This post will cover the 12ax7 to 6n2p toggle switch wiring that I used recently in the Aurora Jupiter Preamp.
Firstly lets cover the key difference between the two valve types.
Typical 12ax7 valves wired for 6.3v AC heaters use pins 4&5 linked together for one side of the filament winding and pin 9 for the other.
6n2p only use pins 4 & 5 separately for each side of the heater winding whilst pin 9 is an internal shield between each triode that should be connected to ground.
so what we are wanting to achieve with the switch is for one position to join pins 4/5 and connect one side of the filament winding to this and to also connect pin 9 to the other side. In the second position pins 4/5 will be connected individually to each side of the filament winding and pin 9 will be shunted to ground.
Firstly here is the picture from the earlier article showing the easiest way to wire the filaments using 3 separate pieces of bus wire which can easily be straightened just by gripping the end in the chuck of a drill and slowly spinning it until it is straight.
And just for extra clarity here is a diagram showing the respective connections.
I have considered making a small switch mounted PCB as wiring mini DPDT toggles can be a bit fiddly. If this is something you would be interested in or you have any other questions let me know in the comments.