FU-50 Single-ended Class A Power Amplifier

FU-50 is a high anode voltage, low anode current power tube. According to conventional design, it can use a higher Ua value with Uamax0V as the limit. Ua@0V is selected here. First, the power supply of home audio should not be too high. Second, in order not to exceed the anode power consumption, the anode current will inevitably decrease when the anode voltage is increased. As a result, the internal resistance of the electron tube increases, the distortion increases, the optimal load impedance increases, and the frequency response of the output transformer is difficult to expand. Therefore, in order to take into account factors such as output power, a 400V anode voltage is selected. The Uamax of FU-50 is 250V. The 150V power supply is selected here because the gate control voltage has a larger linear area within the anode power consumption limit area, which is conducive to the performance of Class A action.

The FU-50 single-ended Class A 7W amplifier circuit is shown in the attached figure. The maximum second grid voltage limit is 250V. 150V is selected in this column, but this voltage must not be simply reduced by a resistor. Otherwise, at the moment of power on, the cathode of FU-50 has not reached the electron emission temperature, and there is no current in the second grid. Its voltage must be too high, which will shorten the life of FU-50. At the same time, the second grid current changes with the input signal, which will also make the second grid unstable and increase the distortion. Therefore, the air-filled voltage regulator VR150 (domestic WY-4 and former Soviet Union CT-4C are the same) is used in this machine to stabilize the second grid voltage at 150V. When the second grid current changes less than 30mA, this voltage is stable.

FU-50 is a large octagonal tube without a tube stem. There is a long positioning pin on the side of the glass shell. The positioning pin is perpendicular to the tube pin, which is the first pin. The tube pins are facing upwards clockwise, which are the first to eighth pins. The seventh pin is an internal isolation piece, which can be grounded when in use. The fourth and eighth pins are 12.6V filaments.

The power source adopts semiconductor diode bridge rectification. Changing the 100Ω resistor can make the anode power supply around 400V. The anode internal resistance of the pentode output tube is relatively high, which means that the anode voltage has little effect on the anode current. Although it is a Class A single-ended tube, there will be no obvious AC noise with simple RC filtering. However, the second grid voltage has a great influence on the anode current, and the power supply ripple must be extremely small. Fortunately, the charging voltage regulator has excellent filtering, so the output static noise voltage of the machine is only below 20mV at the 8Ω end. Because VR150 is connected, when the FU-50 filament is preheated at the moment of power-on (it reaches the rated electron emission temperature about 35 seconds after FU-50 is powered on), the voltage regulator vR150 immediately generates a glow discharge of about 30mA current. Therefore, the semiconductor rectifier of this machine also needs to use a high-voltage delay circuit to keep the power-on impact voltage within the allowable limit of FU-50. Once the electron emission temperature is reached, the second grid generates current, and the current of the charging voltage regulator increases the voltage drop of the 6.2kΩ resistor at the same time, maintaining the second grid voltage of 150V unchanged. If you think the glow discharge tube is troublesome, you can also use a Zener diode instead, but it should be a high-voltage voltage regulator with a Zener voltage of 150V and a maximum Zener current of not less than 45mA.

The primary impedance of the output transformer of this machine is relatively high, and the primary inductance must be increased, otherwise the low-frequency response will be limited. It uses E32 high silicon steel sheets, with a stacking thickness of 50mm, EI sheets are inserted in one direction, and the air gap is 0.2mm.

The primary winding is wound with 3500 turns of φ0.17m enameled wire, the secondary 8Ω winding is wound with 128 turns of φ0.64mm enameled wire, and 52 turns of φ0.51mm enameled wire are used between 8 and 16Ω.