If your using stock system to keep battery charged enough using DC for headlight, night charge rate (green/red) is even lower than daytime (green) charge rate. If you want to use battery for lights, you will need to increase currant available, not loose it! As stock rectifier good for 10 amps DC, I'm not sure if using the yellow headlight (ac) to replace the green/red key posisition 11 night charging wire wire would be too much for the standard rectifier. Anyone know if the yellow headlight wire from mag puts out too much for the rectifier? I'll test it as well as my "white" extra charge (I was thinking to use) on my 73 motor. I'm using a solid state aftermarket unit designed for an old Honda, but I'll plug the old rectifier in to test.

To use the diode bridge and get full wave rectification you'd need to float the ground on the charging coil and that can get tricky with the multiple ouputs.

That said, a diode bridge has been used and work on stock system if you just use one of the diodes... Pntrtbl did it several years ago on his RT1 and posted about it here I think...

You connect the charge wire from the stator to either of the AC inputs (often marked with a squiggly ~). Then hook the battery side to the "+" terminal. Don't connect anything to the other AC terminal or the ground "-" terminal. Best to tape off the unused terminals and bolt the rectifier case to something metal with good contact as a heat sink.

I am using a bridge rectifier (KBPC5010) and had to connect the both AC sides and float only the DC negative to get the system to function. The Charge wire from stator to one of the AC terminals the Other AC terminal to the frame (ground), the DC positive terminal to the red battery wire, DC negative terminal not connected (floated). I believe you do not get fullwave rectification with this method, as you have half of the wave disconnected. This is just like using in a Stanley DE45 rectifier, half wave rectification. The advantage is the bridge rectifier is ten times cheaper and 5 times more durable.

Good info Mark! I had wondered about the ground, and that makes sense. As simple as the wiring on these bikes might be, it still confuses me. Maybe modern lighting is best reserved for modern bikes?

I've been running my set up with the headlight (LED) powered by the battery for a bit now with mixed results. It still flickers a bit and seems like I need to ride the bike with the headlight off for a while before it will come on strong. It's strange to me because I have no issues with the tail light (not LED) even though it's a 12v bulb. Now I'm leaning towards buying myself a 12v conversion for Christmas and going that route.

Hate to revive an old thread (but doing it anyway).
Any updates on this? Did you go to 12v?

Struggling with what direction to go with my 1973 DT3. I have a 6v LED and a 6v half-wave rectifier, but I don't want to burn out my lighting coil.
I've been reading all the posts that are still available on this subject, but still don't have a clear idea of the best route to take.

Is a voltage regulator addition necessary when using a rectifier for the headlight?

I ended up doing the 12v conversion and using a conventional sealed beam unit. 35 watts, I believe. I’ve found that was bring enough for me. A LED would definitely be better, and maybe I’ll fiddle with it more this winter, but the 12v sealed beam is good enough for now. 

The 12v conversion I bought from Rex’s Speedshop was an easy install and worth the money. Just my two cents.