Public Class Form1
    Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles TextBox1.TextChanged
        If TextBox1.Text = String.Empty Then
            TextBox6.Clear()
            Exit Sub
        End If
        Select Case Integer.Parse(TextBox1.Text)
            Case 0 To 60
                TextBox6.Text = "DHTG-65"
            Case 61 To 300
                TextBox6.Text = "DHTG-90"
            Case Else
                TextBox6.Clear()
        End Select
    End Sub
End Class