The simplest black level clamp consists of a signal diode (1N4148) reverse-biased to ground from the output line of the input buffer above and a 4k7 resistor in parallel with it. That forces the sync tips to be at (gnd – threshold voltage of the diode), which shifts the black level of a 2x amplified video reasonably close to ground. Add that and you can connect the two circuits together and see how they work. They should be very good as far as the signal quality goes (maybe not broadcast quality, but no visible signal degradation). Don’t forget good power supply bypassing, use at least 220u of electrolytics and 100n ceramic caps near the transistors on both circuits
