Wiring 6SN7 in Crack - Verifying Pin Assignments

[HF9]

Grainger posted this very useful information in the Foreplay III section a couple years back and I wanted to verify the pin assignments were the same in the Crack if wiring up a 6SN7 in place of the 12AU7.

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.

[Paul Birkeland]

Yes, this is correct.

[Grainger49]

Whew!  I sent one Bottlehead two wrong sets if pin assignments.  I think I posted this after I got it straight.

[HF9]

Are dropping resistors necessary for the Crack running a 6sn7? If so, where would they be installed?

TIA!

[Laudanum]

Graingers quote was based on the FPIII.   As far as I know, just replacing the socket and rewiring it (or using an adapter) are required to use a 6SN7 in place of the 12AU7 in Crack.   The socket adapters that are sold on ebay are simply 9 pin to octal adapters with internal rewire for the pins and they work fine.   Someone smarter will correct me if Im wrong.