The Vacuum Tube Diode

Vacuum Tube Power Diodes – 5U4, 5Z3, and 5R4 Popular in 1940s and 1950s

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).

Typical Vacuum Tube Diode Construction. The schematic diagram shows the heater, also known as filament. Frequently the heater is not drawn. When the heater and cathode are one then it shows the upside down V, as in illustration, for both.

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.

The circled A represents an amp meter while the lines with – and + on each end represents batteries.

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.

Full Wave Rectifier. Notice as one plate becomes negative with reference to the center tip the other will be positive so current would flow. When the voltage reverses the other plate will be positive and the current will then flow through that plate. Thus current flows during both half of the cycles. The filter and load are missing from this illustration but the electron flow would be through a load which is attached to the filter attached to one side of the cathode/heater.

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.

Bridge Rectifier. As the voltage becomes negative on the top of the transformer it will be positive on the bottom. The third diode down will conduct current and the top diode will conduct forming a complete circuit through the load and filter.
The bottom diode and the second diode down prevents electrons flowing which will prevent a short circuit. When the voltage reverses then the opposite diodes will conduct. The filter consists of two chokes and two capacitors in a pi network configuration. The chokes store energy in a magnetic field which becomes stronger as the current increases then as the source voltage drops the collapsing field will reduce the amount of voltage drop reducing the ripple. The capacitors charge as the source voltage increases and then feeds voltage back as the source voltage drops. This how the filter turns the rippling AC to 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.



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