Aperiodic surveillance receiver
SSR-201 – nicknamed watchdog – was an
aperiodic receiver, also known
as a crystal receiver or non-selective receiver,
developed during WWII (~1942) by
James Veatch and William Hoffert of the
Radio Intelligence Division (RID) of the
United States Federal Communications Commission (FCC),
and manufactured by Kann Manufacturing Co. in Baltimore (Maryland, USA)
for use by the Office of Strategic Services (OSS)
It was first used by RID
and OSS for counter-espionage
The device measures 43 x 25 x 13 cm, weights 8.4 kg and has a leather grip
at the top. It is housed in a professional metal cabinet, finished in black
wrinkle-paint, that has the appearance of a regular civil (domestic)
amplifier of the era.
The receiver is non-selective, or aperiodic, which means that it
receives the entire radio band for which it was designed,
at once (0.05 to 60 MHz).
By design, aperiodic receivers are insensitive and should be placed in
the proximity of a suspected transmitter (100 - 500 metres).
The SSR-201 is suitable for detecting AM, R/T and CW signals.
Aperiodic receivers are a great help when the operating frequency of a potential
clandestine radio station is unknown, as it will detect any transmission in its
vicinity. Once a transmission has been detected, it can be monitored, whilst
a regular receiver is used to determine the exact frequency.
The device shown here was used after WWII in The Netherlands,
by the Bijzondere Radio Dienst (BRD) (Special Radio Service) 1 —
the agency that was tasked with finding illegal transmitters.
Its last serious operational task was
during the 1950s, when BRD-officer Daan Neuteboom used it to monitor
a suspected Russian vessel – that had docked a Dutch harbour –
for potential clandestine activity (espionage).
A variant of the receiver – known as the K-series 2 – was housed in a brown wooden
enclosure, but is otherwise nearly identical.
An estimated 200+ units were made .
All controls of the SSR-201 are at the front panel, as shown in the diagram
below. The device is fully self-contained and has a built-in speaker.
It should be powered from the 110V AC mains, but can also be
– for example from a car battery – using an (optional) vibrator pack.
Antenna and ground wires should be connected to the
at the upper right.
All controls are arranged along the lower edge.
The volume control – at the centre – acts as the main power switch.
It switches the LT voltage (6.3V) for the filaments of the valves.
Approx. 15 seconds later, the HT voltage for the anodes of the valves should
be switched on with the plate volts switch at the left.
Full setup and alignment instructions can be found in the
Two versions of the device are known: (1) the original SSR-201, housed
in a metal black wrinkle paint enclosure, and
(2) the so-called K-series that was housed
in a wooden enclosure. The one shown here has serial number 45 and is currently
the only known surviving one of this version.
It has two hinges and locks for fitting a metal cover (that is missing
from the device shown here).
At the start of World War II, in 1939, the US Government knew
it was lacking vital intelligence from intercepted radio traffic, in particular
between Germany and a number of South American countries. Furthermore they
were aware of the dramatic rise in clandestine radio activity by Nazi
spies all over Europe. It prompted US President Franklin D. Roosevelt
to allocate a US$ 1.6 million budget for the foundation of a
National Defense Operations section (NDO) within the FCC
The FCC recruited new personnel for the NDO – mainly from radio amateur ranks – and began setting up fixed and moble monitoring and intercept stations
throughout the country, from September 1940 onwards. Later that year, the NDO
was renamed Radio Intelligence Division (RID) and became a full law-enforcement
agency, under the guidance of FCC inspector George Sterling.
By the time the US entered WWII – after the attack on Pearl Harbor –
the FCC was fully prepared for war and had monitoring stations at strategic
locations in and around the United States. As the FCC had neither the budget
nor the personnel to provide continuous surveillance of clandestine activities,
it was decided to develop a receiver that would automatically respond to
any signal in its vicinity – regardless the frequency – and make it available
to the OSS,
the FBI and the Police.
From the requirements, two RID engineers
– James Veatch and William Hoffert, assisted by William Fellows –
developed the aperiodic receiver.
RID was the first to use the receiver for locating illegal
transmitters, soon followed by the OSS
— the forerunner of the CIA — and the US Navy.
In 1947, Veatch and Hoffert registered it as
US Patent 2,513,384,
and SSR-201 receivers found their way to sister
organisations in liberated Europe, like in the Netherlands, where our
SSR-201 was used for several years by the newly established
Special Radio Service (BRD) to monitor illegal activity.
➤ Further information by Dan Flanagan (off-site)
At the beginning of WWII, most US manufacturers were loaded with defense
contracts and were not able to handle (or interested in) orders for small
production quantities. Consequently, the FCC had to find another way to get
the SSR-201 manufactured, and finally managed to persuade Mr. Manuel Kann
— a radio ham from Baltimore with call sign W3ZK — to work from his
For the remainder of the war, Kann worked day and night to produce
parts for the FCC's Adcock direction finders, and manufacture the SSR-201 receivers
— all from the basement of his home and assisted by engineers from
local radio stations . After the war, he teamed up with Charles Ellert
– a former FCC man – and opened the Kann-Ellert electronics
store in Baltimore City.
➤ Further information by Brian Harrison 
Below is the block diagram of the SSR-201. At the far left – directly connected
to the antenna – is a grid-leak detector built around a 1G4, followed by a
DC-amplifier that feeds a vaccum tube bridge – also known as a VT bridge or
differential amplifier – built around two 6G6 valves.
The upper arm of this amplifier (active) is fed to the output amplifier (6V6)
that drives the speaker.
In combination with the upper arm, the lower arm (inactive) serves two
purposes: (1) it drives an optical electron ray tube indicator (magic eye)
— used here as a field strength indicator —
and (2) it drives a gate circuit that passes the tone from
an AF oscillator to the output amplifier, allowing CW signals to be
heared through the speaker. At the far right is a detector/amplifier
which drives a relay. It can be used to start/stop a recording device,
such as a tape recorder or an undulator.
As the original circuit diagram
— printed in the K-series manual —
is of poor quality, we have recreated it below.
Please note that there are small differences between the original SSR-201
and the K-series.
At the top is the
110V mains power supply unit (PSU), which consists of a mains transformer with
three secondary windings, from which the LT (6.3V AC) and HT (420V DC)
voltages are derived. These voltages are routed via an octal socket at the
rear of the device, to allow it to be powered by an external vibrator pack.
This can be useful in a mobile environment.
In normal operation, two loop wires must be present in the octal socket
(here shown in red). The HT line is first filtered, and then converted to
several stable voltages, using a VR105 and a VR150 stabilizer.
Some of these voltages are critical and may have to be readjusted
after several years.
SSR-201 circuit diagram
A the bottom left is a grid leak detector (1G4) that is connected
directly to a wire antenna. The output of the detector is passed via a
DC amplifier (6SQ7) to a so-called vacuum tube bridge, or VT bridge,
consisting of two 6G6 valves. The leftmost 6G6 is the active arm of the
bridge, whilst the right one is the inactive arm. The tuning indicator
(6U5) is connected between the outputs of the two arms of the bridge.
Sound from an A3 (AM) transmission is taken from the active arm.
At the top right is a sound oscillator (6SL7) that can be used to make
CW (morse) signals audible, or for detection of a nearby RF signal in an
unmanned setup. The oscillator is enabled with S2, and its signal is
passed via a gate (6J5) and a transformer (T1) onto the audio line.
The sound is finally amplified in the output stage (6V6) and passed to
the loudspeaker at the bottom right.
An additional circuit consisting of a detector and a DC amplifier (6SL7)
takes the audio output signal and uses it to drive a relay, which
can be used for starting/stopping an external recording device.
Although the later K-series version is based on the same circuit, there
are small differences, particularly in the selection of valves and a
number of resistor values. These might be later improvements, but may
also be related to wartime shortages of the initially selected valves.
K-series circuit diagram
In this version, the DC amplifier that sits at the output of the grid
leak detector, is built around a 6F5, whilst a 6C5 is used for the
detector/gate circuit. Furthermore, the tone oscillator is built around
a 6SC7 rather than a 6SL7. As a result, the wiring to the valve sockets
is different as well.
The SSR-201 is housed in a metal case that measures 43 x 25 x 13 cm
and weights 8.4 kg. All parts are mounted on a metal frame that is fitted
to the front panel.
The interior can be accessed by removing 14 screws from
the edges of the front panel, after which the frame can be extracted.
The image on the right shows the front with the frame, after its removal
from the enclosure. The construction is very similar to regular domestic audio
amplifiers of the era, and the components are all taken from
non-military (civil) production.
The chassis accomodates a large 110V AC mains transformer (the large black part
at the corner), a selection of thermionic valves,
or vacuum tubes, and several electrolytic capacitors.
Mounted at the front panel (aside the speaker),
is an Electron Ray Tube
– also known as a Magic Eye
or Tuning Eye – that is used a a signal strength indicator.
All passive components are located in the
bottom section of the chassis,
that is protruded by the valve sockets.
Unlike in domestic equipment – where most components were soldered
directly to the valve sockets – the parts inside the SSR-201 are neatly mounted
on separate pertinax circuit boards that are wired to the valves sockets.
Note that several resistors are adjustable. Also note that some parts
have been replaced by modern ones, probably at an earlier restoration attempt.
When we obtained our SSR-201 — in March 2020  — it was in non-working
condition. In fact, the device had never been switched on again after it
was decommissioned in the 1960s. The cap of the fuse holder had been removed
and the mains cord was locked inside the case, probably to prevent the device
from being accidentally plugged into the 220V mains in continental Europe.
As the rubber mains cable had become brittle over the years, it was decided
to replace it with a similarly-looking modern neoprene alternative, so that it
could be connected safely to a power source. As the fuse holder was incomplete,
it was swapped for a similar one of the same size.
After gradually applying a mains voltage – using a VARIAC – it became clear that
one of the large capacitors (3 x 8µF) was rapidly becoming hot and caused the
receiver to draw more current that it should. As an intermediate solution it
was decided to replace it with modern alternatives.
As one of the rotary switches at the front panel had been replaced by a
toggle switch during an earlier repair, it was decided to replace it by a
suitable rotary switch again, and find a matching knob. The result is shown
in the image above, with the removed toggle switch laying in front.
After this, the device was powered up again and the mains current was observed.
This time it stayed within the specified limits and no further anomalies
were spotted. Next, a first test was carried out
by applying an AM signal of sufficient strength from an RF
directly to the antenna/ground terminals at the front panel.
It worked, but it appeared difficult to find a
proper balance of the bridge. Furthermore, the magic eye was
always fully open or fully closed. After checking the bridge circuit it
was found that one of the 10k anode resistors was broken.
After replacing the 10k anode restors
(both, as a safety measure),
the bridge worked again and the magic eye produced a proper indication
of the signal strength. The WIMA paper capacitors – that had not been
replaced as part of the earlier repair – were swapped for modern alternatives.
As the coarse balance potentiometer had to be set almost to its rightmost
position for a proper operation, the balance voltage was readjusted with the
potentiometer at the rear of the device.
As the audio output level was rather low, the circuit around the the gate valve
(6J5 or 6C5) was checked. It turned out that the modulation transformer (T1)
was broken and had become high-impedant at the primary side.
Testing the device with a temporary replacement, confirmed
that it was indeed broken. This incident might be
related to the defective 10k anode resistor (6G6).
The audio transformer has a 1:3 ratio and was made by the
Standard Transformer Corporation (STANCOR) in the US. Although such parts
are extremely rare these days, a NOS replacement of exactly the same type
— A-63-C — was found on eBay. Several weeks later it arrived and was fitted
in the position of the old transformer. The receiver is now fully
In the process of restoration, the following work has been carried out:
- Front panel damages repaired
- Rubber feet replaced
- Phase potentiometer repaired (was binding)
- Mains power cord replaced (was brittle)
- Fuse holder replaced (was partly missing)
- Toggle switch replaced by rotary switch (oscillator)
- Electrolytic capacitor (3 x 8µF) replaced
- 10k resistors to the 6G6 anodes replaced
- WIMA paper capacitors replaced
- Modulation transformer replaced
This video was taken several years ago by Steve Ellington (N4LQ),
and shows the SSR-201 with serial number K70 — at the time part of the
collection of Brian Harisson (KN4R) — in full working order.
Footage via YouTube (retrieved April 2020) .
Other known aperiodic CIA receivers
Sensitivity> 10 mV (2 mV on optical indicator)
FrequencyWideband, non-selective, 50 kHz to > 60 MHz [A]
Supply110V AC, or 6V DC (via optional external vibrator unit)
Dimensions430 x 250 x 130 mm
SSR-2011G4, 6SQ7, 6J5, 6G6 (2x), 6SL7 (2x), 6V6, 6U5, 5Z3, VR105, VR150
K-series1G4, 6F5, 6C5, 6G6 (2x), 6SL7, 6SC7, 6V6, 6U5, 5Z3, VR105, VR150
OSSCrypto Museum 1
FCCAntique Wireless Museum (AWA)
Collector Brian Harisson ➤ Video
Formerly in the collection of Museum Jan Corver .
Copy of original manual kindly provided by Brian Harrison KN4R .
- Cor Moerman, SSR-201 aperiodic receiver, S/N 45 - THANKS !
Museum Jan Corver. Received March 2020.
- Louis Meulstee, Aperiodic Receiver type SSR-201
Wireless for the Warrior - Volume 4
- Louis Meulstee, Aperiodic Receiver type SSR-201
Wireless for the Warrior - Volume 4 - Supplement, Chapter 1.
Version 1.02, April 2015.
- Brian Harrison KN4R, The SSR-201 'Watch-dog' Aperiodic Receiver
Electric Radio Magazine, August 2014. 1
- Dan Flanagan W3DF, History of the Original W3DF
4 September 2016. Retrieved March 2020.
- George E. Sterling, The History of the Radio Intelligence Division
Before and During WWII, 1940-1945
— Edited by Dan Flanagan .
- CIA Memorandum, DF Equipment - FCC and FBI
4 January 1952. Partially approved for release 13 September 2012.
- Steve Ellington (N4LQ), WWII Spy Locator Receiver - Aperiodic
YouTube, 11 May 2014.
- George E. Sterling, The Hunt for Axis Agent Radios 2
CIA, Studies in Intelligence, Volume 3, No. 2. Spring 1960.
Article reproduced here by kind permission from the author.
Originally published in August 2014 in
Electric Radio Magazine.
Approved for release on 17 December 2004.
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© Crypto Museum. Created: Thursday 19 March 2020. Last changed: Monday, 14 March 2022 - 12:16 CET.