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Author Topic: 12AU7 To 6SN7 Pin Assignments  (Read 2423 times)

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Offline Grainger49

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12AU7 To 6SN7 Pin Assignments
« on: November 17, 2011, 10:58:26 AM »
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.
« Last Edit: November 17, 2011, 02:09:15 PM by Grainger49 »