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.
One note of interest. That large heatsink in the middle is the DC power supply for the filaments. I've never seen any documentation on this board. Gonna assume it may be some custom job.