Does the US government prevent tyranny today?
This is, to some degree, a matter of opinion. There are, of course, those who believe that the US government is actually engaged in tyranny or is at least moving in that direction. My own view is that the way that our government is set up prevents tyranny, even if our government itself is not actively suppressing anyone who is trying to tyrannize us.
Our government is meant to prevent tyranny simply through the way in which it is set up. It does not need to actively hunt out would-be tyrants. Instead, our Constitution automatically makes it hard for tyranny to occur. This continues to happen today.
Our Constitution prevents tyranny through its system of separation of powers and checks and balances (and to some degree through its specific guarantees of rights). Because the Constitution divides governmental powers up between the various parts of government, it is very hard for any one part of government to take enough power to become tyrannical. In this way, we are protected from tyranny not by any conscious acts of our government, but by the way in which the government is set up.