A theory of current and energy modulations in an electron beam propagating through a grounded drift tube is developed. The theoretical analysis is based on the assumption that each beam segment has a prescribed current profile at the drift-tube entrance. It is shown that the current modulation amplitude decreases, reaches its minimum value and increases as the beam propagates downstream. The current modulation in the linear regime is described by a sum of the forward and backward density waves. On the other hand, the energy modulation in the linear regime is described by the difference between the forward and backward density waves.