Why my DispatcherTimer doesn't work C# WPF

Question

how could I change my Label content to random letter by using DispatcherTimer C#? My DispatcherTimer doesn't execute Letter_Tick() function :|

My MainWindow.xaml

<Label x:Name="myLbl" Content="" FontSize="108" HorizontalContentAlignment="Center"/>

My main class (MainWindow.xaml.cs)

    public MainWindow()
    {
        InitializeComponent();
        letterRandomizer ob = new letterRandomizer(myLbl);
    }

And my letterRandomizer class

class letterRandomizer
{
    Random rand = new Random();
    string newLetter = "";
    Label MyLbl;
    
    DispatcherTimer letter = new DispatcherTimer();
    
    public letterRandomizer(Label myLbl)
    {
        this.MyLbl = myLbl;
        letter.Tick += Letter_Tick;
        letter.Interval = TimeSpan.FromSeconds(2);
    }

    private void Letter_Tick(object sender, EventArgs e)
    {
        MyLbl.Content = "";
        newLetter = Convert.ToChar(rand.Next(65, 90)).ToString();
        MyLbl.Content = newLetter;
    }

}