Steve, register and we'll give you more help than you can stand! Otherwise, nothi... /forums/images/graemlins/grin.gif ... just kidding.
The northern generators are nice units made in the Midwest. You'll find the same generator head used on several manufacturer's models. I have the 13/10 kW unit. Think I saw it recently for $100 off their regular price. Yes the tractor must run at near full speed, but that's what an engine mounted permanently to a generator is going to do anyway. It's not a problem. Also, the tractor govenor keeps the RPMs constant, assuming you have a decent tractor. Slight variations don't matter anyway, as most US equipment can stand several Hz of variation and some as much as almost 20%. Some electronics, computers etc, don't like too much variation, but unless you have a very poorly performing govener, you shouldn't have any problems. The PTO generator is a great invention - no additional engine to start periodically or maintain. There's lots of other info (and opinions) here at TBN, just start searching.