. Are we talking brighter at a given runtime, longer runtime at a given brightness, or a balance of both?
it may simply because the new A2 has a more efficient LEDs
Sorry for my delay, I have been mostly off of the forums for a few days.
You can't really increase brightness in the regulator without changing the design of the bulb. Surefire has designed the A2 to work with MA02 bulbs, so they have to keep in spec with that bulb's capabilities. The regulator in the A2 is a constant voltage regulator, and thus the only way to increase the brightness is to design a bulb to be more efficient at that voltage (LF HO-A2) or to overdrive an existing bulb (Strion).
The 5mm LEDs are lost in the noise of any calculations, they just don't matter that much sitting next to that big old fireball. They're also not part of the regulator circuit.
I have not done any testing on my bench supply, everything here is observed behavior and no real numbers have been gathered or analyzed. Thus it is subject to be completely wrong, but it is what I have observed.
I have a fairly old A2 4-flats (red) that I use around the house, and I recently acquired a really new A2 round (yellow-green) that I was playing with because, well, it's new.
I have always preferred the 4-flats design, I think it looks better, feels better in my hand, and I like the 1/2 on incandescent stage that shows up when the batteries need replaced. The 1/2 on stage, often called the sickly yellow beam, is what this is all about. I've never seen any of my round A2's exhibit this behavior, but I never really thought about it either.
I have observed that you can often double-tap a 4-flats A2 to change it from the 1/2 on stage to the normal incandescent. Generally, when I can no longer double-tap to get the normal beam, I change my batteries.
My Red A2 needed new batteries, and I gave it the two from the newly acquired YG. I was only really interested in the YG A2 for the YG leds
I tossed the old spent 123's into the YG to play with. The incandescent bulb worked. That's not to unusual to get a short burst after a rest period, so I didn't think much of it. It kept working though, so I started using it. I used it for several more days (just taking the dog out, etc) before it finally gave up. When it gave up there was no 1/2 on state, it was just dead. The 5mm LEDs, of course, continue to work just fine.
The conclusion I draw from this, is that the newer regulators are either more efficient, or have their minimum voltage threshold set lower.
An equally valid conclusion to draw from this is that I just happen to have a particularly bad old 4-flats A2 and a particularly good new round one.
I have enough variety of A2's that I can sample several but I just have other projects I'm working on at the moment and it hasn't been that big of a deal.