Grainger49
New member
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.
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.
