Most likely, the 12ax7-a is V1
While the other two are for the PI
V1 is located nearest the input jacks on left of the rear panel.
V1a - (left tube) is the Input Stage for Channel A.
V1b - (left tube) is the Input stage for Channel B.
V2 - (center tube) is the Phase Inverter for Channel B.
V3 -...