WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Looks like its the same as a 2C51 or 5670 but with a higher heater voltage. Searching for those tubes should get you something. I used the 5670 as an input and phase splitter on my 6216 amplifier. My new 6216 design
Like k35kk wrote in the thread you linked to: I think there is no GNFB going on in your schematic. The ac-signal from the secondary of the output transformer is shunted to ground by the 1000uF capacitor (C5).
