The way I see it, the biceps has two functions, to generate bent arm strenght and to stabilize the elbow in straight arm elements, and at least from my experience they being strong in one doesn't mean that you'll be strong in the other, ie. I have pretty decent bent arm strenght (i can do Ido's 9090 one arm iso holds and a OAC on a good day) but my straight arm strenght sucks to strenghten those two fields I utilize three excercies which you may find useful because they are very adjustable to your current level:
-for bent arm strength I do uneven grip chin ups, with the supporting hand usually ~40cm lower than the pulling one, with this exercise you can easily gauge your progress and it isn't as hard on the elbows as the 9090 or OACs, so I find it quite good for laying a foundation for those, however chins have some cons, which braindx stated
- for straight arm strength I use these pushups
currently on me knees with rings ~5-7 cms above the ground, also very adjustable, and I'm seeing some nice gains from them, I also do them planche exercises posted by Gregor with fingers pointing backwards.
hope it helps