By Larry Babcock
This paper originally appeared in
The Antique Radio Gazette, Volume 16, Number 4. Permission to present it here has
been granted by the Antique Wireless Association, and they retain all rights to
its distribution and reproduction. Only not-for-profit personnal use is authorized
for any hardcopy printouts of this page. I want to thank Mr. Larry Babcock for the
permission to use his article based on a talk given by the late Ken Conrad. I want
also to thank Mr. Bill Fizette, AWA President and AWA OTB Editor Marc Ellis for
their support of The James Millen Society.
This article is comprehensive with respect to the early National receivers. However,
it should be noted that since this was based on a talk given by Ken Conrad and that
most of the receivers discussed were those being demonstrated at the time. Therefore,
such popular receivers like the FB-7 and the early NC series are not mentioned.
With this in mind the reader will still find valuable information contained in this
paper that is not found elsewhere.
Want more information on National - Check
out The National Page >
The following article was reconstructed by Larry Babcock from a talk given to
the Niagara Frontier Wireless Association, (N.F.W.A.) by Ken Conrad in 1983. Kenny
was a ardent collector and historian of the National Company prior to his death
in the summer of 1988. The National information presented is all from Kenny. The
information was preserved by Larry Babcock for this report.
Back in the 1930's the outstanding commerical radio equipment was made by the
National Company in Malden, Mass. They had an assortment of sets. Most were for
short wave work. They made the SW3, the SW4 and the SW5. The number in the model
number was the number of tubes in the set. They were 3 to 5 tubes and all were regenerative.
They all had one or two stages of R.F. amplification. The SW3 and SW4 had a single
In the early 1930's National also made the AGS which is highly sought by collectors
these days. There were only a few ever made. Less accessories, and that means less
coils, less power supply, less tubes, less speaker, less everything except the box
listed for $180.00. In those days that was a lot of money, more than 1/3 the cost
of an automobile, so you see why very few were ever made and very few amateurs could
ever buy one. Remember in 1932 and 1933 there weren't very many dollars around.
Only a few affluent hams could buy am AGS.
Back in 1933 the state-of-the-art of short wave reception was very poor compared
to nowadays. Most hams had home made two tube regenerative sets with a detector
and one stage of audio. A few owned the National SW3 which also had an R.F. stage
(plus regenerative detector and an audio stage). The most advanced amateur who has
some cash (a few of them did but not me) spent up to $100.00 for a Hammarlund Comet
Pro. A lot of people remember this set but it was not very good.
It didn't have an R.F. stage. It had poor selectivity and a superhet was not suited
for short wave reception in those days. A good regenerative set like the SW3 could
run rings around a superhet with no problem at all! There was a real need for a
good superhet like the AGS to be built.
In an article from CQ magazine written by James Millen of National it was stated
that "It was in this chilly atmosphere that James Millen who was W1HRX and
other engineers of the National Company commenced to design an advanced short wave
superhet". The initial requirement came from the federal government.
The Bureau of Air Commerce which was the predecessor of the present Civil Aeronautics
Authority was in the process of replacing the revolving light beacons used for air
navigation with a comprehensive ground to air radio network. The communications
system was divided into three groups. First, the ground transmitting equipment was
to be developed and manufatured by G.E. The airborne radio equipment developed and
manufactured by Aircraft Radio Corporation who later developed the SCR 274N. The
ground station receivers were designed and manufatured by National. The BAC and
National signed a contract to produce several hundred AGS recievers to replace the
antiquated sets in use at the time. Incidentally, AGS stands for Aviation Ground
W1HRX started to work on the AGS design and became the leader of development. He
didn't have much to go on. His previous experience was the old Browning Drake set
of several years previous along with the SW3 and SW5 of National. The problems to
get a good short wave set in those days were tremendous.
The first AGS receivers were made on a government contract with a few commercial
sets also sold to affluent amateurs. The difference between the contracted ones
and those sold on the open market was just in the name plate.
There were only between 200 and 400 sets sold to hams. That was very few even in
those days and is why there aren't very many around today. The AGS was very shortly
made obsolete by the HRO. (Actually the FB-7
superhet came shortly after the AGS, which was much more affordable and sold in
comparatively good quantities. - JMS). The AGS used a new
crystal filter just being developed. It was a single conversion superhet. It had
one R.F. and two I.F. stages, an AVC, VFO and audio stages. The crystal filter was
available if ordered special. With it the receiver was named the AGSX. Ken Conrad's
AGS shown when it first gave this talk to the NFWA was the original type without
the crystal filter.
The intermediate frequency was 500 KHz. There is no automatic volume control in
this set. None at all. The volume was adjusted only by the bias control potentiometer
for the R.F. and I.F. stages. AVC was applied to the first detector, a scheme frowned
on today as the ever changing control voltage tends to pull the high frequency oscillator
and causes strong signals to flutter but they got around this. In the AGS they injected
the local oscillator signal into the plate circuit of the R.F. amplifier and inductively
coupled the mixing signal into the first signal. That was how it was done.
Some aspects of the design were amazing. How to control the inductance and capacitance
and make the tuned stages all track, that was a real job and to keep it right on
the button! They figured that plug in coils were the way to go rather than having
the coils permanently wired in. The AGS has five sets of three coils each and on
the panel there is a chart which shows the frequency band covered by each set of
This helped the operator pick out the ground station that he wanted to monitor.
The tuning capacitors were hand measured. The tuning range and oscillator tracking
were corrected by moving a tap on the wire connecting the tuning capacitor to the
plug in coil. That was the final adjustment.
Using hand labor for calibration of course is impossible today because of the price
of labor but in those days an engineer got a salary of $20- to $40- per week! Imagine
that. Technicians earned from 20 to 35 cents an hour. This is why each AGS could
be hand calibrated and the chart was right.
The chassis was made of heavy drawn 1/8" think aluminum. The panel is very
heavy steel firmly bolted to the chassis. This made a very rigid and absolutely
tight assembly. The tuned frequency wouldn't change even if you dropped the set
on the floor. The high frequency oscillator is in the middle of the unit and driven
by a disk dial.
This receiver has a very unusual dial. It is the same as was later used on the HRO.
The dial has five openings and is calibrated from zero to 500. Each time that the
dial is turned passed one of the little openings, the scale is automatically changed.
The number viewed through the hole jumps another fifty points! Each one of the line
segment divisions that just cover a band. You can read to one tenth of a division
directly and each division is equivalent to about 1 KHz so you can get down to 100
cycles direct reading. A heck of a lot of the receivers today don't even get near
this. Imagine, this was over 50 years ago! The AGS was designed to work forever.
You turn one on today and it will work immediately. Even if it has not been operated
for fifty years that set will work!
The SW4 and SW5 both had plug in coils. Both were shown when Ken gave this talk
to the NFWA. Incidentally, before his death, Kenny donated his entire (large) collection
of National receivers to the AWA and they are currently on display in the AWA museum.
The SW3 was introduced by National in 1931. It was a low priced set. Because of
its very light weight, it was adapted for use by the airlines.
At that time other aircraft receivers were heavy things and didn't perform to well.
They had poor selectivity and were bothered by the planes ignition system.
Pan American was about to start flights to South America and needed a very light
weight receiver. The only sets available at that time were the National SW4 and
SW5. They were too big, too heavy and another problem was that coils would absorb
moisture when flying over the ocean. This made the tuned frequency wander all over
In about 1930 the old Boonton Rubber Company started making Bakelite coil form moldings
for their sister company the Boonton Radio Company. The molding powder sold by the
original Bakelite Corp. was based on the use of wood filler which was very hydroscopic.
Molding in those days was done with steam, not electric heat and as a consequence
the molding rooms were extremely humid. The coils forms molded by this process varied
tremendously in Q. Good and bad coils looked just alike. You couldn't tell a good
coils by looking at it. To solve this problem the Boonton Rubber Co. changed from
the old filler to ground mica and electrically heated presses. For the first time
this method made uniform inductors. The new material was called R39 and was registered
with this trademark. This new material was used in making the SW3 starting in the
summer of 1931. During the long and useful life of the SW3 over 10,000 sets were
made and used by everybody including Pan Am. It is still a good receiver today.
The SW3 had tremendous range. You could get coils all the way from 9 to 2000 meters!
The SW series sets are collectors items today and they are good ones. They are so
stable that you can read sideband on them with no problem!
National used separate power supplies for many years because of the heating problem.
With the power supply separate the heat from the transformer and rectifier didn't
raise the temperature of the receiver and change the tuned frequency. With everything
in the same box the temperature would not stabilize for along time and the frequency
would drift until then. The power supply provided about 200 volts to the plates.
Manufacture of the SW3 continued for many years. The original had tubes with 2.5
volt filaments. Later they were made with 6 volt tubes and even with metal tubes
before production finally folded.
The little National 110 (1-10) was also shown when Kenny gave his talk to the NFWA at the Buffalo, NY
meeting. The #110 is a four tube super regenerative set. The tuned R.F. amplifier
uses a very tiny type 954 tube. The detector was a type 955 in a self-quenching
super regenerative circuit. It was transformer coupled to the first audi stage,
a 6J5. The 6J5 was then resistance coupled to the 6V6 output stage. This was unusual.
Kenny said that he didn't know the reason why national designed it this way rather
than using transformer coupling for the added gain, but that was the way it was.
The model 110 had six separate pairs of small clear plastic coils. They are extremely
small and so shaped that they can be picked up and plugged in very easily. If you
picked them up by the coil itself it would probably collapse because they are so
small and delicate. This set will get down to 300MHz. It tunes the one to ten meter
band and it will get down to one meter. This was a long time ago too, way back in
the fortys. (I liked the way Kenny said "down to 300 MHz" rather than
up to 300 MHz. Here's a man who still thought in terms of wave length rather than
frequency like us young guys!).
The model 110 had a separate A.C. power supply or could be operated with batteries.
The manual didn't give the price but did have charts giving the frequency coverage
for each set of plug in coils. It also had the same unusual tuning dial that was
first used on the AGS.
The HRO is my favorite receiver. Some of the commercial airlines used the National
AGS, but others wanted a little better receiver. They wanted one for CW that could
also work with the new Western Electric phone transmitters (voice) that were about
to be installed in aircraft.
Now consider the state of the art in 1933 and 1934. The great majority of hams had
home made regenerative sets or SW3's. A superhet was useless, they thought, for
CW and pretty poor on general shortwave. It was considered a great thing to get
anything on a short wave superhet so National started to develop the HRO. It was
a coordinated effort. This receiver as developed on the east coast and the west
coast of the U.S. at the same time! There was 3000 miles separating the mechanical
and electrical teams! Wasn't that crazy? Wow. The west coast HRO team was coordinated
by Herbert Hoover Jr.. James Millen was in charge of the east coast. They were in
The tool makers required a job number for their overtime slips. Since none had been
assigned, they took it upon themselves to use the initials "H" "O"
"R", or "HOR" abbreviated from "Hell of a Rush!" since
that was the state of affairs. Now known as the HOR the new prototype was carried
by Millen to Pasadena for circuit revision. Bugs were worked out. It was a success.
By this time however, some of the participants had doubts about the name. So they
therefore renamed it the HRO. This change was made just in time to catch the first
ad in December 1934 QST.
But I have digressed from my story. Getting back to the design, Herb Hoover, Jr.
set up an experimental laboratory in his garage under the direction of Howard Morgan
of Western Electric. Howard and his technicians went to work developing a circuit
based on experience gained with the AGS and what the airlines were looking for.
First the specifications were established. It had to have superior image rejection
which called for two stages of R.F., not just one. The selectivity would require
a crystal filter be included. Jim Millen had designed what they called a epicyclic
dial so you could read to one part in five hundred. In other words the band spread
on that dial was over 12 feet on a ham band!
The specifications for the HRO also required very good AVC and an S meter. The designers
believed that plug in coils were the answer. They didn't want band switching and
instead planned to have a new four bank plug in coil catacomb. Each coil deck had
its own calibration chart. To reduce hum and heating effects the power supply was
separate. As a whole, the receiver was years ahead of its time. It is one of the
first examples of system designed equipment otherwise unknown before World War II.
The HRO was very outstanding, a classic. It had two R.F. stages, a crystal filter,
two I.F. stages, AVC, BFO and 450 degrees band spread on any amateur band!
There was a system built into the catacombs. Each coil had two little screw holes
with little machine screws in them. If you put the screw in to the hole on the left
you get general coverage. When you remove the other one you get just the ham band
over the whole dial. It was simple. The screws just shorted a condenser or opened
one across part of the coil. But it worked and it gave trememdous band spread!
At first there was a lot of drift but in working on the problem they found out that
most of the drift was in the oscillator tube and they were able to eliminate the
problem. The HRO was so beautifully made that it would be impossible to build today.
It would cost hundreds of dollars just to make the coil forms, the tuning condensers
and the insulators. There was so much hand work in both the mechanical and electrical
One of the early owners of the HRO was Charlie Kolster. He was so impressed with
the set that he suggested that Jim Millen replace his old two district call sign
with a first district and have 1HRO in honor of the set. But the FCC goofed and
he ended up with 1HRX (I believe that the "W"
letter prefix was already in existance at this time so these calls should be preceded
with that letter - JMS.) That was how the FCC did things.
In spite of the call sign mix up, the set was an instant success.
Braniff Airlines bought a lot of HRO receivers. The set was so far ahead of time
that they found it could be operated unattended. They installed a lot of HRO receivers
on telephone poles spaced 50 miles apart. That way they could get the coverage to
communicate with airplanes all along the route. At that time there was no radar,
or beacons or anything. So they mounted HRO's in boxes on poles every 50 miles along
the main route of the aircraft. These receivers ran continuously 24 hours a day!
They were tuned to thirty one hundred and five kilocycles (KHz) which was the aircraft
The receiever outputs were patched into land lines and could be monitored in Kansas
City, Dallas, Tulsa, Oklahoma City or anywhere. They had continuous voice monitor
of the planes all along the route. That's where the HRO got the big impetus. Also,
the Howard Hughes flight around the world used the HRO. He had two at each ground
station. That is what he thought of the HRO!
The HRO was so designed that with the crystal filter, now this is almost unbelievable,
you could get down to 20 cycles! (Hz) That's the selectivity of the HRO., twenty
cycles! (Not kilocycles) That is better than practically any set made today, and
this was over fifty years ago.
The HRO remained popular until after World War II. By then SSB was coming in and
the HRO was finally on the downgrade. That is the story of the HRO. It was a wonderful
set. I don't care what they make today, what the price is or anything, they don't
make them to have the design, the selectivity and the durability! After fifty years
I can turn one on and she'll still go. They are worth the time and effort to restore.
Ken did anyone ever make a rack mounted HRO like your AGS?
Yes, And they were provided with a door to slide the coils in and out of the set.
or about National Products >