1974 DT250B. If the headlamp bulb is 35w/35w and blows occasionally, will it be less likely to blow if I fit a 45/45w bulb ? (I am working on basis it blows due to volts and not something else like shock / vibration)
(in case people reading this do not know, the headlamp on these bikes runs on AC current not DC and the headlamp will only show light when the engine is running).
I assume that the higher wattage bulb will bring the voltage down a bit, but will give a dimmer light at low engine RPM. Does that sound right?
But will the higher wattage bulb potentially overload the lighting coil and potentially damage the lighting coil?
Thoughts please

For that to happen "blowing", is your battery not in the circuit. I used to run 12 volt in my open circuit old RT1 ride to work bike with an XJ650 headlight that had replaceable bulbs down this side of the world. Just about melted the roadway & never blew. But of course it was dull at low revs around town. That was a lifetime ago & probably get hand smacked for doing something like that these days.
What you need--really really need--"spice girls song--umm", is a single wire 6 volt ac regulator to stop the overvoltage. You'r theory is quite right though, if you can get 45/45. Higher wattage & weaker lights. Battery balances out the system taking some of the ac to dc to charge battery. If ya not running a battery & just a simple lighting system then you 'could' try running the lights off one of the lower output wires from the lighting coils. Probably have about 3 to choose from. If i'm talking rubbish someone will correct me lol. Just thinking--recently i cranked up dads old AT1, which is a magneto AT1 in NZ not electric start DC. & damn it i turned the lights on & blew everything with a rev. P--sd me off coz i should know better. So tipped all my bulbs out & found some then shuffled the wiring under the tank where the mag loom is & swapped yellow for something--green i think & is good & safe now with no battery. I'll fit a regulator oneday & go back to yellow. Hope ya get it sorted.

I recommend installing a regulator.

That was easy DEET. Why couldn't i just say that haha.

+1

Alternatively, I have read on this forum that some folks have had success by fitting a 12v headlight bulb.