Electronic theory for dynamos, and replacement bulb
I have a wheel-driven dynamo system labelled 6V/3W, and I am running just the front light. (Only one dynamo connection works.) My LBS guy predicted I'd burn out the front bulb running just one because there's "more juice" running to it, but he doesn't know any more detail about the electronics involved. My question, which he can't answer specifically, is what specs should I look for in a replacement bulb. His shop has some bulbs which seem like they would work if I were running both lights, but he thought I might need a more powerful one running just the front. I looked in the automotive section of the hardware store, but they only had 12V bulbs, which doesn't sound right at all.
Partly I'm asking to understand the electronics involved. I know that Volts * Amps = Watts, so I'm guessing that the dynamo puts out a constant 6V, with amperage varying with speed. Is that the right theory? So does that mean a 3W bulb can take the 6V at up to 2 amps, but anything above that might burn it out? If I were running two bulbs, would the amperage be split up between them somehow, or would they each still get the same amps? (They would be in parallel, so I think the both would get the same volts, but I don't remember if amps also stays constant in parallel or is lessened.) And does that also mean that even if I were using both bulbs, riding too fast could bring up the amps to put out over 3W, burning out the lights anyway? How do I know if I'm riding too fast?
My LBS guy also said he thought that constantly varying the power like a dynamo does is rough on bulbs anyway. Would you agree?
To cut to the chase, I really don't know what I'm looking for in a replacement bulb, especially running just one bulb on a system designed for two. I also don't know how bulbs are usually spec'd. The ones I saw in the hardware store just said 12V, nothing about watts or amps. Should I definately be looking for 6V, and maybe between 3 and 6 watts? Does it matter what it says about amps?
Where is a good source for replacement bulbs of this nature, Radio Shack? Online?