I thought the pulse phasers are right next to the bussard collectors on that front wall of the nacelle.
I used to think the same thing, there's that little pair of cylinders on the flat front face, with the big box in between. But, no, it's the big pair of greebles on the edge of the nacelle where it meets the "saucer," one above and one below. I know some people have thought it was both, I'm sure I've seen fan art with four pairs of phaser-pulses in olden-times.