10H. The speakers are custom-built full-range bookshelves, utilizing two 6.5-inch 8Ω flying horns salvaged from an old radio. The base is from a dismantled switching power supply aluminum housing, measuring 300mm x 160mm x 80mm. Apart from the power switch and volume potentiometer, the front panel includes a stereo jack of φ3.5mm and φ6.5mm, enabling input of various audio sources like CDs, MP3s, MP4s, mobile music, and FM radio. Use high-quality shielded cables and ground one end of the signal path to avoid hum issues.

This compact amplifier delivers 3.5W of output power, making it perfect for enjoying music in the study or bedroom. It can also serve as a computer desktop amplifier, offering a delicate and pure sound with a sweet and engaging tone. Both high, mid, and low frequencies perform well, providing a personal listening experience superior to 6P1 or 6P14. The signal-to-noise ratio is relatively high, and there’s no noticeable hum when your ear is close to the speaker. The entire build uses second-hand components available at local electronics markets, costing under 100 yuan. For those interested in budget-friendly audio projects, this is definitely worth trying out.