To start understanding the operation of the vacuum tube we must start with the diode meaning two elements or two electrodes. These two electrodes are the cathode and the anode more commonly known as the plate. There is the requirement of a heater which may also serve as the cathode but many diodes have the heater inside a sleeve so it will heat the sleeve which is the cathode. Though the heater is separate from the cathode it is not counted as an element.
As the cathode heats up to red hot the molecules are vibrating so vigorously they cause some to the atoms to lose electrons. These free electrons form an electron cloud around the hot cathode. When a negative potential is placed on the cathode the field will become greater, that is to say more intense and more electrons inter into the electron cloud.
If the cathode was not in a vacuum the electrons cloud would be dissipated by the gas molecules. Thus it is placed inside an evacuated envelope (glass or metal).
A plate is also included inside the envelope. The term plate, I fear, gives a miss conception of what it looks like. The plate is usually a cylinder that is placed around the cathode (see the above drawing). If a positive charge is placed on the plate the free electrons will migrate across the gap to the plate thus producing a current flow.This process is known as the Edison Effect.
If a negative potential is placed on the plate that is negative with reference to the cathode thus leaving a positive cathode and negative plate current will not flow. This is the principle of rectification. The part of the cycle that causes the cathode to be negative and the plate positive current will flow but when the reverse happen and the plate is negative and the cathode positive current will cease thus turning the AC into a rippling DC voltage.
When the diode is used to produce DC from AC filters must be added to eliminate the AC ripple. The raw DC output is a single diode will rise and fall as the AC voltage rises to a peak and then fall back to zero volts. Then there will be no current flow during the second half of the cycle. Thus if only one tube is used it is called a half wave rectifier. Four diodes can be used in a bridge rectifier circuit or by using a center tap on the transformer two tubes can be used to make a full wave rectifier. A smaller filter can be used when a full wave rectifier is used to produce a clean DC voltage.
Vacuum tube rectifiers are not common in modern ham radio equipment but when I became an amateur radio operator in 1960 they were very common in new amateur radio equipment.