I'm not familiar with the 6633A, but most decent linear power supplies use a transformer with multiple voltage taps. They select the tap that will allow them to provide the required output voltage while minimizing the wasted power. Does the main transformer look like it has a ridiculous number of connections? If so, it probably uses that kind of switching.
If you can measure AC current, you can also compare the input and output power levels to see how much power is being dissipated internally.
Ed