Index is outside bounds of array

When I run this it says that index was outside of bounds for the array but it is not. If I change public static ConectorRec[] ch = new ConectorRec[74]; to ConectorRec[] ch = new ConectorRec[75]; then they are not all referenced to an object and throws other errors on other parts of my code.

using System;
using System.Drawing;
using System.Windows;
using System.Windows.Forms;
using System.Collections.Generic;

namespace Dots
{
    public static class Conectors
    {
        public static ConectorRec[] ch = new ConectorRec[74];
        public static void intitialize()
        {
            ch[0] = new ConectorRec(new Rectangle(10, 20, 10, 40));
            ch[1] = new ConectorRec(new Rectangle(20, 10, 40, 10));
            ch[2] = new ConectorRec(new Rectangle(20, 60, 40, 10));
            ch[3] = new ConectorRec(new Rectangle(60, 20, 10, 40));
            int t = 0;
            int tt = 1;
            for (int i = 4; i<73; i++)
            {
                t++;
                if (t == 1)
                {
                    ch[i] = new ConectorRec(new Rectangle(50 * tt + 20, 10, 40, 10));
                }
                if (t == 2)
                {
                    ch[i] = new ConectorRec(new Rectangle(50 * tt + 20, 60, 40, 10));
                }
                if (t == 3)
                {
                    tt++;
                    ch[i] = new ConectorRec(new Rectangle(50 * tt + 10, 20, 10, 40));
                    t = 0;
                }
            }
            ch[74] = new ConectorRec(new Rectangle(10, 70, 10, 40));
        }
    }
}

please tell me what I am doing wrong and how to fix this error.