Amplifier circuit made by parallel connection of FU-50

I recently checked the information and found that the triode connection method of FU-50 pentode electron tube has a characteristic curve very similar to that of 300B, but its output power is about 7W. If two tubes are connected in parallel, the output power can reach 14W, which is completely sufficient for a 30m2 room, and the cost is only 1/5 of that of 300B (including the tube socket).

The circuit is as shown in the figure. Each channel uses 4 tubes, and each level is connected according to the triode method. The advantage of the triode connection method is that the signal distortion characteristics are basically even harmonic distortion, and the tone is gentle, sweet and very pleasant to listen to.

The pre-stage voltage amplifier is performed by 6N11, which is connected into an SRPP circuit with an amplification factor of about 29.

The driver stage uses the power tube EL-34, which is connected as a triode for voltage amplification. The screen voltage of this tube is about 250V, the screen current is 20mA, the gate negative voltage is -20V, and the gain is about 7.5. The output voltage amplitude is 20Vx7.5=150V.

The output stage is made up of two triode-connected FU-50 tubes connected in parallel.

Work in Class A state. After the two tubes are connected in parallel, the load impedance is only 1.25kΩ, which can reduce nonlinear distortion and improve the damping of the playback system. Strengthen the control of the speaker. FU-50 screen voltage 480V, screen current 80mA. Grid negative voltage -65V, self-biased, load impedance 2.5kΩ, two tubes in parallel load 1.25kΩ, total output power 14W. A 1kΩ resistor is connected in series in the grid circuit to prevent self-excitation.

The driver stage adds negative feedback to the pre-stage through an RC network to widen the passband and improve the transient response of the entire machine.

The high-voltage DC circuit is provided by the whole machine after diode rectification and CLC filtering. Two delay circuits are used, and the delay time is 60 seconds and 70 to 80 seconds respectively. After 60 seconds of starting up, the DC high voltage is charged to each filter capacitor through the current limiting resistor. After 70 to 80 seconds, the high-voltage circuit works normally. The high voltage achieves the purpose of "soft start" and prevents high voltage shock (power supply circuit omitted).

Each filament winding is wound in parallel with two wires and grounded in the middle. This method can significantly reduce the AC noise.

The ground wires of the two channels are made of φ2mm silver-plated copper wire, each forming a loop. The ground wires run as follows: input interface--volume potentiometer--preamplifier--drive stage--power stage--load. Finally, they are connected to the chassis with the filter capacitor.

Each transformer must be shielded, especially the output transformer close to the power transformer, which has a significant effect on improving the signal-to-noise ratio of the whole machine.

The audio output transformer is made as follows: the core is made of high-quality silicon steel sheet with a width of 40mm and a thickness of 50mm. The primary winding is wound with φ0.31mm high-quality enameled wire with 3500 turns.

The secondary is wound with φ0.96mm enameled wire with 39 turns (4Ω) or 323 turns (8Ω), and the primary is wound in four layers and the secondary is wound in three layers. The core is inserted in the same direction, and a 0.2mm thick paper is placed in the middle of the EI.