Those 'bottle' generators are notoriously inefficient (causing a
lot of drag, even when you're not using the light,) and usually of pretty poor quality. They also can't stand much in the way of side-thrust, so I don't think that they should be used as a chain tightener... There are some German wheel-rub generators which are pretty efficient, and are much higher quality, if you can find them.
In addition, the output voltage is very sensitive to the generator RPM, so if you're running at much over 10 MPH, they tend to blow the bulbs in the lights.
Since a MB can run quite a bit faster than 10 MPH, you'll probably want to make the rub wheel larger - maybe glue a short piece of thick rubber tube around the rub wheel, or even make one of wood or plastic. (Just be sure that the new 'tire' has a rough surface to avoid slippage. This larger diameter 'tire' on the generator will reduce the drag you experience.
The other alternative (although more expensive) is to get a
dynohub & build a front wheel around it. They're very efficient (meaning that you will barely notice that they're there, even when peddling with the lights on,) have virtually no drag when they're not being used to power the lights, and their output voltage is much more stable, over the whole speed range than a bottle-type generator. The one I've got even has an internal drum brake built-in.