Well, it depends.
What species of tortoise do you have? If they're a forest tort (Redfoot, Yellowfoot, Burmese Brown/Black, Forset Hingeback) then you do not need a UVB bulb.
You need to make 10% of their diet animal protein (low fat cat food) mixed with with powdered calcium carbonate. Forest torts uptake calcium via the D3 in animal protein vs. sunlight like arid torts.
If you have an arid environment tort, you do need a UVB bulb.
My Redfoot's have never had a UVB bulb and are perfectly healthy, produce 5 clutches of eggs each year, healthy hatchlings and have only have teh tiniest amount of pyramiding from spending they lives (12+ years old at this pont ) in Minnesota.
My Russian's and Greeks have always had UVB bulbs and are perfectly healthy as well.