Most evidence is anecdotal unfortunately . . .
. . . but for what it's worth I've been running an AMC twin at 12v for over 20 years and a lot of cumulative miles, standard windings, nowadays with one of Mike's DVR2s. There's no way of stepping up the gearing on a pinion driven unit and I ran it in London for several years with a 60/55W halogen. It was fine, although heavy traffic and low revs did obviously tend to drain the battery a bit. Before that, when it was still 6v in the 1970s, I ran a 60/55W light as well - and it lasted for ages until the dynamo eventually said 'Enough!' and threw wires off the commutator. After the conversion to 12v, I used a JG reg, which was as good as you could get back then (and still works). But not as good as the DVR2 in my opinion. Dynamo output is perfectly adequate. That bike has its magneto - so I've never had to think about powering a Boyer unit as well (which might worry me a bit, not having Mike's skills with electrickery!). Nowadays it's on a 45W halogen in a Cibié concave reflector, which is awesome. The only thing I used to find, less so now with the DVR but still now and then, is that it sometimes needs a good handful of revs to get the dynamo to cut in. That could be because of the traces of oil that tend to work their way into the dynamo on that bike, or maybe the level of remanent magnetism is a bit low?
On another AMC twin I have recently put in a '12v' so-called armature and field, so 60W max safe output, and it supports a 45W lamp easily with an old fashioned tail, same as on the other one. Also with a mag and a DVR2. Works fine, and of course cut-in revs are as per original, which is the point of it. I am beginning to prefer this set-up - winter will tell me if I really do - April is a way off!
My A runs at 6v, also with a DVR, and more modest 35W halogen, standard tail lamp (and magneto of course). It would support a 45W easily, same as the others, but I have an aversion to lots of amps doing the rounds if I can help it. The lights aren't bad at all but that Cibié spoils me for anything, even moderns. The toothed-belt drive (SRM) has needed no adjustment (and the compartment isn't full of oil!). I installed it with enough slack to twist the belt to vertical from horizontal, which I think was what I was meant to do. However, the taper on the drive pulley went awol early on as Mike mentioned and I used the original steel dynamo drive sprocket centre set in the reworked pulley to sort it. No problems since.
Overall, I trust all three bikes to illuminate the road and to charge. All have fuses here and there and I run them all positive earth, just so I don't muddle myself up when playing. It's interesting having one of each of the three main variations on the E3L theme, and I'd say they're all OK if they're properly regulated and if the harness and battery are OK. But I definitely prefer 12v, and when the A needs a battery, I'll convert that too. Meanwhile, I'm very grateful to Mike for having come up with a great box of tricks.
