Question: I like doing bent-over rows. However, I've quickly outgrown my 50lb dumbbell, and am not strong enough yet to do pullups. I haven't any other equipment.

What else can I do to get to the point where I can do pullups?

Answer: 
Since your goal here is to do pull ups, I'm going to focus more on that then just upper back strengthening as the title requests. There are three major factors that you should focus on improving: weight, strength, and technique.

1. **Lose weight.** Doing cardio to burn fat will make pull ups more manageable, since you have less total weight to physically pull up. You will see biggest gains in this area if you are overweight, but if you are average/skinny, then focus elsewhere.
2. **Assisted Pull Ups**. This has the benefit of building strength and practicing technique. You can accomplish this via a pull up machine, which helps support X amount of your weight, so you can effectively do pull ups at a significantly lighter weight than your bodyweight, and then progressively increase that amount until you have the strength to do unassisted. Requires gym/machine to do.
3. **Resistance Bands**. This is another way to simulate a pull-up without having to do full bodyweight. Attach resistance bands to the ceiling/doorway, sit down, and mimic the pull up. Builds strength, less so for technique. Can be done at home with heavy resistance bands.

![resistance band pull up](https://i.stack.imgur.com/yn56g.jpg)

4. **Negatives**. Basically you use a chair or other material to help get your chin above the bar, and then slowly lower yourself back down. This helps get you more accustomed to holding your weight and helps improve your strength and technique. Can be done at home with pull up bar and chair.
5. **Pulldowns**. This will help strengthen the muscles, and can be progressively loaded far more than a dumbbell. Downside is it requires a machine.

![enter image description here](https://i.stack.imgur.com/bXqMq.jpg)
Assuming that you have no other accessibility to additional equipment, as your description implies, I would recommend two things.

**Different angles**:

I would try varying the angles at which you exercise your back with the dumbbells. Varying the angles will allow you to strengthen supporting muscles and tissues that should help in your quest to perform pullups. For example, you may want to look into performing [incline bench pullups](https://www.youtube.com/watch?v=mPRNmG2CxuY). The important aspect is that your back is at a varied angle. Even if you were to try to raise your back slightly while doing one-arm dumbbell rows, that would help. Most importantly, make sure to protect your lower back by not putting it into an **unsupported** position.

**Body weight movement**:

Additionally, I would look at performing something like [reverse bench pulls](https://www.youtube.com/watch?v=fk7AdWvlt4Y). Granted, you may not have a bar and exercise ball, but this movement can be done with your feet firmly on the floor and a simple solid immovable table or some other object that you can use to pull yourself up to. Just be extra careful if you use a table that it will not fall on you.

Again, given your limitations, I think the most important thing for you right now is *variation* of movement *with what you currently have*.