- Aug 2, 2011
Pin 4 must be connected to the voltage source in each case. The heaters are connected within the tube, but you must have 18-19V going through them. If pin 5 is connected to ground (correct), the only way that will happen is if you connect pin 4 to the voltage source.
Actually 12.6 V for a 12AU7 build.
The pins are not connected together as there is no dot on the schematic. They need 12.6 V across them. You connect one pin to ground and the other to pin 3 of the MOSFET.