The Envelope

The envelope of a tube may be made of ceramic, metal, or glass. Its major purpose is to keep the vacuum in and the atmosphere out. The main reason for this is that the heated filament would burn up in the atmosphere. There are other reasons for providing a vacuum, but the important thing is to realize that a tube with a leaky envelope will not function properly.

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.

Symbols

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 completely.

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.

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). The plate voltage-plate current characteristic for a given diode is a measure of exactly how much plate voltage controls how much plate current.

This is often called the E_p - I_p characteristic.

The E_p - I_p characteristic for a given diode, is determined by design engineers using mathematical analysis and laboratory experiment. You, as a technician, will never need to do this. However, you will use the results obtained by the engineers. You will also use your knowledge of the diode as you analyze equipment malfunction. Assume that we have the circuit in figure 1-12.

(The filament has the proper voltage-even though it isn't shown on the diagram.) Our purpose is to determine just how a changing voltage on the plate changes (or controls) the plate current. The method is as follows:

Figure 1-12. - Determining diode plate characteristic.

Starting with zero volts from our variable dc voltage source, increase the plate voltage (E_p) in steps of 50 volts until you reach 400 volts. At a each 50-volt step, measure the milliamperes of plate current (I_p) that flow through the meter. Record the I_p meter readings, step by step, so that you may analyze the results.

Assume that table 1-2 shows our results. While we could use the table, a more normal procedure is to plot a graph of the values. Such a graph is called an E_p - I_p CURVE and is shown in figure 1-13. Each tube has its own E_p - I _p curve, which is available in commercial tube manuals and in many equipment technical manuals. Each curve will be different in some respects from every other curve. The shapes, however, will be similar.

Table 1-2. - E_p - I_p

Values Obtained by Experiment

Figure 1-13. - E_p - I_p characteristic curve.

The E_p - I_p curve in figure 1-13, although just an example, is typical of real plate characteristic curves. You may learn certain characteristics that apply to both diodes and other tubes by studying it.

First, look at the part of the curve to the left of point A. Because it is not a straight line, it is referred to as NONLINEAR. Note that a change of 150 volts (0-150) caused a change of 10 mA of plate current (0-10). In comparison with the straight-line part of the curve, between points A and B, this is a relatively small change in current. The smaller the change in current, the flatter the curve.

In explaining this NONLINEAR portion of the curve, let's go back just a bit to electron emission. The electrons emitted by a cathode form a cloud around the cathode. This cloud is called the SPACE CHARGE. The closer the space charge is to the cathode, the more densely packed it is with electrons. In our example, the lower plate voltages (0-150 volts) over this part of the curve exert a pull on only the outer fringe of the space charge where there are few electrons. This results in relatively few electrons flowing to the plate.

Now look at the center portion of the curve between A and B. This is known as the LINEAR portion because it is nearly a STRAIGHT LINE. Over this portion, a change of 50 volts E_p causes a change of 10 mA I_p.

The reason for the increased change in plate current for a given change of plate voltage also has to do with the space charge. With a higher plate voltage (over 150 volts), the attraction from the plate begins to influence the DENSER part of the space charge that has greater numbers of electrons. Therefore, a higher current flows for a given voltage than in the nonlinear part. The curve becomes steeper. In our example, this linearity continues to about 300 volts, point B.

Lastly, let's look at the top portion of the curve. The plate current plotted here is produced by the higher plate voltages. However, the amount of current change for a given voltage change is greatly reduced. The reason for this again involves the space charge. At about 300 volts, almost all of the electrons in the space charge are flowing to the plate. A higher voltage cannot attract more electrons because the cathode cannot produce any more. The point where all (or almost all) available electrons are being drawn to the plate is called PLATE SATURATION or just SATURATION. This is one of the limiting factors of every tube.

You can see from the analysis that the most consistent control of plate current takes place over the linear portion of the E_p - I_p curve. In most applications, electron tubes are operated in this linear portion of the characteristic curve.

Plate Resistance (R_p)

One tube parameter that can be calculated from values on the E _p - I_p curve is known as plate resistance, abbreviated as R_p. In a properly designed electron tube, there is no physical resistor between cathode and plate; that is, the electrons do not pass through a resistor in arriving at the plate. You may have wondered, however, why the variable dc voltage source of figure 1-12 didn't blow a fuse. Doesn't the plate circuit appear to be a short circuit-a circuit without a load to limit the current?

The fact is, there is a very real, effective RESISTANCE between cathode and plate. It is not lumped in a resistor, but the circuit may be analyzed as if it is. The plate resistance of a given tube, R_p, can be calculated by applying Ohm's law to the values of E_p and I_p. Figure 1-14 is a typical diode E_p - I_p curve. The plate resistance has been figured for R _p under three different conditions, as follows:

Figure 1-14. - The E_p - I _p characteristic curve for a diode.

Remember that 1 mA = .001 ampere; therefore 40 mA =.040 ampere.

Solution:

The other two indicated values of R_p were figured in the same way.

You should note that there is very little difference in plate resistance when the E_p and I_p values are taken from the linear portions of curves. Check this out with values taken from the linear portion of figure 1-13.

R_p (with a capital R) is the effective resistance offered to direct current.

PLATE RESISTANCE IN GAS DIODES. - Gas diodes are a type of tube that we have not yet discussed. They are mentioned here only because of their plate-resistance characteristic.

Instead of a high-vacuum environment, some tubes have small amounts of gas introduced in the envelope vacuum during manufacture. Argon, neon, helium, or mercury vapor are commonly used.

When a certain minimum voltage is placed on the plate, the gas molecules in the envelope ionize. This happens by a process that will be explained when gas diodes are studied. The positive ions tend to cancel some of the effects of the space charge that influence plate resistance in a vacuum tube. This canceling reduces internal plate resistance to a relatively low, constant value. In applications that require a large plate current, the low plate resistance of a gas-filled diode has an efficiency that cannot be approached by a high-vacuum diode.

This and other characteristics of gas tubes will be covered later.

Q.10 Vacuum tubes are designed to operate in what portion of the E_p - I_p curve?
Q.11 What value can be calculated from the values found on an E_p - I_p curve?