This tube shown above is from the SMECC
Pages 57-59, cont. Pg 103-105
E. Hunter, WECT of Wimington, NC
sent this article to InfoAge - THANKS!!!
THE SECRET TUBE
THAT CHANGED THE WAR
Today it's junk - a bargain - priced surplus special -
it is also history, the WW II tube no one knew about
By WILLIAM I. ORR, W6SAI
RADIO AMATEUR saw the dull glint of glass in the bottom of the dusty box and
immediately plunged his hand into the receptacle, searching for the unknown
object that caught his attention. Grasping something, he slowly drew forth
a curious, large misshapen radio tube. Holding the dusty object up to the
bare light bulb dangling from a faded sign that read "UR CHOICE -
29c," he examined his find carefully. Puzzled, he turned to the
proprietor. "Hey, Sam! What do you know about this tube? Can I
use it on two meters?"
Sam," owner of the radio junk shop, took the tube and examined it as if it
were a fine jewel. He sighed. "Who knows? Buy it! I don't know
what it is, but you can't go wrong for twenty-nine cents!"
| WHERE SHALL WE start the story of the curious tube?
On a June morning twenty years ago in Normandy? Or before that, at
the Panama Canal, or
years later on the slope of a numbered hill in Korea? It's a strange
tale of a
unique tube, an Army major and American ingenuity-a true story whose ob-
solete residue was finally found by the inquisitive amateur in a surplus
Panama, 1940: America is not yet at war, but it
is obvious to some that we soon will be. The Panama canal is a tempting
and vulnerable target from the air. Radar, the radio eye, had been
invented a few years before, but the only available equipment worked on
the relatively low frequency of 110 megacycles, and then not very well.
The safety of the canal could not be trusted to this primitive,
unsensitive gear which showed an almost complete blindness in detecting
A decision is made to construct a small number
of radically new and powerful radar sets capable of locating and
detecting small planes, and to put these sets aboard picket ships
located in the approaches to the canal. Laboratory ex-
|periments show that a good frequency for the new sets would be 600
megacycles, but no available tubes can produce the required power at
what was then regarded as an unusually high frequency.
By a stroke of fortune of the kind that often
changes history, a radar tube is invented by young Major Harold Zahl of
the Army Signal Corps that can produce the power required. A prototype
of the vital search radar employing the major's radically new tube is to
be secretly built and tested as fast as humanly possible.
On the M.S. Nordic off the New Jersey coast:
The vessel is equipped with the new radar, and testing is going forward.
Suddenly, a German submarine, intent on spying, surfaces close by. It
does not go unnoticed, and as the sub's periscope turns, it sees a
destroyer closing in together with a blimp overhead, both carrying depth
charges. The sub crashdives as the depth charges drop. The new radar and
those aboard the Nordicshaken up by the explosions-are safe. The tests
continue. The search radar can detect a single bomber over one hundred
powerful high-frequency Zahl tube was used in critical radar applications-to
detect low-flying aircraft, and to trace the sources of deadly mortar barrages.
|miles away with the radar antenna mounted only fifteen feet
above the surface of the water!
THE SECRET, revolutionary canal radar
equipment was so successful that the Air Force asked the Signal Corps to
repackage the equipment into a lightassault type radar which could be
airlifted to a battle zone and then hand-carried to the front. A
prototype of the repackaged radar was built in February, 1943. To prove
it was air-transportable, the unit was loaded aboard a. bomber at the
Newark (ICJ.) airport and flown to Florida. It was up and in operation
four hours after it arrived at Orlando.
This cleared the way for a crash program to
construct a small number of the secret radars (by now called the
AN/TPS-3) for immediate shipment to critical war theatres. Twelve sets
were built at Camp Evans Signal Laboratory in New Jersey with the aid of
GI operating crews who later flew into combat with the equipment. The
AN/TPS-3 could be assembled and put on the air by a crew of four men in
thirty minutes. (Continued on page 103)
The AN/TPS-3, known as
"Tipsy Three" is shown below installed in a tent. It was the
first radar set to operate at high power in 600-megacycle range.
The Zahl tube and its inventor, Dr. Harold A. Zahl, now director of
the Army's Research and Development Laboratories, Ft. Monmouth, N.J. The
radically new tube-four triodes in parallel with tuned plate and grid
lines to make it an oscillatormarked a point of departure for modern
tube designs containing resonant circuitry within the tube. Fortunately
for the Allied cause during World War II, the Germans never obtained a
Zahl tube intact, or guessed its secret. It was, without doubt, one of
the factors that won the war and saved countless lives.
|The Secret Tube
(Continued from page 59 )
The first twenty-five production units followed by many more-were
built by Zenith Radio Corporation, and went to England and then to the
beaches of Normandy.
A part of the Normandy radar-support operation
ended in tragedy-and it was feared that the set and its tube had fallen
into enemy hands-when four radarcarrying gliders crashed during the
ill-fated Arnhem expedition. Fortunately for the Allies, the destruction
of the sets was so complete that there was little left for the Germans
THE SUCCESS of the "Tipsy
Three," as it was known to its operators, was due to the secret
tube invented by Major Zahl. Essentially four triode tubes connected in
parallel, the tube envelope also contained tuned plate and grid lines
which made it an oscillator. As much as 250,000 watts peak power could
be extracted' from the tube during a radar pulse. Because of the plate
dissipation and cathode emission required to produce the 250-kilowatt
pulse, the anode elements of the secret Zahl tube ran red hot.
Once the tube had been proven, Major Zahl
brought a hand-made version of his invention to Eitel-McCullough, Inc.,
a pioneer manufacturer of high-frequency transmitting tubes located near
San Francisco. He asked the engineers of the company if the tube could
be massproduced on a crash basis. The entire resources and ingenuity of
the company were thrown into a program of producing Zahl tubes in
quantity, and in secrecy. The production tube also produced in
appreciable quantities by Machlett Laboratories-was designated the
The exact number of VT-158's produced during
the war is no longer known, but it is said that at one time the entire
output of the Tantalum Defense Corporation was being used to make the
heat-resisting elements of the secret tube. Many problems were
encountered in mass-producing the revolutionary
|VT-158, but the tube was soon given the unconditional Joint Army-Navy
(JAN) approval and placed on the "Preferred List."
Doctor Zahl, now the Director of Research at
the Army's Electronics Research and Development Laboratories, Ft.
Monmouth, N.J., wrote recently, "Within my recollection, this tube
passed through its entire life cycle of usage without ever having been
the subject of an unsatisfactory report from the field. Eitel-McCullough
did a superb job in the production-design of this tube. Even now, I
wonder how they did it."
THE TUBES, still unknown to the
public and the enemy, saw action in the Pacific Theatre as well as
Europe. In Doctor Zahl's article, "One Hundred Years of
Research," published in the October, 1960, IRE Transactions on
Military Electronics, he said, "But with all the assistance total
mobilization brought (to the development of new electronic systems)
there were many problem areas where the most learned hesitated to
travel, lest the war be over before the problem could be solved-if it
could be solved at all. Riding high in this category was the location of
enemy mortars, the deadly devices which caused the majority of our
"The problem was one of finding metal
objects the size of a small tomato can, loaded with explosives and fired
at our troops in bursts of hundreds, with nothing more complicated than
a large shotgun shell at the bottom of a piece of iron pipe. Finding
these clouds of deadly torpedo raindrops coming unannounced from miles
away was the first part of the problem; the next was to establish
definitive trajectories, trace the various shell paths back to their
points of origin and by coincidence methods, to saturate these
coordinates with overwhelming counterfire so that peace and quiet would
prevail in these particular areasand many thousands like them!"
With Major General R. B. Colton challenging his
scientists and engineers, and with Captain John Marchetti leading the
design group as he had previously done with the AN/TPS-3, Signal Corps
Research took on the mortar locating problem when much talented advice
said there was no solution. Within six months
|the problem. was solved. Under the personal` urging of 'General
Stilwell to hurry the equipment into emergency overseas freight, Captain
Marchetti's task force of twenty Signal Research scientists worked for
an unbroken stretch of ninety-six hours-to the verge of collapse on the
, first prototype radar unit. The deadly problem of enemy mortars had a
solution-the Zahl tube used in the AN/TPQ-3 mortar radar set.
During the Korean conflict, the Army again
called on the aging Zahl tube and the semi-obsolete AN/TPQ-3 mortar
radar-both resurrected from World War II.
THE ZAHL TUBE is no longer manufactured, but the concept has
not been forgotten. While the once-secret, revolutionary VT-158 may now
be found in dusty surplus bins, work is still being done on powerful new
ultra-high frequency radio tubes that contain the resonant circuitry
within the tube.
One, the new X-841D giant klystron tube,
designed for multi-megawatt, frequency-agile radar, is a modern
descendant of the secret Zahl tube. Using six integral cavities resonant
in the 400megacycle region, this eleven-foot, 1000pound giant is the
latest development in the long, continuing search for more power at
higher frequencies that started
in Panama so many years ago. -®-
Always say you saw it in - POPULAR
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