From this book:
The aura of success that results from high quality care is of great help.
I also found the sentence:
You are great help to me.
in the Longman dictionary. Can I change this into:
You are of great help to me?
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You are a great help to me.
You are of great help to me.
Both the sentences are grammatical.
There's no difference in meaning between these sentences. The noun "help" in the former has been used in the sense of a person who helps somebody to do something. In the latter, it has been used as an act or process of helping somebody.
However, the former is more common.
The first sentence is not correct with no article (“a”), as stated in the original question:
❌ No: You are great help to me.
By the way, the negation of this sentence works fine with no article:
✔️Yes: You are no great help to me.