Looking at the configuration of the amp, I'd guess that it's a direct copy of the original Vox Night Train NT15 head (though with their own circuit board layout). Do a web search and you will be able to download a copy of the circuit diagram for it e.g. https://elektrotanya.com/vox_night_t...wnload.html#dl

No adjustable bias as far as I can see on the power valves.