A Portable Thyratron Tester
1957 Radio & Television News Article
thyratron is not necessarily a familiar type of vacuum tube to most
RF and microwave electronics practitioners unless they happen to be
involved in radar, imaging (x-ray), particle accelerators, etc.†
It is basically a high speed, high current switch used in pulse forming
networks for firing magnetrons (via a high-voltage transformer). Both
the S-band airport surveillance radar and the X-band precision approach
radar I worked on in the USAF employed thyratrons. The X-band radar
had been modified by the time I came on the scene to use a solid state
thyratron (one of the earliest adaptations), but the S-band radar still
used its original vacuum tube thyratron. While I don't recall for certain,
the thyratron in the thumbnail image is the one it used. The accompanying
ruler is 12" long to give you an idea of the size. They used to burn
out and have to be replaced fairly regularly on our mobile radar units
probably from being powered up and down so often, and from the occasional
over-the-road trip en route to a temporary runway
February 1957 Radio & TV News
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio & Television News articles
I have already posted. All copyrights (if any) are hereby
Being a lifelong electronics nerd and woodworker, I actually
made a lamp out of a spent S-band thyratron tube (no photo, unfortunately).
I turned an oak base on a lathe and then mounted the tube in the middle.
Three 3/8" diameter brass tubes were bent to conform to the side of
the vacuum tube and were joined with a brass strip at the top. A threaded
lamp socket nipple went in the middle of the junction and the entire
assembly was soldered. The bottoms of the tubes were epoxied into holes
in the wood base spaced 120° apart around the tube's perimeter.
The lamp cord was threaded through one of the brass tubes and routed
out the side of the base to a plug. The bulb socket and harp for holding
the shade topped off the lamp. My parents were the lucky recipients.
While I regarded it as a fine example of artistic juxtaposing of classic
wood and modern microwave electronics, they likely considered it as
fitting for public display as Ralphie's mother did the "major award"
his father won from an entry in a crossword puzzle contes which was
the lady's fishnet-stockinged leg lamp (A
Christmas Story). My lamp's current status is MIA - probably buried
in the back yard like Mr. Paker's lamp.
† RF Cafe visitor Jimmy C. pointed
out another common use of the thyratron, "I would like to add Television
Broadcast Transmitters using
IOTs. The purpose of the thyratron is to remove the DC High Voltage
from the IOT when there is an arc. The Thyratron actually shorts the
high voltage to ground. Not sure how it handles 32,000 volts to ground
but it does. It operates very fast. That is how it protects the IOT
See all available
vintage Radio News
A Portable Thyratron Tester
By W. Philbrook
A commercially available instrument uses several unique circuits to
test specialized tube types.
Figure 1. - The Alectric thyratron tester.
There are receiving tube checkers
of almost every conceivable variety on the market but there has been
no portable, easily usable tester for thyratron tubes. Why this condition
has existed is difficult to say because, with the volume of industrial
electronic equipment in current use, there should be an excellent sales
potential for such a device. Now a tester has been developed which not
only permits the rapid and accurate checking of a thyratron tube's characteristics,
but does so with a number of unique circuits. Before we examine these
circuits, it may be desirable to review the basic operation of a thyratron
Thyratrons are gas-filled tubes in which the flow of current
is either at zero or at the saturation value. The grid voltage, usually
negative, keeps them cut off until such time as it is desired to trigger
them. At that time, the grid voltage is made relatively positive and
electrons are thus permitted to flow from the cathode to the plate.
Once this movement starts, even in a minute degree, the molecules
of gas enclosed in the tube tend to become positively ionized. This
condition of positive ionization neutralizes the holding or controlling
voltage on the grid. This action completely opens the dam, so to speak,
to the electrons being emitted by the cathode and permits current to
surge freely through the tube to the plate. Once this flow has started,
the grid loses all control over the rate of flow and - as long as the
plate voltage is kept above a certain critical minimum - all electrons
leaving the cathode travel to the plate.
This, roughly, is how
a thyratron tube operates. And to determine the operating condition
of such a tube obviously requires a special type of tester. Let us therefore
note the various thyratron features which are important and see how
the tester shown in Fig. 1, made by the Alectric Mfg. Co. of Kenosha,
Wis., accomplishes its testing function.
One of the first things
to observe is that most thyratrons operate with alternating voltage
on the plate (or anode). This is done so that the grid may resume control
of the tube after it has been fired or triggered. There is no point
in having a tube in the circuit over which no control at all is possible,
and the simplest way to achieve this control is to apply a.c. to the
plate. When the plate voltage goes negative, conduction ceases, the
ionized gas de-ionizes, and the control grid is able to prevent the
flow of electrons from cathode to plate. The tube is now ready for the
next triggering pulse.
Hence, if you examine the schematic diagram of this tester, Fig. 2,
you will see that a.c. voltage is brought to the plate from one of two
points: either the variable-voltage transformer, T1
or the step-up transformer, T3.
Voltages from 0 to 200 volts are obtainable from T1
while 0 to 3000 volts (peak) can be obtained from the secondary of T3.
The purpose of the low a.c. voltage is to check thyratrons at their
rated currents - in this case, either 1, 2.5, or 6 amperes. By using
a low voltage and proportionately low-valued resistors in the plate
circuit, the wattage requirement and, with it, the heat dissipation
can be kept within reasonable bounds. When checking 2.5 and 6 ampere
tubes, even under these conditions, it is still necessary to place the
plate load resistors in a separate box. If a higher plate voltage were
used, the load resistances would be correspondingly higher and the wattage
requirements would raise the cost of these units excessively. Hence,
low plate voltages are used for certain tests where maximum rated anode
currents are desired.
Figure 2. - Complete schematic for the Alectric specialized
On the other hand, when a thyratron tube
is being checked for its grid-plate characteristics, we can use a high
plate voltage and employ high-valued load resistances in the plate circuit
to keep the current down. In this test, it is simply a question of establishing
"trigger points" or critical grid voltages, not to determine what the
peak plate current is. More on this test presently.
of the a.c. voltage from T1
to the plate of the thyratron tube to be checked can be readily followed
from Fig. 2. The start can be made at T1.
From the center tap, point B, the line leads to switch S2,
which for the purpose of this test is turned to the a.c. position. From
S2, the path goes to point
C, then through S2-4, which
is now closed, to point D, then E, and finally through R7
to the plate of the thyratron.
The other side of the circuit is
completed from point A through S1-2,
which is now closed, to F and from here to the center tap of the filament
Now, as a measure of tube
reliability, a direct measurement is not made of the tube's peak plate
current. Rather we measure the voltage drop across the tube when the
latter is conducting at full current. This voltage is known as the arc
drop and its value is sought because an increase in the arc-drop voltage
is the most outstanding indication of the end of life of a thyratron
tube. However, a single arc-drop reading in itself will not indicate
the life factor; rather what is needed is a series of readings over
a period of time to anticipate the end. See Fig. 3. The technique could,
in a way, be compared to the predicting of weather conditions by taking
comparative readings on a barometer.
It is suggested that a
reading be taken after about every 400 hours of operation. As the arc-drop
value begins to rise, a shorter interval should be observed - say every
200 hours as the tube approaches the end of its useful life.
To be most effective, the arc-drop reading permits the end of life to
be predicted so that the tube can be removed from operation before costly
work stoppage occurs. A reasonable amount of arc-drop may well establish
the limits at which such tubes should be removed to prevent emergency
Figure 3. - The arc-drop voltage of a thyratron increases generally
with the age of the tube, but may vary during life. It is an
indication of tube condition.
Once a tube is placed in operation, its arc-drop
voltage will vary throughout its life. Typical variations are shown
in Fig. 3. With tube "C" the rate of increase of the arc drop accelerates,
but the curve is easily recognized. As a result, the tube can be replaced
before this reading reaches its published limit. The steady linear rise
with tube "B" is also easily recognized. As happens with some tubes,
the limit is reached early with tube "A" but this condition is only
temporary. The arc-drop voltage reduces below the limit again. However,
the upward rise is soon resumed, so the tube should be changed the first
time the limit is reached.
When measuring the arc-drop voltage,
simply placing an a.c. voltmeter across the tube would produce an erroneous
indication. This is because on one half-cycle, when the tube is conducting,
we would be measuring the true arc-drop value; however, on the other
half-cycle, when the tube is non-conducting, the meter would be subject
to the full applied a.c. voltage. The average of these two readings
would be much higher than the true arc-drop value.
In the Alectric
tester, this difficulty is overcome by inserting the current coil of
a wattmeter in series with the plate of the tube to be checked, while
the voltage coil of the meter is placed across the tube, from plate
to cathode. Since tube current flows only during one half-cycle, the
wattmeter will be affected only during this period. During the subsequent
negative half-cycle, no plate current flows, the current coil is inactive,
and the meter is not actuated. Furthermore, the plate circuit resistance
is so chosen that only 1 ampere flows through the current coil. This
permits the meter scale to be calibrated directly in volts representing
the arc-drop voltage. Additional switches (S3
and S4) and load resistors
(R17 and R18)
permit tubes with plate currents up to 6 amperes to be checked. (Although
6 amperes is the maximum current available, tubes through the 16-ampere
rating are tested at the 6-ampere level.)
Another characteristic of a thyratron is its
critical anode starting voltage. This test is made at a specified control-grid
voltage, usually on the order of 4 volts positive on the grid. The anode
voltage, which is d.c. now, is slowly increased from zero until the
firing point of the tube is reached. This value can be compared to that
given by the manufacturer. Most of the time, the tube-data sheet will
list the anode starting voltage for an average tube of a given type
and also for the tube within the type with the highest starting voltage.
The d.c. voltage required for this test is obtained from selenium
rectifier SR1, resistor
R4 and filter capacitor
C1. For the test, S2
is placed in the d.c. position and the d.c. voltage developed across
C1 is fed, via S1-4
to the anode of the thyratron tube to be tested. (S1-4
is closed for this test.) The voltmeter is placed in the position indicated
by the dotted lines from the anode of the tube, through R37
and R16. Readings are taken
on the low 100-volt scale, since the anode starting voltage seldom exceeds
50 or 60 volts when the control grid is 4 volts positive.
necessary positive grid voltage for the thyratron is obtained from the
network consisting of T4
the two selenium rectifiers connected across T4
R25 and R24.
The control grid is directly connected to the center tap of R24.
The center arm of R-g; goes to the filament of the thyratron via a center
tap on filament transformer T •. When the movable arm of R25,
is exactly at its center position, there will be no difference of potential
between the grid and filament. Turning the arm of R25
in one direction produces a nega\tive grid potential; rotating the arm
in the opposite direction produces a positive grid voltage. This arrangement
is simple and quite effective.
Critical Grid Voltage
To understand the purpose of the next test, that of
determining the critical grid voltage, let us refer to a tube characteristic
chart. This is shown in Fig. 4 for a C3J tube, but it is typical in
form for a wide variety of thyratrons. In the section of the graph labeled
"No Breakdown In This Area," no combination of certain a.c. anode voltages
on the left-hand side with certain negative grid voltages shown at the
bottom will cause the tube to fire. To reach the firing point of a tube,
we must move into the shaded area. Some tubes may have to be driven
deeper into this area (meaning either higher plate voltages or less
negative grid voltages) before they are triggered, but they should fire
before they reach the extreme right-hand edge of the shaded section.
If this does not occur, some defect or variation from normal is indicated
and the tube should be replaced.
Beyond the shaded area and
to the right of it, the control grid is generally so positive that almost
any positive anode voltage will trigger the tube. Operation in this
area is not sought because control of the tube is either difficult,
variable, or impossible.
To test a thyratron tube for its critical grid voltage, high-valued
a.c. anode voltages are required. These are obtained from the secondary
of transformer T3, where
voltages having r.m.s. values to 1500 volts are available. In carrying
out this test, S1-1 and
S1-3 are closed, while S1-2
and S1-4 are open. Switch
S2 is in the a.c. position.
The voltage developed across the secondary winding of T3
reaches the plate of the test thyratron via R5
and R7. R5
is purposely made high in value so that the anode current will be kept
below 40 ma. This is done because there is no desire to check the ability
of the tube to produce its peak current; rather, all we wish to do is
determine its critical grid voltage at a certain anode voltage. By keeping
the current low, it is possible to use a low-wattage, inexpensive resistor
for R5. We obtain the same
value for the critical grid voltage whether a large or a small current
flows after the tube has been triggered.
Figure 4. - Tube characteristic chart for the C3J, a typical
thyratron, shows the range of combinations of grid and anode
voltages that will cause firing, also the combinations of these
voltages that will cause breakdown of the tube.
Now to the test itself.
It could be carried out by fixing the anode voltage at some value and
then slowly reducing the negative grid voltage-making it more positive
- until the tube fires. This would be done by slowly rotating R25
until the neon light in the plate circuit flickered on.
the same characteristic can be determined automatically because of the
presence of C3 and R38.
Initially, R25 is set until
it is 4 volts positive with respect to the filament. The a.c. anode
voltage is then slowly increased until the tube fires. When this happens,
the surge of current through the circuit charges C3,
counteracting the initial +4 volts on the grid. During the next positive
half-cycle of a.c. anode voltage, the tube firing point is governed
by the combined voltage from R25
and C3. This combination,
after a few cycles, attains an equilibrium level which is the critical
grid voltage for that value of applied anode voltage.
now change the anode voltage, by adjusting T1,
then the total grid-to-filament voltage will reestablish itself at another
equilibrium value which will represent the critical grid voltage for
that anode voltage. For example, if the anode voltage is raised, the
voltage across C3 will increase,
effectively making the grid-to-filament potential more negative than
it was before. Conversely, if the anode voltage is lowered, the average
voltage developed across C3
In essence, the network formed by C3,
R38 , and R25
swings the tube's operating point a minute distance above and below
the firing point. The range is governed by the values of these components
(i.e., the overall time constant). Here it is chosen so that the meter
needle recording the critical grid voltage remains quite steady as the
anode voltage passes through its positive and negative half-cycles.
Critical Grid Current
The final characteristic
of the tube to be checked is the critical grid current. This is the
infinitesimal grid current that flows as the critical grid voltage is
approached. It starts at the grid and flows down through R22
when push-button PB7 is
opened. The voltage developed across R22
is negative on the grid side of the resistor and positive on the other
side. Furthermore, this voltage adds to that provided by R25
To see how
this critical current itself is measured, note first that R22
is a 1-megohm resistor. Since the critical grid current is in microamperes,
the value of voltage developed across R22
is equal to the grid current in microamperes. When PB7
is closed, and the system is set up so that the critical grid voltage
is indicated automatically, then the value of the total grid voltage
is revealed by the grid voltmeter. This meter is connected between the
bottom of R22 (and hence
is not affected by any voltage that may develop across R22)
and the center tap of the filament transformer. If, now, PB7
is opened, the critical grid current will flow through R22
and develop several volts here. This will alter the total grid-to-filament
potential and drive the grid more negative. To bring the overall voltage
back to the critical grid value point, the voltage across C3
will decrease by an amount equal to that brought into the circuit by
in C3 voltage will be reflected
in the grid voltmeter reading since the latter, remember, measures both
C3 voltage and that developed
by R25. Thus, the change
in reading on the grid voltmeter when PB7
is depressed represents the critical grid current in microamperes.
This covers the operation of the tester in general and the tests
it performs. Some odds and ends still remain, such as the VR-150 which
is placed in parallel with C3.
This tube serves to protect C3
when switch S8 is first
opened and C3 is being charged
initially. Voltage surges of a thousand volts or more frequently occur
at this time. These would destroy C3
unless the latter had a sufficiently high breakdown voltage value. Since
C3 possesses a high capacitance,
using a unit with a high surge rating would be extremely costly. The
difficulty is solved much more economically by having the VR-150 tube
In the anode voltmeter circuit at the left, four
ranges are obtained using only three push-buttons. With all buttons
open, the voltmeter is on its 2000-volt range. For the 1000-volt range,
the button so marked is depressed. The same is true for the 100- and
200-volt ranges; that is, the desired range is brought in by depressing
the associated button. The circuit is so set up that, when either the
1000- or 2000-volt ranges are in use, depressing the 100- or 200-volt
buttons accidentally will have no effect on the meter.
tester will also check phanotron tubes. These are tubes which are essentially
thyratrons without a control grid. Hence, the tests are considerably
simplified for them. Tests usually include arc-drop voltage, anode starting
voltage, and an interelement short-circuit check.
purpose of any tube tester or analyzer is to permit a decision to be
made on the condition of the tube. It is not a practical matter to construct
a tester like the conventional radio-tube tester where a meter reads
"good" or bad." In a thyratron, there are many factors other than the
tube's ability to conduct a given quantity of current that determine
its acceptance. That is why all of the foregoing tests are provided
for and all should be performed if a true picture of condition is to
September 30, 2013