Installation of electron tube differential amplifier circuit

After connecting a 6N8 to drive a FD422 single-ended tube amplifier, the circuit did not work properly.

I checked the voltage on the cathode of 12AU7 and it was more than 20 volts (the voltage did not drop after adjustment with a potentiometer), but the installation failed! This was all because I was skeptical about whether the 62kΩ cathode resistor was correct when I read the article (usually the resistance value connected to the cathode of the electron tube is from tens of ohms to 2kΩ), and I thought 62kΩ was too large. After analysis, I thought the reason was that the negative grid voltage could not be adjusted to the voltage value for the electron tube to work normally, so I looked for articles about current negative feedback. Finally, I found the answer in the article "Searching for the Beautiful Sound of 6J1+6C16" published by Mr. Tian Qingsong. After I suddenly realized it, I changed the 62kΩ cathode resistor to 510Ω+5l0Ω. After connecting in series, connect to the ground, connect one end of the 200kΩ resistor to the V1 grid, and weld the other end to the series point of the 5l0Ω resistor to form current negative feedback and grid negative voltage. After plugging in the electron tube, power on and measure the working voltage of each point. There is a normal voltage of about 8V on the cathode, but the sound from the speaker is not music but loud noise. The circuit has self-excitation. When the V2 grid resistor R5 is disconnected, the self-excitation disappears, and the speaker immediately emits loud music. The installation is successful! This differential amplifier circuit can indeed suppress current noise. The ear can't hear the noise at a distance of about half a meter from the speaker (the signal-to-noise ratio is acceptable). After actual listening, the sound quality is very good, and no distortion can be heard; the sound quality characteristics of the differential amplifier circuit assembled with 12AU7 are clear sound, good low-frequency strength and cohesion, and elasticity. The circuit is shown in the attached figure.