October 1935 Short Wave Craft
Wax nostalgic about and learn from the history of early electronics. See articles
from Short Wave Craft,
published 1930 - 1936. All copyrights hereby acknowledged.
Just the other day I was using the familiar analogy that relates
water pressure, hose diameter, and flow rate to electrical voltage,
resistance, and current, respectively, in an explanation to my daughter
regarding why the water flow rate in her rebuilt house
(original burned down a year ago)
is not what it was in her original house. The cause, I surmise,
is due to use of the plastic PEX tubing which has a smaller inside
diameter than the old copper pipe. The submersible pump and holding
tank supply the same 50 psi as before, but since that pressure
now has to force the water through a path inside the house that
has more resistance to the water flow, the delivery rate to fixtures
is now lower. When I hold the contacts closed on the control relay,
the most I can get out of the pump is about 55 psi. Raising the
pressure will require replacing the submersible pump, which is probably
a good idea since it is a who-knows-how-old 115 volt, ½-HP
job that sits about 100 feet down in the ground. A 1-HP version
that operates on 230 volts will be an easy swap since it can use
the same size electric cable as the existing one (12-2). Guess who
gets to do that task?
I mention all that because this article's author goes to great
lengths to create mechanical analogies based on hydraulic systems
and little elfin men throwing electrons around inside a vacuum tube.
It's definitely a unique approach, and the drawings are pretty good
See Part 1,
Part 2, and
Radio Amateur Course: No.2 - How the Vacuum Tube Works
is the second lesson in the Radio Amateur Course, which has been
especially prepared for the readers of Short Wave Craft. Future
lessons will take up "inductance and capacity," and explain how
oscillatory circuits work.
The entire radio industry as it stands today, owes its success
and magnitude to the electron tube, or vacuum tube, if you prefer;
the well-known bulb which is used in every type of present-day radio.
The electron tube is not only used today for radio, but in other
industries and serves a vast number of other purposes.
For instance, with the aid of the photoelectric cell (a special
form of vacuum tube) or electric eye, color measurements are made
and it is possible to match colors perfectly with this instrument,
where previously the entire matching of colors was dependent upon
the human eye.
It is the vacuum tubes used in radio, however, which we intend
to discuss in this lesson. The starting point in the construction
or analyzing the construction of the electron tube, is the source
of electrons, which is technically termed the cathode. This cathode
is made of a material which when heated gives off a quantity of
electrons. In Fig. 1, we see the filament type cathode, wherein
the wire used for constructing the filament contains a certain amount
of material which will provide (emit) electrons when heated by an
electric current passing through it.
If this filament were exposed to the atmosphere and heated it
would decompose or "burn out," as it is commonly termed. However,
when enclosed in a glass envelope from which all air has been exhausted
or removed, this filament will glow for a long time without damage.
Diagrams above-Figs. 1 to 4A, show filament and
cathode heater units; how current flow is opposite to the electron
flow (3); rectification of A.C. passing through an electron tube
(4); hook-up of full-wave rectifier tube (4A); detail of tube elements
In Fig. 2 we have what is termed an indirectly heated cathode,
which consists of a small tube through the center of which is run
the heating wire or resistor. The outside of this tube is usually
coated with some metal oxide. When it is heated to a sufficient
temperature, electrons are then emitted from the outside coating
and it is entirely independent of the heater unit.
As the electrons are emitted from this cathode they form what
is termed an "electron cloud" around the cathode and within the
envelope in which it is enclosed. These electrons can only be attracted
to some object which is positively charged. Now, if we insert in
this tube and around the cathode, a piece of metal or some other
conducting material, and charge it positively, we can draw or attract
the electrons to it.
In Fig. 3 we show the action which takes place in a tube having
a cathode and a plate or anode, as the plate is technically termed.
By connecting a battery between the anode and cathode with the positive
terminals of the battery connected to the plate, we attract electrons
to this plate or anode. Current will then flow between the anode
and cathode, remembering always that the current flow is also opposite
to the electron flow.
If we insert a meter in series with the circuit we will find
that it will show the amount of current flowing in the circuit.
This tube, as shown in Fig. 3, is known as the diode or one having
two elements. If we remove the battery from the circuit and connect
the negative side of the battery to the plate and the positive side
to the cathode, no current will flow, because the negatively charged
plate will reject the electrons.
Effect of A.C. on Tube
So far, we have considered a constant polarity of voltage applied
between the cathode and anode. Now, if we were to apply an alternate
voltage between these two elements (see Lesson 1 for explanation
of alternating current electricity), the plate will be alternately
charged positive and negative, which means that current will only
flow through the circuit during the period when a positive voltage
is applied to the anode. When the anode side of the circuit becomes
negative, current does not flow. In Fig. 4 we illustrate what is
Simple hydraulic analogy to illustrate
how the grid in an electron tube acts like a valve to regulate
or control the amount of current (water) passing between
the plate (motor) and the cathode (pump). The water reservoir
in the analog diagram corresponds to the "B" battery or
power supply unit in a radio circuit.
The input circuit is indicated as alternating current while the
output circuit shows current flowing in only one direction during
half of the time of the input cycle. We have flowing in the output
circuit then, an interrupted direct current or what would otherwise
be termed half-wave rectification. All tubes of the diode type are
therefore termed half-wave rectifiers. The 281 is an example of
this type of tube.
By using two anodes we can obtain full-wave rectification. This
is shown in Fig. 4A. A rectifier of this type is termed a full-wave
rectifier and an example is the 280 tube.
Returning to Fig. 3 we can readily understand that as we change
the degree of positive potential (voltage) applied to the plate,
we will change the volume of electrons which are attracted to it.
A low potential will attract a small amount of electrons, while
a high potential or high voltage will attract a greater number of
electrons. An important point to bear in mind is that a negative
potential repels electrons, while a positive potential attracts
them. (Unlike charges attract and vice versa.)
How the "Grid" Works
To have a better control over the amount of current flow in the
plate circuit of the vacuum tube, we may insert a third element,
known as the grid. Tubes having three elements are termed triodes,
the prefix "tri" meaning three. This grid consists of a form of
screen between the anode and cathode through which electrons must
pass in order to reach the plate.
This grid being located nearer to the cathode or source of electrons,
will have a greater effect upon the electron stream when it is charged
either positively or negatively. In Fig. 5, we have the same circuit
as in Fig. 3, excepting for the addition of the grid. Because of
the great effect this grid has upon the electron flow, it is called
the control grid.
Fig. 5 shows action of triode tube; 5A,
how A.C. input is changed into a rectified, pulsating direct current
by an electron tube; 6, arrangement of the elements in a screen-grid
tube; how elements are arranged in a pentode at Fig. 7.
We may now apply either a positive or a negative potential to
this grid and obtain a change in plate current or a change in the
number of electrons reaching the plate, because if the grid is charged
negatively, it will tend to repel or retard the flow of electrons
between the cathode and the plate. This grid can be made so negative
(biased) that it will entirely cut off the flow of electrons, reducing
the plate current to zero.
As this grid becomes more positively, charged, an increase in
the flow of electrons to the plate will take place. That is, providing
the potential (voltage) of the grid is not as great as that of the
plate. As this grid becomes entirely positive, relative to the cathode,
it will then collect a certain amount of electrons from the stream
and return them to the cathode, causing current to flow in the grid
In receiving circuits the output of the vacuum tube depends upon
the plate current change, that is, the increase and decrease in
amplitude or, more simply stated, the magnitude of the change. So,
we can readily see that by using this control grid, which is located
close to the filament, we can effect great changes in the current
flowing in the plate circuit of the vacuum tube with relatively
small changes in the potential of the grid and thus obtain considerable
amplification in radio circuits.
The simple analogy above is an attempt
to show the action taking place in the vacuum or electron
tube used in radio circuits. Note that the shutters (corresponding
to the grid in the tube) control the number of balls hurled
through it toward the target (or plate). In a similar way,
the grid in a vacuum tube controls the amount of current
passing from the plate to the cathode (or filament).
In Fig. 5a we show what happens when A.C. is applied to the input
circuit of a triode, biased (bias usually means applying a fixed
negative or positive charge, independent of the signal voltage,
to the grid of the tube) so that the plate current is of fairly
low value, but nowhere near the cut-off point. We show the input
signal to the grid as alternating current, where it rises above
and falls below the zero mark. As the input signal swings the grid
more positive, or better stated - less negative - the plate current
begins to rise above what is commonly termed the "no-signal" (static)
plate current value; that is, the normal value of plate current
with no applied signal.
This constitutes one-half of the cycle of the input signals.
On the other half of the input-signal cycle, the grid becomes more
negative, causing the plate current to fall below its normal no-signal
value. (See previous explanation under "How the Grid Works.") Now,
in the plate circuit, we have apparently the same wave form as the
input signal. The input signal was A.C.; however, A.C. does not
flow in the plate circuit of the tube. This fluctuating replica
of the input signal is termed the alternating component of the plate
current. ("Plate current" is the current flowing through the circuit
from plate to filament, or heater, when the electron stream is established
by heating the filament.)
If we were to connect earphones in series with the plate circuit,
we would be able to hear the incoming signal reproduced and amplified
in the plate circuit, that is if it was of low enough frequency
to come within the range of the human ear.
The fluctuating plate current or the alternating component of
the plate current would cause the diaphragm of the earphone to vibrate
due to the varying current flowing through the phones and the change
in the magnetic pull on the diaphragm.
So long as the voltage of the incoming signal does not exceed
the value of the bias battery, there will be no grid current flowing,
because the grid will never go completely positive. On the positive
half of the input signal the grid, in reality, becomes just less
If we were to insert a resistor (R) in series with the plate circuit,
the fluctuating current flowing through this resistor would cause
a voltage drop across the resistor, varying directly with the plate
current. The ratio of this varying voltage drop to the input signal
voltage, is known as the gain of the tube or the voltage amplification.
Tubes Have Capacity Between Elements
In all types of vacuum tubes, we have in reality a number of
small condensers in that there is a definite electrical capacity,
for instance, between the plate and the grid, between the grid and
cathode, and also between the plate and cathode, for the simple
reason that each of these elements can be likened to the plates
of a small condenser (current absorber). The grid to cathode capacitance
is termed the input capacitance. The output capacitance is the capacity
between the plate and cathode. In many very "high-gain" circuits,
it is necessary to neutralize the plate to grid capacity in order
that energy will not be fed back from the plate circuit to the grid
This can be accomplished either by external methods of neutralizing,
which will be explained in a later lesson, or by inserting a shield
or a screen between "the plate of the tube and the grid. This is
commonly termed the screen-grid and tubes having a control grid
and a screen-grid, together with the anode and cathode, are termed
This screen-grid is so designed that it will effectively shield
the plate from the grid. While the plate to grid capacity of a triode
may be as great as 8 mmf., the plate to grid capacitance of a screen-grid
tube may be reduced to a value as low as 0.007 mmf. This screen
must be constructed so that it will not materially obstruct" the
flow of electrons between the cathode and plate; therefore, it is
made in the form of wire mesh.
It also must not be negatively charged because the flow of electrons
would also be impeded. Therefore, a positive potential is in most
cases applied to the screen-grid in order to accelerate the flow
of electrons to the plate. This screen being an electrostatic shield
must be bypassed with a condenser to the cathode in order to be
grounded, in so far as high frequency currents are concerned.
The voltage applied to the screen is usually lower than the plate
voltage. The stream of electrons going to the plate being greatly
accelerated by the screen-grid, may strike the plate at such a terrific
speed that they will dislodge other electrons, which may be attracted
to the screen, which is the nearest positively charged element.
This is known as secondary emission and limits the output capabilities
of the tube. This condition can be overcome by inserting between
the screen and the plate another element which will not obstruct
the flow of electrons to the plate but prevent them from returning
to the screen.
In order to accomplish this, the third grid or suppressor is
usually connected directly to the cathode in order that electrons
dislodged from the plate may continue back via the suppressor to
In some tubes such as the types 34 and 39 this suppressor is
connected directly to the cathode of the tube internally. However,
tubes such as types 57 and 58 have a separate pin on the base for
this suppressor grid, in order that in special circuits a positive
or a negative voltage may be applied to it. The values, of course,
will be dependent upon the circuit requirements. In large transmitting
tubes of the pentode type (pentode is a name given to all tubes
having 5 elements), this suppressor is positively charged to the
order of 30 or 40 volts.*
Hydraulic analogs showing action of half- and
also full-wave rectifiers, the detector tube in a receiving circuit
acting as a rectifier. The first diagram shows how a single-action
pump and a check valve permits water to pass through the pipe up
into the tank on each half stroke, while any counter water pressure
is prevented from passing back into the pump by the check-valve.
The second diagram shows how two half-wave rectifiers (pumps), with
the aid of two check-valves cause a "full-wave" pressure to be developed
in the main water line. When one is not working, the other is.
*Some excellent books covering the electron
theory and the operation of electron tubes are:
"Electrons at Work," by Charles R. Underhill.
"Radio Receiving Tubes," by Moyer & Wostrel.
The RCA Tube Manual also contains a wealth of information covering
the operation of various types of vacuum tubes.
"Principles of Radio Communication," Prof. John
"Modern Vacuum Tubes and How They Work," by
Posted April 23, 2015