6sn7 vs 12sn7
 

Would a 6sn7 work in place of a 12sn7 ? I know the voltages are different but what about the currant.

Depends upon where you are going to use it. A 6SN7 filament current is 0.6 amperes. Substituting it in a series string set with 0.3 ampere tubes to replace a 12SN7, it will only have about 3 volts across the filament so won't heat up properly.

According to this
http://www.radiostation.ru/tubes/6SN7.pdf

They are both 3.78 watts; 6SN7 draws 600mA at 6V, 12SN7 draws 300 mA at 12 V.

So the answer generally is no. In a parallel circuit, it will draw double current at 12 volts and burn out. In a series circuit, it will decrease the total resistance so that the string draws somewhat more than 300 mA, but this will still be too little for the 6SN7.

A tube with a heater centertap like the 12AU7/AT7/AX7 family, can be configured for either 6 or 12 V.
http://cdn.instructables.com/F6L/JVN...EM0B.LARGE.jpg