When I started, I wasn't able to do pull-ups and even negatives were too hard. I used to use resistance bands with the door attachment but wasn't really happy with the results I was getting. I use this pull up assist with my pull up bar and I'm much happier with it. It's not mimicking a pull up motion like the door attachment method does; it's reducing the weight you're lifting when doing the same motion as a real pull up. You can use different strength bands or reduce the tension as you get stronger.
I bought some pull up bands from Amazon that have really helped me increase my rep count. I think it assists 100lbs, but you can remove bands to decrease the amount of assist it gives you.
I got from 0 to 3 using this guy
and then from 3 to 15 largely through losing weight (185 to 155).
I got a pullup revolution to combat this problem. I have a lat bar too, but there are so many different types of pullups that I felt like I was constantly switching weights (my machine is a bit cumbersome), and sitting down in the middle of the workout to use the machine also wasn't working for me. Plus, pull-ups work a lot of muscles that can slack off when you're on a lat bar; it's just not the same. This helped a lot. It's not perfect; it gives too much assist at the bottom of the pullup and not enough at the top, but it's a great way to start. Just another option to consider.
This assist is awesome. I weigh 240 and it really helps. On my third round of P90x
http://www.amazon.com/gp/product/B003KNC1MO/ref=oh_details_o02_s00_i01?ie=UTF8&psc=1
Hands down the best solution. You can also use it as a push up assist as well.
You can give this a try
Also, the Lifeline USA Pullup Revolution can help you do assisted pull ups along with doing negatives. I also have the Iron gym pullup bar. They work great together.
Have you considered getting the Lifeline USA Pullup Revolution to help you do assisted pull ups along with doing negatives?
Get these: http://www.amazon.com/Lifeline-4-PUR-USA-Pullup-Revolution/dp/B003KNC1MO/ref=pd_sim_sg_8
I have them, they are great to help with assisting you to lift easier.