6V to 12V Converter

R1, R4 2 .2K 1/4W Resistor 
R2, R3  4.7K 1/4W Resistor 
R5  1K 1/4W Resistor 
R6  1.5K 1/4W Resistor 
R7  33K 1/4W Resistor 
R8  10K 1/4W Resistor 
C1,C2  0.1uF Ceramic Disc Capacitor 
C3  470uF 25V Electrolytic Capcitor 
D1  1N914 Diode 
D2  1N4004 Diode 
D3  12V 400mW Zener Diode 
Q1, Q2, Q4  BC547 NPN Transistor 
Q3  BD679 NPN Transistor 
L1  See Notes
Notes
1. L1 is a custom inductor wound with about 80 turns of 0.5mm magnet wire around a toroidal core with a 40mm outside diameter.

2. Different values of D3 can be used to get different output voltages from about 0.6V to around 30V. Note that at higher voltages the circuit might not perform as well and may not produce as much current. You may also need to use a larger C3 for higher voltages and/or higher currents.

3. You can use a larger value for C3 to provide better filtering.

4. The circuit will require about 2A from the 6V supply to provide the full 800mA at 12V.