You may have already seen this, but here is a link that might offer up some more clues: http://www.audiocircuit.com/9120-valveamplifier-circuit/Commercial/NYAL-NYA/9120CMNYA.htm
This site actually shows a schematic and there's an email address at the bottom so you might be able to ask for it: http://www.meta-gizmo.com/Tri/nyal/nyal_art.html
This site actually shows a schematic and there's an email address at the bottom so you might be able to ask for it: http://www.meta-gizmo.com/Tri/nyal/nyal_art.html