The silver spot you will sometimes see on the inside surface of the glass envelope of a vacuum tube
is normal. It was caused by the "flashing" of a chemical during the manufacture of the tube. Burning the
chemical, called the GETTER, helps to produce a better vacuum and eliminates any remaining gases.
ELECTRICAL PARAMETERS OF DIODES
Thousands of different tubes exist. While many of them are similar and even interchangeable, many
have unique characteristics. The differences in materials, dimensions, and other physical characteristics,
such as we have just covered, result in differing electrical characteristics.
The electrical parameters of a diode, and any tube, are specific. In the process of discussing these
parameters, we will state exact values. Voltages will be increased and decreased and the effects measured.
Limiting factors and quantities will be explored and defined. The discussion will be based on simplified
and experimental circuits.
It is important for you to realize that practically all of the parameters, limitations, definitions,
abbreviations, and so on that we will cover in these next paragraphs will apply directly to the more
complex tubes and circuits you will study later. Diode parameters are the foundation for all that follows.
You have learned to use letters and letter combinations to abbreviate or symbolize electrical
quantities. (The letters E, I, and R are examples.) We will continue this practice in referring to tube
quantities. You should be aware that other publications may use different abbreviations. Many attempts
have been made to standardize such abbreviations, inside the Navy and out. None have succeeded
Table 1-1 lists electron-tube symbols used in the remainder of this chapter. The right-hand column
shows equivalent symbols that you may find in OTHER texts and courses.
PLATE VOLTAGE, D.C. VALUE
PLATE SUPPLY VOLTAGE, D.C.
GRID BIAS VOLTAGE, D.C. VALUE
GRID BIAS SUPPLY VOLTAGE, D.C.
INSTANTANEOUS PLATE VOLTAGE
INSTANTANEOUS GRID VOLTAGE
A.C. COMPONENT OF GRID VOLTAGE
A.C. COMPONENT OF PLATE VOLTAGE (ANODE)
D.C. PLATE CURRENT
D.C. PLATE RESISTANCE
Table 1-1.Symbols for Tube Parameters
Plate Voltage-Plate Current Characteristic
You know that a positive voltage on the diode plate allows current to flow in the plate circuit. Each
diode, depending on the physical and electrical characteristics designed into the diode, is able to pass an
exact amount of current for each specific plate voltage (more voltage, more current-at least to a point).