Audio Note Gaku-on - Manual (Page 9)

Looking from the back of the amplifier, insert the
5R4WGB valves into the two empty
sockets located on the left, situated between the black rectangular transformer
covers and the loudspeaker connection binding posts. Locate the rib on the central
pin of the valve and align with the valve base (it can only be inserted one way due to
this locating rib) and with gentle but firm pressure insert the valve.
Finally, insert the
VT25 / 10Y into the front socket. The valve can only be inserted
one way.
Check one more time that all of the 5 valves are correctly fitted, and
repeat the process for the second amplifier.
VALVE INSTALLATION continued…