For pure Tricep strength, you're already doing the best exercises:
Dips
Close Grip Bench
If you are going for PURE strength then I'd do the following:
Weighted Dips:
3-5 reps, 5 sets, 2 minute rest between sets, lower 3 seconds, pause for 1 second at the bottom, 2 seconds coming up
Close Grip Bench:
Weighted Dips:
3-5 reps, 5 sets, 2 minute rest between sets, lower 3 seconds, pause for 1 second at the bottom, 2 seconds coming up
Reasoning:
lower reps for strength
5 sets because the lower the reps, the more sets required to trash the muscle
adequate rest between sets to recover
slow and controlled tempo to get the most out of the movement through good form and new stimulus
Most people don't pay attention to tempo, but it really does make a difference. For example a guy who does rows like he's starting a lawn mower will be lifting the weight for 10 seconds. Then a guy who slows down the movement, and it takes him 35 seconds. They did the same amount of reps, but the time under tension was very different. So there will be a different training effect.
Good luck