To get a smoother flow, many transformers are designed with a center tap on the output winding. This results in a reference point at the 0 voltage point in the AC sine wave. But still not perfect. We are still getting a rising and falling voltage at twice the rate as before. There are two methods used in tube amps.
Inductors look like small transformers, but only have two connecting wires.
Inductors store energy in the form of a magnetic field. When current flows through the windings in the inductor, energy is stored in the magnetic field. When the amount of current coming in through the windings drops, the magnetic field collapses and produces current in the wires, hopefully long enough to maintain a constant level of current. In reality, the current produced by the collapse isn't enough to completely smooth out the pulsing. Capacitors store electric potential, much the same way as a rechargeable battery does.