Vacuum Tube Theory 101
Vacuum Tube Basics
Types and Theory of Operation
There are several types of tubes still in use today. They are categorized depending on how many electronic elements are inside the tube. In this article we will discuss the differences between the types and theory of operation of these vacuum tubes.
Diodes have two elements, Cathode and Anode (Plate). In many cases the Cathode and the heater is the same element hence the term direct heated cathode. When the cathode is heated a cloud of electrons are excited off of the element and its surrounding the Cathode. When a large Positive potential (high voltage) is applied to the Anode the electrons surrounding the Cathode are drawn across the gap between the two elements. What makes this type tube useful is current will only flow in one direction from Cathode to Anode, thus turning AC to DC. These types of tubes are not used as much anymore because they have been replaced by solid state diodes that are more reliable and cheaper.
The next category of tube is the Triode. This type tube has three elements inside, Anode (Plate) Cathode and a Control Grid. The Cathode and the Plate (Anode) work the same as they do in the diode tubes. The Control Grid is inserted between the Cathode and the Plate. When a small negative voltage in respect to the cathode is applied to the Control grid the current flow between the plate (anode) and cathode is reduced. So a small variance in the negative voltage will cause a large variance at the plate.
So this is how amplification is achieved. Depending on the setting of the negative voltage (Control grid bias) on the control grid determines the operating class of amplification of the circuit. The classes are A, AB1, AB2, B, and C. Class A: the tube conducts during the entire cycle of the input signal. AB1 & AB2 the tube conducts between 75% and 99% of the cycle of the input signal. Class B the tube conducts between 75% and 50% of the cycle of the input signal. And in Class C the tube conducts less than 50% of the input signal cycle. Changing the class of amplification increases the efficiency of the tube. Class A is 30% efficient, AB1 & AB2 40%, B is 50% and C is 60%. There are many more considerations to determining which class of operation an amp will be operating that we won’t go into at this time.
The next type of tube is the Tetrode. This tube has all the elements of the two previous types with an added element. The added element is the Screen Grid. This element is inserted between the Plate and the Control Grid. About half of the positive plate voltage is applied to this grid to enhance the amount of current flowing through the tube. With the added Screen Grid the tube will have a greater amplification factor than a simple triode. One must be careful not to apply a positive voltage that is too high to the screen grid as to have much current flow through the grid or the tube will be damaged.
The next type of common tube is the Pentode. This tube has a third grid added between the plate and the Screen grid. The Suppresser Grid is usually internally connected to the Cathode. This element is added to the tube to suppress current flow in the screen grid. Without this element installed, electrons bounce off of the Plate and go back to the screen grid and increase the current flow through this element. The suppresser grid being much more negative pushes the electrons back to the plate to be conducted through that element rather than the screen Grid.
by Kevin “O.B.P.”, SWL #2