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GK-III b
Speech scrambler - not in collection

GK-III b was a portable speech inverter, or voice scrambler, developed around 1942 by Siemens & Halske for use by the German Army during WWII. The battery-powered valve-based device was housed in a standard-size panzerholz carrying case, and could be connected to an existing (field) telephone line or radio set. According to the text on the lid, it is also known as Iv.-Gerät 1 , which probably means Invertier-Gerät [1]. The official name was Kleiner Leitungsverzerrer GK III.

The image on the right shows a typical GK-III b, which is held in the collection of the Foundation for German Technology CDV & T [1]. It is housed in a typical panzerholz case – similar to the ones used during the war for German radio sets and for Hell­schreibers – that could be carried on the back. It has a removable lid at the front, behind which all controls and connections are located.

The device is powered by three batteries (-3V, +2V and +90V) that are installed in the bottom compartment of the case. The rest of the case (approx. 2/3rd) is taken by the actual scrambler.
  
GK-III b Sprach-Invertierer

The GK-III uses a simple frequency inversion scheme, based on a fixed mirroring frequency of approx. 2000 Hz, and is very similar to the American A3 and the British Frequency Changer, both of which were used by the Allies during WWII. The GK-III was used as a safety measure against occasional or (un)intentional eavesdroppers – such as the exchange operator or a field engineer working on the lines – but was no match for a professional interceptor. This was known to the Germans as they themselves were able to solve the British and American scrambler systems [4].

According to Michael Pröse in [4], the GK-III was used by the German Army (Wehrmacht), the German Police (Polizei), the Nazi party (NSdAP) and by various departments of the German State. As most systems were used over telephone lines, there was little chance of enemy interception.

  1. Iv.-Gerät = Invertier-Gerät (inversion device).

Help wanted — At present, not much is known about the Siemens & Halse GK-III speech scrambler. Although it is mentioned in Michael Pröse's dissertation of 2004 [4], the year of introduction and the production quantities are currently unknown. We are specifically looking for circuit diagrams and for service documentation, but any other information would also be most welcome.  Contact us
GK-III b Sprach-Invertierer Front view Empty panzerholz case with room for the GK-III b and the batteries Front panel controls Adjustement Space for the batteries KF-III b removed from the transport case Text on the outside of the lid Circuit diagram - affixed inside the lid
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GK-III b Sprach-Invertierer
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Front view
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Empty panzerholz case with room for the GK-III b and the batteries
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Front panel controls
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Adjustement
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Space for the batteries
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KF-III b removed from the transport case
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Text on the outside of the lid
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Circuit diagram - affixed inside the lid

Features
All controls and connections are located at the front panel of the GK-III, as shown in the diagram below. About 2/3rd of the case is taken by the actual scrambler, whilst the remaining space (at the bottom) is used for the batteries. The batteries are automatically connected to the circuits as soon as the lid of the case is removed. A red switch at the centre is used to turn the device on.

Controls and connections on the front panel of the GK-IIIb

A voltage meter, just above the ON/OFF switch, allows the battery voltages to be checked. Furthermore, recessed controls (below the ON/OFF switch) are used to select the desired telephone system (OB or ZB) and adjust the audio volume of the receiver circuit. The tele­phone line and the tele­phone set should be connected to the banana terminals along the lower edge.

ON/OFF switch Socket for connection of telephone set Socket for connection of the line Jack socket (line) jack socket (telephone set) Warning - The enemy is listening Three valves installed behind the warning lid Lid switch
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ON/OFF switch
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Socket for connection of telephone set
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Socket for connection of the line
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Jack socket (line)
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jack socket (telephone set)
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Warning - The enemy is listening
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Three valves installed behind the warning lid
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Lid switch

Setup
The GK-III was an in-line device, that was connected between a (field) telephone set and the (field) telephone line. The diagram below shows the basic setup of a simple point-to-point connection, with one GK-III b unit connected at either end of a regular (field) telephone line.


A two wire telephone set was then connected to the Fernsprech-Apparat terminals of the GK-III. A recessed switch on the front panel of the GK-III, allowed selection between Local Battery (OB) or Central Battery (ZB) systems. Signalling (i.e. the bell/calling voltage) 1 bypasses the system.

  1. German: Weckspannung (wake-up voltage).

Block diagram
Below is the simplified block diagram of the GK-III b, based on the drawing that was found inside the lid of the device featured above. The circuit is pretty straightforward and is very similar to the British wartime Frequency Changer, also known as Privacy Set and later also as Secraphone.

The circuit has two branches: one for transmission and one for reception. The signal from the connected telephone set (left) is first filtered, so that only signals in the 0-1900 Hz range are kept (blue). The resulting signal is then passed to a ring mixer, where it is added to the 2000 Hz sinewave from a fixed oscillator. At the output of the mixer, the sum and difference of the two signals are available, with the difference (red) being the inverted version of the original signal.

Block diagram of the GK-IIIb voice srambler

The signal is then passed through another low-pass filter, so that only the inverted signal (red) is presented at the output (right). The reception path (i.e. the lower branch) works in a similar way. The inverted signal (red) is first filtered and then passed to a 2nd ring mixer, where it is added to the 2000 Hz oscillator signal, after which the sum and difference are available at the output. After low-pass filtering it again, the un-inverted signal (blue) is passed to the telephone set (left).

At the top of the block diagram is a bypass switch that is operated in parallel with the ON/OFF switch. It ensures that – when the device is switched off – the telephone set is connected directly to the line. At the bottom is a mystery circuit that is marked as Umg., which probably means Umgang (bypass). Although it is currently unclear what this circuit does, it seems likely that it is used to pass the ringing voltage (bell) directly from the line to the telephone set and vice versa.


Interior
The GK-IIIb is housed in a standard panzerholz case that measures approx. 44.5 x 36 x 22 cm and weights no less than 25 kg (without the batteries). The interior can be accessed by loosening four large (red) bolts at the corners of the front panel, and using the two large grips to extract the metal frame with the electronics from the case. All circuits are mounted to the front panel.

Looking at the interior from the rear, the frame can roughly be divided into three sections: the receiver at the left, the transmitter at the right and the valves (tubes), controls and connections at the centre. The transmitter and receiver each consist of a series of shielded (metal) modules of which the terminals are visible from the outside.

The circuit is build with just three RV-2P800 valves (tubes): one for the 2000 Hz oscillator, and one for each of the audio amplifiers. The rest of the unit consists of passive parts, like resistors, capacitors and many low-pass filters.
  
The three valves seen from the interior

At the centre of the upper edge of the front panel is a push-button (shown below) that detects whether the lid of the panzerholz case is in place. According to the metal tag, this is the Deckel­schalter (lid switch). It ensures that – when the case is closed – all battery wires are disconnected.

The image on the right shows a close-up of the lid switch, which consists of four individual sets of contacts. Two power lines (+2V and +90V) are also routed via the ON/OFF switch. When OFF, the telephone set is connected directly to the line. In the ON position, power is applied to the circuits and the phone is routed via the device.

The RV-2P800 valves are mounted at the centre of the device, behind the front panel, in such a way that they can be accessed through a hinged lid at the front. As the extra safety measure, the lid holds an eavesdropping warning message.
  
Lid switch

The mixers are made with very early GL-101/6 copper-oxide rectifiers (diodes) [2] – made by Siemens – of which four are used in each mixer. On his website CDVandT.org, Arthur Bauer shows that in the GK-IIIb he obtained in early 2019, these diodes were all gone, probably due to ageing of the material. They could simply be replaced by four small signal diodes (e.g. 1N4148) [1].

 More on Arthur's website (off-site)

KF-III b removed from the transport case Rear view of the interior Interior Right side - electronics - receiver Left side - electronics - transmitter Three valves seen from the front The three valves seen from the interior GK-III ring mixer. Photograph courtesy Arthur Bauer [1].
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KF-III b removed from the transport case
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Rear view of the interior
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Interior
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Right side - electronics - receiver
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Left side - electronics - transmitter
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Three valves seen from the front
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The three valves seen from the interior
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GK-III ring mixer. Photograph courtesy Arthur Bauer [1].

Specifications
  • Dimensions
    44.5 × 36 × 22 cm
  • Weight
    25 kg
  • Power
    -3V, +2V, +90V
  • Batteries
    HT 0-3V-90V, LT 2B19
  • Valves
    3 × RV-2P800
  • Carrier
    2000 Hz ± 50 Hz
Known serial numbers
The serial number of the GK-IIIb is usually engraved in a metal plate that is fitted at the front panel (at the bottom right). Furthermore, a tag with the full production data can be found at the inside of the front panel, close to the upper edge. The following data is available:

  • Type: G K III b
  • Rel.Sk.: 11 Rel.Sk.590/03b
  • Zchng. Nr.: 11 Rel.trgb.11b
    Zchng = Zeichnung (drawing), trgb = tragbar (portable)
  • Fabr. Nr.: 424158
    Fabr. = Fabrikage-Nummer (manufacturing number)
It is currently unknown how many GK-IIIb units were manufactured. In order to get an indication of the production quantity, will would like to capture as many serial number as possible. So far, the following serial numbers have been observed:

  • 302174
    unreadable
    Private collector Austria (green case)
  • 423988
    Wa.A. 577
    Private collector Austria
  • 424028
    ?
    CDV & T Netherlands
  • 424158
    Wa.A. 577
    CDV & T Netherlands
Glossary
OB   Orts-Batterie
Local battery system. In English known as LB.
Umg.   Umgang
Bypass (not confirmed).
ZB   Zentral-Batterie
Central battery system. In English known as CB.
References
  1. Arthur Bauer, GK-III b — Iv.-Gerät
    Foundation for German Technology. Retrieved April-May 2019.

  2. R. Tramm & F. Bath, Der Siemens Kupferoxydul-Gleichrichter
    Siemens & Halske, 6 December 1935. 1

  3. Wikipedia (Germany), Feldtelefon
    Retrieved May 2019.

  4. Michael Pröse, Chiffriermaschinen und Entzifferungsgeräte oim 2.WK
    Dissertation, December 2004 (German). Deutsche Geräte zur Sprachtarnung, p. 172.
  1. Document kindly provided by Arthur Bauer [1].

Further information
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Crypto Museum. Created: Monday 29 April 2019. Last changed: Sunday, 12 May 2019 - 19:37 CET.
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