It has often been asked how to use the whole hole in the FP III, install an octal socket and use 6SN7s instead of the stock 12AU7s.
Here is a run down of tube pins that are equivalent:
12AU7 Pin wired to 6SN7 Pin
1 2
2 1
3 3
4 7
5 No Connection
6 5
7 4
8 6
9 8
Also see the Sticky post about alternate tubes and dropping resistors for the heater supply in this folder.
Notice that pins 4 and 5 are shorted together in the FP III using the 12AU7. That folds the 12AU7 heater in half, making it a 6.3V heater like the 6SN7. Nothing is free in electricity, the current is doubled from the standard 12AU7 heater current, when it is wired for 12.6V.
There is a bias circuit to "float" the heater closer to the cathode voltage. On page 30 of the manual it is soldered to pin 9 of the stock tube socket. That red wire goes to pin 8 of the 6SN7 socket.
Please Sticky this if it is useful to enough Bottleheads.
