No. 7 →
← No. 6AA
The system is housed in a metal enclosure in black wrinkle-paint finish,
that measures 585 x 360 x 185 mm and weights 41 kg. At the top is a
hinged lid that gives access to the connections, the valves and the
vibrator pack. The remaining building blocks are in the lower part of the case.
The device was usually installed under a desk or in
an adjacent room, and was connected to a converted telephone set –
the voice terminal – that was placed on the user's desk.
In most cases, the voice terminal had push-buttons to
switch between normal and scrambled speech.
The device uses the principle of inversion of the audio spectrum,
also known as frequency domain speech scrambling,
or speech mirroring.
High tones become low tones and vice versa, resulting in unintelligible
speech. All the person at the other end has to do, is invert the
spectrum once more, to make it intelligible again. Although this method
is not very secure — it is not a real encryption device —
it was believed to be good enough to protect a
conversation against a casual eavesdropper, such as the exchange operator
or a telephone engineer working on the lines.
At least 3 variants of the 6AC were made, identified by
a suffix to the model number.
The one featured here is variant '/3'.
The device was introduced in January 1944,
and was in production until 1957, after which it was succeeded
by Privacy Set 7 and eventually, in 1962, by the fully
transistorised Privacy Set 8.
The 6AC/3 unit featured here, was manufactured by the
Telephone Manufacturing Company (TMC)
in London (UK), and was marketed under the name
➤ Overview of the various models
The diagram below provides a quick overview of the features of the
Frequency Changer 6AC/3. The device is shown here with open lid and a
wartime SA 5063/1 voice terminal placed in front of it.
At the upper edge of the right side panel is the
mains power receptacle. The
mains plug has to be
removed before the lid can be opened, to ensure that the user can not touch
lethal voltages. The desired mains voltage is selected with
a wire strap at the far right. In addition,
there are five configuration straps for selection between operation from
the mains (M) or a 12V battery (B).
The wiring to the subscriber line and the voice terminal is fed into the
device via a circular hole in the left side panel,
and connected to the
terminal block at the left. In the simplest
configuration, the Frequency Changer is connected directly to the subscriber
line, in parallel with a regular telephone set with dial, that is used
to initiate a call.
A simplified telephone set 1 is connected to the voice circuits of the
Frequency Changer. It is only used for its handset (microphone and speaker).
Once the call has been established by means of the regular telephone set, both
parties lift the handset of the simple terminal, and put the handset of the
regular telephone set on-hook.
They can now continue their conversation in secure (scrambled) mode.
At the end of the conversation, both parties put the handset of their simple
terminal on-hook to disconnect the subscriber line.
In most cases however, a more advanced setp was used, in which an
SA-50xx voice terminal is connected to the subscriber line,
in parallel to a regular telephone set with a dial that is used for initiating
The voice terminal has two buttons, marked SECRET and NORMAL (or similar),
so that it can be used for both clear and secure calls.
The SA-5063/1 is a good example of this.
Advanced configuration with wartime voice terminal
In clear mode, the terminal can accept incoming calls from the regular
telephone set. When switching to secure, the subscriber line is routed
via the Frequency Changer, whilst the microphone and speaker of the
terminal's handset are connected directly to the voice circuits of the
Frequency Changer. This routing is controlled by the
inside the voice terminal.
When the Frequency Changer was used on an automatically switched network
— which was rarely the case during WWII — it was also possible to use
a voice terminal that was fitted with a dial, and connect it directly
to the subscriber line. The telephone is then used in clear-mode to
initiate the call, after which both parties switch to secure. A good
example on this website is the SA-5030.
Using a voice terminal with dial
In practice, the Frequency changer and its voice terminal were connected
as shown in the diagram below. A junction box, such as the Block Terminal (BT)
No. 20/8 or BT No. 6, was used to connect all parts together. The subscriber
line is routed to the SA 50xx voice terminal, and the
303/A Key Unit inside the
SA 50xx determines whether the line is connected directly to the telephone
or via Frequency Changer. In the latter case, the
Key Unit disconnects
the handset from the telephone and connects its microphone and speaker
directly to the voice circuits of the Frequency Changer.
Note that the carbon microphone of the handset is responsible for keeping
the subscriber line engaged. When it is connected to the Frequency Changer,
the latter 'steals' the DC voltage — required for a proper operation of a
carbon microphone — from the subscriber line. This load is enough to signal
to the exchange that the line is 'in use'.
Unmarked simplified telephone set, based on the design of Telephone
No. 162 or No. 232.
The bare Frequency Changer (i.e. the actual voice scrambler) is housed in
a metal enclosure in black wrinkle paint finish that measures 585 x 360 x 185 m
and weights no less than 41 kg.
The unit would normally be installed in a cupboard, under a desk, or in an
adjacent room. Once installed, there is no need to have quick access to
it, as it has no controls. It is powered from the AC mains or from a 12V battery. At the top is a hinged lid that provides access to the serviceable
parts (valves, vibrator, fuses, etc.).
In some cases, the unit was supplied with a simple voice terminal, that was
based on GPO Telephone No. 162 or 232.
The microphone and speaker element of its
handset are connected directly to the voice circuits of the
In this configuration,
the scrambler is connected directly to the subscriber
line, in parallel with a regular telephone set that was used to initiate calls.
Once the call is established, the 162-unit is used for the secure conversation.
➤ More information
The Frequency Changer 6AC/3 can be powered directly from the AC
mains network in most countries. Once the
desired voltage has been
selected and the five configuration straps
are set to 'M', power
can be applied to the 3-pin Bulgin receptacle
at the top right.
The required plug was delivered with the set.
The image on the right shows a mains power cable that is suitable
for the AC mains network in continental Europe.
Depending on the circumstances, each device came with a
selection of spare parts, such as spare valves, a spare
vibrator pack, light bulbs, power connectors and fuse wire.
It allowed technicians to carry out simple repairs
in the field. The image on the right shows a selection
of spare parts that were found with the device featured here.
In addition to the spare parts shown above, the devices
featured here also came with full service documentation, comprising
technical details and descriptions of the various building blocks.
The handbook even contains the complete circuit diagram of
the three variants or groups:
6AC/1, 6AC/2 and 6AC/3 [A].
➤ Download the handbook
➤ View the circuit diagram
Below is the block diagram of Frequency Changer 6AC/3.
At the far right is the 2-wire subscriber line, which is usually connected
in parallel to a regular telephone set with a dial. At the far left is
the handset of the voice terminal. The audio signal from the microphone is
applied directly to a ring mixer (UD), where it is added to the 2500 Hz
signal from the central oscillator (VA).
At the output of the mixer, the sum
and the difference
of the two signals is 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 low-pass filtering it (UC), only the mirrored
signal remains, which is then amplified (VB) 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 (UA)
produces two images, of which the lower one is the mirrored
version of the received signal. After amplification (VC), it is passed
through a low-pass filter (UB), so that only the lower
part remains (blue). This is a copy of the original audio signal.
The spectrum diagrams illustrate what happens.
The inverted (scrambled) audio is shown in red.
Note that this only works, of course, when both parties use the same
inversion frequency of 2500 Hz.
It is also important that the two low-pass filters (UB and UC)
are of a higher order and have a sharp cut-off. This means that
the baseband (everything up to the crossover frequency) is passed
unattenuated, and that everything above its crossover frequency
is sufficiently suppressed. Together with the ring mixers, the
low-pass filters are responsible for the overall audio quality.
Although the Frequency Changer, or frequency inverter, offered
reasonable protection against an casual (un)intentional eavesdropper,
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 classic case of security by obscurity.
➤ Check out the original block diagram
Frequency Changer 6AC/3 is housed in a heavy (41 kg) metal enclosure.
The interior consists of two sections: the upper 1/3rd that contains the
user-serviceable parts, and the lower 2/3rd with the
parts that cannot be serviced by the user. The upper section
is accessed via the hinged lid.
Each strap has two possible positions: 'M' for mains and 'B' for battery.
Below the straps are four fuses: two (1/5A) for the primary and two (3A)
for the secondary power circuit. The fuses consist of a bakelite base
with a removable insert that contains a bare fuse wire, which can be replaced.
At the centre is a circular vibrator unit that is used when the device is
configured for use with a 12V DC power source. It converts the DC
to an AC voltage, which is then transformed to the required HT voltage
for the anodes of the valves.
The vibrator unit is installed in a metal enclosure that contains additional
filter parts, to prevent harmonics of the vibrator's switching frequency
from leaking into the Frequency Changer's audio circuits. A broken vibrator
can easily be replaced by the user.
To the left of the vibrator, close to the edge of the base panel,
is the power lamp.
When the device is closed, the power lamp is visible through a red lens
in the front side of the top lid. It is lit when the unit is powered.
The lower section contains the parts that should only be serviced by a
qualified enigineer. It is covered by a metal plate that is held in place
by two screws.
After loosening these screws (in the upper left and right corners),
the front panel can be lifted out, and the remaining circuits
The four large grey building blocks are the low-pass filters and
the two ring mixers; one for the transmitting side and one for the
receiving side. They are shown in the image on the right.
The mixers are fitted with potentiometers, that
allow adjustment of the balance (i.e. suppress the carrier
Surrounding these four units, are the smaller
building blocks, such as the audio transformers
and the power circuitry.
UARing mixer (receiver)
UB2350 Hz low-pass filter (receiver)
UC2400 Hz low-pass filter (transmitter)
UDRing mixer (transmitter)
UEOscillator tuned circuit (adjusted at 2500 Hz)
UFOscillator output transformer
VAOscillator valve CV1052 (carrier)
VBAmplifier valve CV1502 (transmitter)
VCAmplifier valve CV1502 (receiver)
XBAudio transfomer (input of VB)
XCAudio transformer (output of VC)
XDAudio transformer (input of VC)
For many years we had been looking for the war-time version of the
Frequency Changer, the device that
used for private conversations. We had found the
post-war transistorised version,
but older valve-based variants appeared to be unobtainable.
Very few were made and even fewer have survived. But in May 2021 our
patience was rewarded, when we were offered two complete 6AC/3 units
in mint condition, that had been found in an attic in Brussels (Belgium) .
First the primary side of the mains transformer was checked.
It has a 53Ω DC resistance, which seems about right.
On the contacts of the mains receptacle however – which are
connected to the transformer – the resistance was infinite.
It turned out that all four fuses in each of the scramblers were broken.
This was not caused by a short circuit in the devices, but
by galvanic corrosion caused by the potential difference that
exists when two different metals are in electric contact with each other.
Over time, this had caused the thin fuse wire to corrode and break.
Unlike modern fuses, that can be replaced easily, the fuses of the
6AC/3 contain a short piece of fuse wire that is held in place by
two screws. This allows the fuse to be rewired when it is broken.
But as bare fuse wire is something of the distant past, it may be difficult
to find a suitable supply.
No power cables were supplied with the devices, but fortunately
the 3-pin Bulgin plugs were stowed inside the units, so
mains power cords could easily be made.
The settings of the five configuration straps
were checked – all five must be set to 'M' (mains) – and the wire
straps of the voltage selector at the right were configured for an
AC mains voltage of 250V to avoid damage. 1
In the meantime, a Google search had revealed that fuse wire was still available from
a variety of sources in the Far-East, and for very reasonable prices too.
We ordered spools with 2A and 3A. 2
When it arrived several weeks later, the four fuses of each device were
rewired, and mains power was gradually applied to the two devices by
means of a VARIAC. During this process – that is necessary to reform the
capacitors – a thermal camera was used to confirm that no parts were
overheating. When we were certain that this was not the case,
the voltage was increased to 230V.
At this point, a faint 2500 Hz tone started to emanate from the devices,
which indicated that the main oscillators were working. Next it was time
to hook the devices up to telephone sets and an exchange,
so that they could be tested.
As we want the setup to look as authentic as possible, we made a seven-wire
braided cord (shown in the image on the right) and connected one end to
the terminal block of the Frequency Changer.
The other end was fitted with
a 7-pin XLR plug, 3 so that it could easily be connected to the
telephone line and to the voice terminal.
As the configuration of the terminal block inside the 6AC/3 is slightly
different from the 6AC/1 and 6AC/2, the internal wiring of this block
was modified so that it is now compatible with the other models. This
involves the removal of the orange wire from terminal (6) and connecting
it in parallel with the orange wire at terminal (4). It provides the 3V
DC for the carbon microphone. 4
Next, the SA-5030 telephone set from our collection was
internally rewired as per wartime instructions, and fitted with an
8-wire braided cord that terminates in a BT-20/8 junction box.
This box is also fitted with a braided line cord, and with a 7-wire
braided cord for connection of the Frequency changer. The latter
terminates in a 7-pin XLR female connector that mates with the
male connector of the Frequency Changer.
This way, the Frequency Changer
and the telephone set can easily be connected and disconnected.
The wiring is shown in the image on the right.
After connecting the Frequency Changer to the wiring of the
(via the XLR connectors), the line cord was connected to the local
PABX for a first test. The SA-5030 can make outgoing calls and
accept incoming calls, whilst a regular POTS telephone set was
used at the other end.
In normal mode — the default after initiating a call —
speech in both directions was loud and clear, indicating that
the SA-5030 was wired up correctly.
After switching to scrambled mode — pressing the SCRAMBLE button
on the SA-5030 — speech became unintelligible as expected.
High tones became low ones and vice versa.
Next, the other one was wired up and connected to a suitable
telephone. It was tested against the regular telephone set, but there
was a problem. There was a strong residual 2500 Hz tone on the line,
even when it was not in scrambled mode.
When tried in scrambled mode, both clear and scrambled speech was
heard at the same time. Something was clearly wrong, so the device was
put on the workbench for further investigation. After studying the circuit
diagram and doing some tests, it became clear that the transmission
path was fine, but that the ring mixer in the reception path – the
demodulator – had a fault.
One half of the mixer didn't work at all, causing an imbalance of the
two transformers, resulting in all kinds of side-effects, such as
injecting the 2500 Hz oscillator signal back into the line.
After removing the demodulator (block UA)
from the device, it was extracted from its enclosure, as shown in the
image on the right. Although the demodulator is well-constructed, this
particular unit was probably built on a friday afternoon or a monday
morning, as none of its screws had been tightened properly. Because of
this, both transformers could be moved feely by half a cm.
In addition to this, the tightly bundled wiring is extremely stiff,
and offers little support for movement. As a result of this, two wires
that were soldered to the primary transformer (UAE) were broken. After
refitting the wires and tightening all screws, the unit was reassembled
and externally connected to the device.
This time it worked as expected. After mounting it back in place,
both 6AC/3 units were completely re-aligned as per service manual and
tested – with the original wartime telephone sets – on the local PABX.
They now work flawlessly in either direction.
The units produce a clear voice signal, both in clear and in scrambled mode,
with an excellent intelligibility. It is truly amazing that all electronic
parts – even the capacitors – have all survived.
Although the official mains voltage in Europe in 230V, it is
often much higher than that. The mains voltage at our location
is 241V and sometimes even higher.
Ordered from AliExpress. 1.5A fuse wire was not available,
so we choose 2A wire instead.
Although this is a modern connector, it was choosen for the quality of
its contacts, as well as its dimensions. The XLR-7 plug
fits through the cable hole at the left side of the Frequency Changer.
The original wiring of the 6AC/3 terminal block is really weird
and seems to be illogical. By modifying it as described, the DC
voltage for the handset's carbon microphone is taken from the
- No power cables
- All fuses broken due to corrosion
- Decomposition of the plastic cradles of the voice terminals
- Strong 2500 Hz tone on the line
- Superficial cleaning
- New power cables made
- New braided interconnection cables made
- Fuses rewired
- Wiring of terminal block slightly modified
- Demodulator of one unit repaired
- Both unit completely re-aligned
When repairing and/or aligning a Frequency Changer 6AC, it may be
difficult to inject a tone of, say 400 Hz, from an AF generator
directly into the circuit. Likewise, it is difficult to connect a
modern oscilloscope directly to the circuit, as it connected to the
same ground (earth). In such situations it is recommended to use a
standard 600Ω 1:1 transformer as a galvanic separation.
Simple test tools used for testing and repairing Frequency Changer 6AC
The diagram above shows examples of simple circuits that can be used
for injection of an AF signal (left) and for measuring (right).
Note that the parts shown in grey (C1, R1, X1) are optional. C1 should
be inserted when the circuit carries a high DC voltage. R1 should
be added when measuring the signal on the line terminals, when the
device is not connected to a subscriber line.
In many cases it may be useful to connect a high impedance crystal
earphone (X1) in parallel to the output of the transformer,
so the the AF signal can be monitored directly. This is particularly
useful when the circuit is connected to the line terminals, whilst
the balance potentiometers of the demodulator (UA) are adjusted
for minimum leak of the 2500 Hz carrier back into the line.
Power should be applied to the device via the 3-pin Bulgin connector
that is located at the top right. It is positioned in such a way, that
it can only be accessed (via a hole in the top lid) when the top lid is
closed. In the same vein, the plug must be removed before the lid can be
- AC (N) Neutral
- AC (L) Live
- Ground (E) Earth
It is important to verify that the correct mains voltage has been configured
on the screw terminal block at the right, before applying a mains AC voltage.
The device can also be powered from a 12V DC battery, in which case an
internal vibrator pack is used. Confusingly however, the same 3-pin Bulgin
connector is used for connection of the battery voltage. Furthermore, it
requires 5 wire links to be altered. Always check the current configuration
before applying mains power.
For service engineers, an extra 3-pin Bulgin connector is available inside
the device, that is connected in parallel to the Bulgin connector at the
top right. A dummy plug is installed to cover its contacts. It allows the
device to be powered when the lid (and the front of the case) is open.
For connection to the AC mains, the built-in power transformer has a
primary winding with a number of taps, that allow it to be configured for
virtually any AC mains network in the world.
The windings are brought out to a screw terminal block that is fitted in the
upper section, to the right of the fuses. By connecting the red and black
wire to two of the terminals, the following voltages can be selected.
The terminal numbers are shown in brackets (e.g. 1-6 for 250V).
- 100V (2-3)
- 110V (1-3)
- 200V (2-4)
- 210V (1-4)
- 220V (2-5)
- 230V (1-5)
- 240V (2-6)
- 250V (1-6) ← our current setting
It is important to select the appropriate voltage prior to connecting the
device to the mains, as the unit will be activated as soon as the mains
is plugged in (there is no power switch). As the power in continental
Europe has been raised at the beginning of the century from 220V to 230V AC,
an incorrectly configured power transformer may saturate and eventually
break down. In practice, mains voltages are often even higher.
A mains voltage of 240V is no exception.
Whenever possible, select the next higher voltage to avoid saturation.
At the top left of the Frequency Changer is a
double terminal block
to which the subscriber line and the telephone instrument (i.e. the
voice terminal) should be connected. The subscriber line is connected
to the lower terminal block (pins 1-2), whilst the microphone and speaker
elements of the voice terminal's handset are connected to the upper
terminal block (pins 4-7). As follows:
- Line (B)
- Line (A) 1
- LB 1
- Microphone (2) 2
- Microphone (1)
- Speaker (2)
- Speaker (1)
The diagram above shows the wiring of the 6AC/1 and 6AC/2
models. The wiring of the 6AC/3 is slightly different (pin 6 is used
to pass a DC voltage for the carbon microphone) but it is
advised to change it, so that the becomes compatible with the other
models. Remove the orange wire from pin (6) and connect it in parallel
with the orange wire at pin (4).
➤ See the circuit diagram
In a Local Battery (LB) installation, a 3V battery should be
connected between pin 3 of the terminal block (+) and 2 of the
BT-20/4 of the instrument (-).
Omitted in a Local Battery (LB) installation.
Below is the internal wiring diagram of the
SA 5030 voice terminal.
At the bottom right are the (A) and (B) terminals of the subscriber line.
Directly above it, is the wiring to the terminal block of the Frequency
Changer 6AC. The make-before-break (MBB) switches KA (1-4) are part of
the 303/A Key Unit
that is controlled by the 2 (or 3) push-buttons on top of the
They allow the telephone set to be used for plain as well as secure
calls. In secure mode, the speaker and microphone of the telephone's
No. 164 handset
are routed via the Frequency Changer 6AC.
During WWII, the dial armature was often omitted from the
SA 5063/0 and SA 5063/1 telephone sets,
as the British Government used a private telephone network
– completely separated from the public switched network –
that was patched manually.
An extra push-button (KB) could be fitted (between the other two)
for the RECALL feature that was available on some automated exchanges.
Below is the circuit diagram of the simplified TMC-232X telephone set.
It has been modified for direct operation with the Frequency Changer 6AC,
and is used for its handset only. Note that the two contact pairs of the
hook switch are used for disconnecting the microphone and also for disconnecting
the DC voltage that is needed for the carbon microphone, when it is on-hook.
Note that a different connection pattern is required when using the telephone
with the Frequency Changer 6AC/3, as the latter has a slightly different
internal wiring of its terminator block.
To allow the Frequency Changer and suitable telephone sets to be tested, used
and demonstrated in various configurations, without altering the fragile
vintage wiring of the devices all the time, Crypto Museum has defined its own
standard, involving inline 7-pin male/female
In this standard, an 8-point junction box BT 20/8 is used as the
central hub. The SA 50xx voice terminal is fitted to the BT 20/8 via
a fixed 8-wire braided cable. The subscriber line is also fitted to the
BT 20/8 via a fixed 2-wire or 4-wire braided cable, whilst a fixed
7-wire braided cable with an XLR7/F connector
at the end is present for quick (dis)connection of the Frequency Changer.
The Frequency Changer itself is fitted with a fixed 7 or 8-wire braided cable
with an XLR7/M connector at the end. This allows the Frequency Changer
to be disconnected from the setup without opening it and unscrewing the
wires from its terminal block or from the
Below is the pinout of the XLR7/M connector on the cable that is
fitted to the Frequency Changer.
The wiring order is identical to the
order on the terminal block inside the Frequency Changer 6AC.
- Line (B)
- Line (A)
- LB (or unused)
- Microphone (H)
- Microphone (L)
- Speaker (L)
- Speaker (H)
The XLR7/M plug fits exactly through the cable hole in the left side of the
Frequency Changer, allowing the braided cable to be stowed inside the device
when it is in storage or in transit.
The CV1052 is an indirectly heated pentode, introduced in 1940 for use in
audio circuits. Apart from AC and DC applications, it was also used in car
audio systems, either as a class 'A' amplifier or in push-pull configuration.
It is equivalent to the EL32. It is also electrically
(but not physically) equivalent to the EL2.
It was one of the standard valves of the UK's
General Post Office (GPO).
➤ CV1052 datasheet
Years1944 - 1957
PrincipleSpeech frequency inversion
Pivot freq.2500 Hz
Remote2-wire telephone line, 600Ω (default or 300Ω (option)
Local4-wire interface (handset)
CircuitsOscillator, amplifier (2x), mixer (2x)
Valves3 x CV1052 (EL32)
Mains100, 110, 200, 210, 220, 230, 240, 250 V AC 1 (50-60 Hz)
Battery12V, 1.7A DC 2
Dimensions585 x 360 x 185 mm
?C & E Museum, Kingston, Ontario, Canada
U1444/3/1459Crypto Museum, Netherlands
U1444/3/1467Crypto Museum, Netherlands