The scrambler was housed in a wooden or metal
enclosure that was usually placed under the table or in an adjacent room.
The voice terminal was placed on the desk of the calling
parties. It came in several flavours, but was usually based
on an adapted standard black 394 or 396 telephone,
modified with a
164 handset in Jade Green.
Due to wartime shortages, black handsets were sometimes
painted lime green as an alternative.
The image on the right shows a typical
SA5063 voice terminal
with green 164 handset, that was used with the Scrambler from 1943
onwards [1].
During World War II,
many scrambler systems were installed to prevent
accidental or intentional eavesdropping. Initially, the public
telephone network was used for this, but the War Office later
established its own private network, that was completely independent
from public exchanges. 3
The WWII image on the right shows British Prime Minister
Winston Churchill
behind his desk at the Cabinet War Rooms4 just 10 feet below street level
under the New Public Offices 5 at Whitehall. To his left is
Royal Navy Captain Richard Pim who uses the Scrambler Phone on Churchill's desk.
In 1938, after a survey by the Office of Works, this building was thought
to be suitable for use as a temporary office in the event of war.
It was hastily converted into a reinforced 6 temporary command center and
became operational in August 1939, just before the outbreak of
WWII.
The Frequency Changer (scrambler) was developed in 1939 at the
Post Office Research Station at Dollis Hill (North-West London).
It was first deployed
at home (in the UK) and in the field (abroad) in early 1940, with modified
4-wire variants being available for use over radio, and tropicalised
versions for areas with high humidity. During the war, most
units were manufactured at the GPO's Holloway factory in North-London
(manufacturer code FH).
After the war, production was moved to other manufacturers, including TMC
in London – who marketed the system as Secraphone.
Green handsets were used to distinguise secured lines from regular ones,
especially during the war.
In the 1960s telephone sets with no distinct colour were used.
In GPO terminology, the voice terminal is commonly known a the
telephone instrument.
Most of the War Office's private telephone networks were of the
Local Battery (LB) type and were manually switched, which is why
the vast majority of voice terminals does not have a dial.
The Cabinet War Rooms — today known as the Churchill War Rooms —
are now part of the Imperial War Museum
(IWM) in London (UK) and are open to the public. It also houses the Churchill
Museum [16].
This building now houses the Treasury.
Although this 'bunker' was reinforced several times during the war,
it hardly offered any real protection against a direct bomb hit.
The diagram below shows how the scrambler
works. At the right is
the 2 or 4-wire line to the exchange or to a radio. At the left
is the voice terminal, which can be an individual microphone and
speaker, a handset or a telephone. The audio signal from the
microphone is first attenuated to reduce its dynamic range.
It then passes a low-pass filter, so that
only the 20-2000 Hz part of the spectrum is fed to a ring mixer,
where it is added to the 2500 Hz signal from an oscillator.
At the output of the mixer, the sum
and the difference
of the two signals are available, with the difference being the
mirrored version of the original signal (here shown in red).
This means that low-frequency tones have become high-frequency
tones and vice versa.
After filtering it again in a low-pass filter, only the mirrored
signal is left, which is then amplified and delivered to the line.
The bottom half of the diagram shows the reception path, which is
more or less the same, but in reverse direction. The mixer
produces two images again, of which the lower one is the mirrored
version of the received signal. After filtering, the original
audio signal is recoverd, which is then amplified and delivered
to the speaker. The spectrum diagrams should illustrate what happens.
Although the Frequency Changer – actually a frequency inverter – offered
reasonable protection against (un)intentional eavesdropping,
such as the exchange operator or a service engineer working on the
lines, it was no match for a professional interceptor. All one had to
do, was find the inversion frequency and mirror the spectrum again.
A classical case of security by obscurity.
The diagrams below show the different configurations in which the scrambler
could be used. In all cases 1 , the push-buttons on top of the telephone set
(i.e. the voice terminal) are configured in such a way that in SECRET mode,
the microphone and speaker are wired directly to the scrambler.
The diagram above shows the most common configuration, in which a regular
(switched) 2-wire subscriber line is used to connect the two parties. In this
situation, the fork circuit, or hybrid, inside the scrambler unit
combines the transmit and receive circuits into a single 2-wire line.
In addition, when 4-wire leased lines are available, it is possible to
avoid the use of a fork circuit and use two separate 2-wire lines for
transmit and receive. Although this solution provides the
best possible audio quality, 4-wire switched lines were hardly used as they
were very expensive.
It was also possible to use the scrambler system over wireless (radio) links.
In that case, the units were used in 4-wire configuration, so that the
voice circuits could be wired directly to the transmitter (TX) and receiver
(RX). The diagram above shows a full-duplex scrambled radio link.
With some versions of the scrambler it was possible to connect a standard
(2-wire) telephone set instead of the (4-wire) handset by using an extra
fork circuit, but in practice this was rarely done.
Voice terminals
Below is an overview of the various (converted) telephone sets that
were used in combination with the Frequency Changers (scramblers).
Note that many variations exist, often caused by shortages during
the war. In the overview below, we are heavily drawing on the information
supplied by Andy Grant in the UK, most of which was published
in an article in Telecommunications Heritage Journal in the Summer
of 2017 [1]. Many thanks to Andy for permission to use this information.
In the text below, Telephone Set Type Number is abbreviated to Tele.No.
The chassis of a Tele.No. 396 was sometimes used as a replacement for the 394.
Telephone No. 740 was used as an alternative to the 710.
Telephone No. 162
— wanted
The first scrambler sets that were released in early 1940, used a
green bakelite Tele.No.162 as the voice terminal for scrambled
conversations. The call was initiated with a standard black
telephone set of the No. 328 range,
that was also connected to the Frequency Changer [1].
Introduced in August 1940 for use on CB/Auto installations.
It consists of a Tele. No. 328, fitted with a Key No. 303A
switch assembly, a 6-wire line cord ending in a BT20/8
connection box, or Block Terminal, and
a green Handset No. 164.
The set was usually fitted
with two push-buttons – in front of the handset –
labelled SECRET and NORMAL (later: SCRAMBLE and NORMAL).
Introduced August 1940 for use on LB
(CBS 1, 2 & 3) and
Magneto systems.
It consists of a Tele. No. 394, fitted with a Key No. 303A switch
asssembly, an 8-wire line cord, a BT20/8 connection box,
and green handset 164.
It is fitted with two push-buttons, labelled
SECRET and NORMAL.
Introduced in early 1943 for use on LB
(CBS 2 & 3) and
Magneto systems, and on for use on long lines with
CB/Auto systems.
It consists of a Tele. No. 394 with a Key No. 303A switch assembly,
a 12-wire line cord ending in a BT No.6 connection box,
and a No. 164 Jade Green handset [1].
Introduced in February 1944 for use on LB
(CBS 2 & 3) and
Magneto systems, and on for use with long lines on
CB/Auto systems.
It consisted of a modified Tele.No.394, fitted with a
Key No.303B switch assembly, a 12-wire cord ending in a BT No.6
connection box, and a green 164 handset.
In 1964, two years after the introduction of the transistorised
Privacy Set No. 8, the GPO gradually switched from the bakelite
SA-5030
(based on the 300-series)
to modified sets from the 700-series,
which had a plastic body.
The first model was based on Telephone No. 710 (the successor to the 706),
and had provisions for up to four push-buttons. Two of these buttons
were used to switch between clear and scrambled speech.
In 1968, Telephone No. 740 was introduced as the successor to the 710.
It has a slightly modified design and an improved electric circuit, but is
otherwise nearly identical.
Note the unusual combination of a black body and a brown handset, which was
probably done to indicate that this device was used for a special
(i.e. non-standard) purpose.
There were several versions of the scrambler, depending on the application
and on advances in technology. During WWII, all scrambler mainframes
were known as Frequency Changer, whilst after the war, from 1957
onwards, the name Privacy Set was most commonly used.
In addition, the names Privacy Unit and Secrecy Unit
also turn up in literature every now and then. The British
manufacturer TMC marketed the equipment under the name
Secraphone[1].
Parts of the chassis carry a (potentially lethal) live voltage.
Uses a vibrator pack.
Equivalent to the EL32.
Uses Germanium PNP transistors instead of valves.
Frequency Changer No. 6
— wanted
Introduced in June 1940, for use on CB/Auto installations.
The unit was housed in a wooden case and operated on 200-250V
AC or DC mains power. It is built around CV1732 valves (tubes),
which are 5-pin triodes designed for signal processing (equivalent
to ML4 and VT90/VT129).
No. 6 was short-lived and was succeeded within months
by the improved 6A and 6B versions,
as it had several design issues.
This improved version was introduced in August 1940 for use on
CB/Auto
or LB installations. It was suitable for the
200-250V AC mains only and had an isolated power transformer,
which made it much safer for maintenance engineers.
Like the No. 6 it is built around CV1732 valves.
Introduced in September 1940, this version was nearly identical
to the 6A, but was suitable for the 200-250V AC or DC mains
and had therefore a live voltage-carrying chassis (like the No. 6).
This version was intended for use in areas that still had a Direct
Current (DC) mains power network, with the ability to run from
Alternating Current (AC), once the local mains network
had been converted.
Introduced in 1942 for use on CB/Auto and LB installations.
Unlike its predecessors, it was built around the
CV1052 signal processing valve;
an octal-based penthode equivalent to the EL32.
It came in two flavours: 6AA/0 and 6AA/1, differing only in their
power supplies.
The 6AA/0 was suitable for the 200-250V AC mains, which was used
in most parts of the UK.
The 6AA/1 was more flexible and
could be powered from the 100-110V AC or 200-250V AC mains.
This made the 6AA/1 suitable for use in areas that were still on 110V AC.
Introduced in January 1944, this unit is similar to
the 6AA, but had a more flexible power supply, that could be configured
for the 100-110V AC or 200-250V AC mains, but also for 12V DC.
The 12V DC input uses a vibrator circuit, and is
intended for situations where the mains power is failing or
unavailable. At least three variants of this model were made,
indentified by a numeric suffix to the model number (e.g. 6AC/3).
Introduced in November 1957 for use on CB/ Auto and LB systems.
It is housed in a metal enclosure and was known as Privacy Set
rather than Frequency Changer.
This version was built around VC138 penthode valves — equivalent
to the EF91 — and offers improved performance and security.
It has a modular construction, which made it much more service
friendly in case of a fault or repair.
Privacy Set No. 8 was the first model that was fully transistorised.
It first appears in GPO diagrams from 1962 and was designed for use on
CB/Auto systems only. Like Privacy Set 7, it was also available from
commercial parties. It is arguably the most wide-spread post-war model.
Models 8, 9 and 9A are basically the same device with small production
and circuit changes.
The SA5050 was a relay unit,
that was introduced in March 1944.
It allows up to three SA5063/0 voice terminals, or up to three
sets of SA5063/1 terminals wired in parallel, to be connected
to a single Frequency Changer. Depressing the ENGAGE FOR SECRET
(HOLD SCRAMBLER) on one of the SA5063 terminals, causes the
SA5050 to disable the other terminals for the duration of the call.
Indicator
401CN
In order to inform the other users on the internal network that
the Frequency Changer was in use by someone else and was
therefore unavailable to you, Indicators No. 401CN
were mounted in the vicinity of the SA5063/1 voice terminals.
No. 401CN is a so-called dolls eye status indicator [1].
Connection box BT20/8
...
Connection box BT No. 6
...
Secraphone
From 1957 onwards, the Scrambler was no longer known as Frequency
Changer but as Privacy Set. Also, from the early 1950s
onwards, the devices were no longer made by the GPO but by external
parties, such as TMC who marketed the system as Secraphone.
The image above shows a Privacy Set No. 8 with a transparent voice terminal
that was made for demonstration purposes. In SECRET-mode, the microphone
and speaker of the handset are routed directly to the Privacy Set, via
a 6- or 7-wire cable.
The image below shows a TMC advert for the Secraphone [7].
Genuine GPO No. 394 telephone sets that have been used as part of a
Secraphone secure speech setup, are extremely rare and are very difficult
to find. Many collectors have created a mockup by taking an existing
multi-button 300, swapping the text shield and adding a green handset
to it. If you are a collector and are searching the internet for
genuine 394 scrambler phones, there are a few pitfalls to watch out for.
Here are some examples of phones that have turned up in the past.
In June 2013, the telephone set shown in the image on the right
turned up for sale at Giddings Auctioneers in Leicester (UK).
According to the description it is a GPO Telephone No. 394,
which had been used as part of a scrambler telephone system,
in combination with Security Unit No. 8.
Although this set looks complete and in good condition, we have some
reservations about its authenticity. There are strong indications that
the set has been put together from spare parts and/or from parts from
other telephone sets. First of all, the unit has a dial and, although it
is technically possible, this was very uncommon for terminals that were
used with the Scrambler.
Secondly, and more seriously, the two buttons, marked NORMAL and SECRET,
are labelled the wrong way around. Although various designs of the text
label are known, the SECURE button is always on the left and the CLEAR
button on the right. This phone therefore could be a fake.
Another example of a non-authentic scrambler phone is shown in the image
on the right. The item appeared on eBay in February 2016, but in this case,
the seller made it very clear in her description, that it was a
reproduction [13].
According to the description, it consists of a Telephone set No. 328L
of 1962 vintage, with a green reproduction handset that was added at
a later date.
The scramble/normal plate is genuine and was given to the seller in 1981
when she was an apprentice at GPO/BT,
by a former fitter who used to work in the Houses of Parliament.
On both the above examples, the dials may seem out-of-place, but are
necessary if you wanted to operate the Scrambler via a local PABX,
like in the second case. If the dials were replaced by a genuine blanking
panel No. 3, they would look excellent in any WWII Scrambler Phone display.
Video
An excellent documentary about Churchill's Cabinet War Rooms is available
on YouTube via the link below. It features a combination of archive film
material from the Imperial War Museum's vast collection, and atmospheric
dramatisations filmed inside the actual Cabinet War Rooms.
Frequency Changer 6AC, Privacy Set 7, 8/8A and 9/9A.
Privacy Set 8.
Privacy Set 8/8A and 9/9A, albeit possibly with different dimensions.
In the mid-1960s, TMC was taken over by Pye in Cambridge. In 1967,
Pye was taken over by Philips.
Connections
BT20/8 Block Terminal
Red
Microphone/Speaker (common)
Blue
LINE B (telephone line)
Green
LINE B (Privacy Set)
White
Microphone
Orange
Speaker
Black
Line A
Grey 1
Recall (to PABX when present)
Brown
Ground
The Grey wire was usually identified as SL (Slate).
Nomenclature
The Frequency Changer was known under the following (informal) names:
Scrambler Phone
Frequency Changer
Privacy Set
Privacy Equipment No. 1
Privacy Unit
Secrecy Unit
Secraphone
Glossary
Expressions and abbreviations used on this page:
AC
Alternating Current
Type of current used for most mains networks in the world, typically
with an alternating frequency of 50 or 60 Hz. During WWII, most (but
not all) of the UK had an AC mains network with a voltage of 110 or
230V. Some regions remained on DC for several years.
BT
(1) British Telecom
Arguably the largest telecom operator of the UK. Previously state-owned
and known as the General Post Office (GPO)
or the British Post Office (BPO).
BT
(2) Block Terminal GPO expression for the connection box between a telephone set and the line.
CB
Central Battery system
System in which all the energy needed for transmission and signalling
is delivered by the exchange. No local batteries or hand generators are
used at the telephone end.
➤ More (off-site)
CBS
Central Battery Signalling system
Similar to a CB system, except that the mircophone is powered locally
by a battery at the telephone end. Power for signalling is provided by the
exchange as in a CB system. In the UK there were three types of CBS.
➤ More (off-site)
DC
Direct Current
Type of current typically used in cars, but not on the mains network.
During WWII, parts of the UK still had a DC mains network, whilst
the majority was on AC.
GPO
General Post Office
The state-owned post and telecommunications operator in the UK, before it
was renamed BT and privitised. The GPO was als known as British Post
Office (BPO) and simply as Post Office (PO).
It is currently known as British Telecom (BT).
➤ More
LB
Local battery system
System in which a local battery is used for providing the current for
the speech circuits.
Magneto
Hand-cranked electrical generator that provides electricity for signalling
in an (old) telephone system. In some countries known as inductor,
crank ringer, or wake-up unit.
Private Branche Exchange
Local telephone exchange or switching system, using inside the building
of a private organisation, usually connected to the Public Switched
Telephone Network (PSTN) via Central Office (CO) lines.
PL
Plessey
Manufacturing code used on the body of the telephone sets and also
inside, often stamped on the chassis. The manufacturer's code is also
cast inside the bakelite body.
TE
TMC (see below)
Manufacturing code used on the body of the telephone sets and also
inside, often stamped on the chassis. The manufacturer's code is also
cast inside the bakelite body.
➤ More
TMC
Telephone Manufacturing Company
British telephone manufacturer. Also one of the manufacturers
of the Privacy Sets.
Based in St. Mary Cray (Kent, UK) and London [7].
➤ More
Pandora Blake, GPO Black Bakelite 300 Type Phone with Scrambler button and Green Handset
eBay seller phased_001, item 301880390186. Retrieved February 2016.
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Crypto Museum. Created: Tuesday 28 January 2014. Last changed: Wednesday, 23 April 2025 - 10:02 CET.
![Churchill in the Cabinet War Rooms during WWII, with a staff member operating his Scrambler Phone. Copyright IWM [4]. Click to enlarge.](img/churchill_col1_large.jpg)
![Block diagram of the Frequency Changer. Click to see the original drawing [1].](files/ps_block.jpg)
![Example of an SA 5031 voice terminal. Photograph kindly supplied by Andy Grant [1].](https://www.cryptomuseum.com/img/sa5031.jpg)
![Telephone No. 710. Copyright Gildings Auctioneers [10].](img/phone710.jpg)
![Relay unit SA5050. Photograph via Andy Grant [1].](https://www.cryptomuseum.com/img/sa5050_small.jpg)
![Advert for the Secraphone by Telephone Manufacturing Co Ltd. (TMC). Via Sam Hallas [6].](https://www.cryptomuseum.com/img/secraphone_adv.jpg)
![Telephone No. 394. Copyright Gildings Auctioneers [11].](img/phone394.jpg)
![Image of a reproduction Secraphone terminal as sold on eBay in February/March 2016 [14]](img/repro_large.jpg)
