Preamplifier circuit made of composite screen triode electron tube 6C5P

The circuit of this machine is shown in the figure. The audio signal directly reaches the grid, 6C5P is connected to form a common cathode amplifier circuit, and the screen uses capacitor isolation and coupling output, which is conducive to stability (focusing on high frequency) +39kΩ (focusing on medium and low frequency) Two resistors are connected in series as the 6C5P screen load resistor.

The power supply part adopts full-wave tube rectification. This machine adopts 6Z4 tube rectification and inductor filtering.

2. Component Selection

6C5P is a wide-band low-noise voltage amplifier triode. Domestic products include Nanjing 6C5C, Shuguang 6C5P, etc. Foreign products include 6C5, 6C5GT, 6C5C, etc. The author uses Nanjing 6C5C. The resistor is a domestically produced green carbon film resistor from the 1960s and 1970s. This resistor has a sweet tone and is magnetic.

The power transformer and choke are products of Shanghai Radio Factory 27. The power transformer has a power of 100W, and the choke is 10H50mA. The power transformer can also directly use a five-lamp or six-lamp radio power transformer.

3. Assembly, debugging and audition

By adjusting the cathode resistance of 6C5P, the cathode voltage of 6C5P can be set to 4V.

The cathode current is about 4mA. The equipment connected to this preamplifier for audition is: a self-made 10W/300BC single-ended machine. The sound source is Philips DVD. The speakers are self-made 90dBl2-inch three-way floor-standing speakers. The sound is rich in tube flavor and strong in air. The high, medium and low frequencies are fully expressed, and the sound field positioning is accurate.