[K Sykes]

I'm traveling at the moment, so I don't have the exact value. If you have a multimeter, use it to read the resistance (in Ohms) of the bulb you have and get one that is approximately the same value. Other option is to go to radio shack and get the lowest resistance one you can find. I soldered it across one of the sockets, the two ends of the resistor bridging the two wires that go into the socket, The polarity of the LED matters too, so it will only work one of the two ways it can go into the socket. An incandescent
bulb has no polarity, so it does't matter.

I'm home on friday if you want an exact value (or someone with an EE background might jump in)