Preheat oven to 200 degrees. Combine brown sugar, Karo, butter, cream of tartar, salt in a medium saucepan and bring to a boil, stirring constantly. Boil for 5 minutes. While mixture is boiling, place popped corn in a large, buttered pan. Remove sugar mixture from heat and stir in baking soda. It bubbles quickly. Pour over popcorn and stir to combine. Bake at 200 for 1 hour. The best!!