Focused light coming out of a headlamp is a function of the lens, the reflector and the location of the filament. If the filament in your quartz bulb isn't in the same location and orientation as the original 6v bulb, and if the reflector has been modified, then the beam will be out of focus and the light pattern coming from the lens will be compromised. Here is a drawing showing the complexity of light zones in a 36 headlamp lens.