Magnum,
The cable clamp is doing exactly what you want. It is filling your penis with blood, but not allowing it to escape. As a result, your penis is swelling up and stretching to it’s limit. This is why you see the veins poking up, they are full of blood. For gains, this is good and exactly what you are looking for. But since your dick is not conditioned yet, you are developing small blood blisters on the head as capillaries burst from the high pressure. No big deal, they will heal up stronger and the blood will be absorbed in short order and the red spots will go away. Over time, the capillaries will get used to the pressure and will no longer burst.
My buddy Melvin is absolutely right with his advice on building you penis up with something like horses first. Even forceful jelqing slowly while 80% erect will help condition your penis for later, more advanced exercises. Think of PE as a ladder. You start at the first rung, then go up to the next, and so on. If you try to take several rungs at a time, you risk falling off. In PE, falling off means a penis injury. So take it easy, build yourself up, and watch your penis carefully to be sure you are not going too far. You did right posting this question Magnum. The questions you ask, and the answers you get will help you get a huge penis the safe way.
"In an honest Service there is thin Commons, low Wages, and hard labour; in this (Piracy), Plenty and Society, Pleasure and Ease, Liberty and Power; and who would not balance Creditor on this side when all the Hazard that is run for it, at worst, is only a sour Look or two at choaking. No, A merry Life and a short one, shall be my Motto."--Bartholomew Roberts