Abwehr spy radio transmitter
S-87/20 was a clandestine
20 Watt short-wave (SW) radio transmitter, developed
around 1939 at OKW-Aussenstelle Berlin-Stahnsdorf 1 (Germany)
for use during World War II (WWII)
at the central control stations (Köpfe) 2
of the German Intelligence Service,
the Abwehr, where it was introduced in 1939.
From 1942 onwards, the transmitter was produced by OKW Aussenstelle Wurzen
The S-87/20 was a standalone transmitter, that was commonly used in combination
with an existing receiver, such as the
the Radione R3 or the
It was primarily used for communication with agents in foreign countries,
and with front reconnaissance groups.
Contrary to other clandestine transmitters of the era, it
is not crystal-operated but features a VFO (variable frequency oscillator),
built around an EDD11 double-triode valve (tube).
The 20 Watt output power is delivered by two
EL11 or EL12 valves in a so-called push-pull configuration.
The device is housed in a metal case,
measures 25 x 19.5 x 10.5 cm and weights 2500 grams.
It should be powered by an external power supply unit (PSU) that provides
6.3V AC (LT) and +420V DC (HT). It has two front panel selectable frequency
bands: (1) 4.5-7 MHz and (2) 8-12 MHz.
The device is suitable for
only, and the key
is connected in series with the cathode of the EDD11 oscillator valve.
Note the use of civil EL11/EL12 valves, as the Abwehr was not (yet) allowed to use
military parts at the time.
The balanced antenna output is intended for use in combination with a
symmetrical centre-fed open dipole wire antenna, which makes the device
less suitable for field (agent) use, but all the more interesting for fixed stations, not least because of its superior radiation pattern
and much reduced antenna matching problems.
The S-87/20 was produced in small numbers. It is likely that no more than
25 units were made . The device featured here has serial number 6,
which is screen-printed at the top left corner of the front panel.
The S-87/20 was succeeded in 1939 by the more powerful
OKW = Oberkommando der Wehrmacht (Supreme Command of the Armed
Forces) in Nazi Germany during the Second World War.
Aussenstelle = Outpost.
In German known as Funkmelde-Köpfe (radio message head-end stations).
These were fixed Abwehr stations (such as Wohldorf-Hamburg) or temporary
(mobile) stations for a specific operation.
All controls and connections are located at the front panel of the device.
At the top left is a 4-pin socket
for connection of the external power supply
unit (PSU). A 6.3V AC or DC voltage is needed for the filaments of the valves
(LT), whilst +420V DC is needed as HT voltage. At the bottom right are two
banana sockets for connection of the antenna
The device is best used with a centre-fed open dipole.
At the top are two banana sockets for
connection of the morse key (T).
The device is turned ON with the
power switch at the left centre. It controls
the supply of the LT voltage. At the bottom left is the
frequency dial, which
has a 180° scale. A table – for conversion to the actual frequency – was
usually supplied with every unit.
A lever just below the meter (marked U),
is used to select the required frequency band: 1 (4.5-7 MHz)
or 2 (8-12 MHz).
Once the frequency is chosen, the two
knobs at the bottom right should be used to tune the transmitter for maximum
power output, using the meter at the top right as an indicator. The leftmost one
(0-180°) is part of the tuned circuit in the output of the power amplifier
stage (PA). The rightmost one controls the coupling between the tuned circuit
of the PA and the antenna.
The interior of the S-87/20 can be accessed by loosening the four large bolts
at the corners of the front panel, after which the
front panel can be lifted
from the metal case shell. All internal parts are mounted to the front panel,
wich is actually a metal frame divided in two compartments.
The smallest compartment contains the freely adjustable oscillator,
built around an EDD11
with a balanced tuned circuit. It also holds an
array of wire-wound power resistors
that are part of the power supply. A large adjustable
resistor is used for adjusting the current through the EDD11.
The larger section holds the power amplifier (PA), which is built around
two EL11 valves
in a push-pull arrangement. The current through the PA valves
can be adjusted with a
wire-wound adjustable resistor in the
oscillator/power supply section, mounted aside the EDD11 adjustment.
The tuned circuit of the PA section is fully balanced, just like the antenna
loading coil, which is loosely coupled with the tank coil. The latter is
mounted on an isolated adjustable axle, so that it can be moved in and out
of the tank coil by means of the rightmost knob at the front panel.
The image on the right shows a close-up of the main tank coil which is fully
symmetric. At the center – in the space between the two halfs of the tank
coil – is the antenna loading coil, which consists of a cylindrical
pertinax body with two symmetric windings. The two brown wires at the left
are routed to a current transformer
which is mounted to the back of the meter.
The output of the current transformer is rectified by a Sirutor, converting
the meter into a tuning indicator.
The band selector is operated by a small lever
at the front panel, just below the meter (marked U).
The lever is mounted to a metal axle that runs from left to right through
the transmitter's interior. At either end of the axle is a
two-position rotary switch
that shorts-out part of the windings of each coil (the hexagonal ceramic
bodies), thereby effectively selecting the high band (8-12 MHz).
Below is the complete circuit diagram of the S-87/20 transmitter, with
the exception of the power supply unit (PSU), as it was originally published
by Rudolf Staritz . This diagram was taken from the original
wartime drawings but does not fully match with the actual device as we
As the actual circuit is different from the circuit diagram above,
and because we had to restore our transmitter, we decided to recreate
the circuit diagram from scratch. The result is available below.
For clarity we have altered the orientation somewhat, so that the
balanced nature of the circuit is more obvious.
At the bottom right is the power socket with a network of wire-wound
resistors to obtain the desired voltages. The device is turned on
with switch S1 (6.3V AC LT).
At the bottom left is a free-running oscillator,
built around an EDD11 touble triode. It is activated by the
morse key, which is connected in series with the cathode.
The oscillator signal is fed to the grids
of the power amplifier (PA), that is built around two EL11 valves in
The output of the PA is fully balanced, with its loading coil
mounted on an axle, so that it can be moved in and out of the PA tank coil
with a knob at the front panel.
The center of the loading coil is looped through a current transformer
that drives the meter, using a Siemens Sirutor 1 to rectify the alternating
antenna current. This way, the meter can be used as an antenna tuning indicator.
Note the presence of the two neutralisation capacitors Cn that are
cross-connected between the g1 and the anode of the complementary valve.
They are present to compensate the inherent anode/g1 capacity inside each valve
and are typically very small (in the order of 0.5 to 1 pF).
In many cases this is just a wire soldered to the g1 and placed in the
vicinity of the opposite anode.
A Sirutor is an early type of semiconductor diode, which consists
of a series of stacked copper-oxide tables. At the time they were mainly
used in high-voltage low-current and high-frequency (HF) applications .
When we received our S-87/20 transmitter, it was not in working condition.
Some of the large wire-wound resistors appeared to be broken, and in a earlier
restoration attempt – to replace a couple of physically damaged resistors –
some wires to the PA valves had been misconnected.
At one corner of the oscillator compartment is an
array of five wire-wound power resistors,
that are used to create the internal voltages for the
valves, all derived from the main +420V supply.
Three of these resistors appeared to be broken (i.e. ∞ resistance).
As these are the high-ohm resistors (30k and 2 x 20k) this is probably due
to oxidation of the thin resistance wire. Luckily, we were able to repair
the 30k one at the corner, by removing the end caps, cleaning the contacts
and the wire and re-assembling it. It measures as 27k, which is OK for the
The two 20k resistors appeared to be beyond repair, but finding a proper
replacement turned out to be dificult. As they are connected in parallel
(to form a 10k resistor), we decided to replace them with a slightly
larger single 10k resistor from new-old-stock, that is correct for the era.
The image above shows the green replacement 10k resistor that supplies the
EL11 g2 voltage.
The next challenge were the two large adjustable capacitors (in the oscillator
and the PA), both of which were binding and had to be removed for repair.
The one in the PA stage was easily fixed, but the oscillator one
had to be disassembled completely. Eventually both were fully restored.
Whilst the PA tuning capacitor was removed, we were able to take a closer
look at the parts that are mounted below the sockets of the PA valves.
In a previous restoration attempt, the two 20k resistors (connected between
the g1 of each EL11 and ground) had been replaced by modern types, as the
original ones were physically damaged. In the processing of doing this,
the two 10nF capacitors (connected between the cathodes of the EL11 valves
and ground) had accidentally been connected to the g1 of the rearmost EL11 valve.
The two resistors were replaced by era-correct (red) types, and the wiring of
the capacitors was corrected. We also added the missing decoupling capacitor
to the anode voltage rail. The result is shown above (with removed tuning
To repair the tuning capacitor of the oscillator's tuned circuit,
it was necessary to remove the tuning dial, allowing the latter to be
cleaned more thoroughly.
For this, it was necessary to remove the oscillator coil first, as the
mounting nut of the index hairline (shown in the image on the right) is
located under the hexagonal coil.
The tuning dial was disassembled, cleaned and reassembled. The bakelite
knob at the center of the dial was polished in order to remove a couple
of severe scratches. Once the individual parts were ready, the entire unit
was reassembled again. The outer case shell – which had several dents –
was straighted, before the front panel was screwed-in again.
So far, the following has been restored:
- 30k power resistor repaired
- 2 x 20k broken power resistors replaced by a single 10k type
- PA tuning capacitor repaired (was binding)
- Oscillator tuning capacitor repaired (was binding)
- 2 x 20k resistors at PA g1 replaced by era-correct types
- 2 x 10nF capacitor at PA cathode connection corrected
- 1 x 20nF capacitor at 420V rail added (era correct type)
- One EL11 glass glued
- Tuning dial cleaned, bakelite knob polished
- Front panel cleaned and paint partly restored
- Case shell straightened.
- Case shell clained and paint partly restored
The transmitter is powered by an external power supply unit that must
be connected to the 4-pin socket at the top left of the front panel. Only two
voltages are needed: 6.3V for the filaments (LT) and +420V for the anodes
of the valves (HT). The pinout of this socket is a s follows:
Finding a suitable connector for this socket can be very difficult, as they
were probably purpose-built at the time. As a gap-fill solution it is
possible to use female banana plugs and place them over the pins in the
socket. The best solution however, would be to make a replica from era-correct
materials. Below are the dimensions of the plug (in mm). The body
is made of pertinax.
➤ Download drawing as PDF
When inserting or removing the power plug, be careful not to touch the
screws at the front side of the plug as they carry a high voltage that will
cause a serious – and potentially harmful – shock. This may also be the case
after the external power supply unit has been switched off or disconnected,
as the capacitors inside the S-87 transmitter may still be charged.
Always hold the plug by the short sides.
EL11 or EL12 power amplifier valve
EL11 and EL12 are penthodes from the so-called Harmonische Reihe,
developed in 1938 and produced by many manfucturers, including Philips,
Telefunken, Siemens, TeKaDe and Valvo. The EL11 has a maximum anode voltage
of 250V, whilst this is 375V for the EL11/375.
➤ EL11 datasheet
➤ EL12 datasheet
➤ EL12/235, /350 and /375
EDD11 is a double triode, introduced in 1938 by Telefunken, mainly for use
as an audio amplifier in early car radios. In the S-87/20 it is used as a
balanced oscillator, which forms the device's variable frequency
➤ EDD11 datasheet
HT voltage420V DC
LT voltage6.3V AC or DC
ValvesEDD11, 2 x EL11 (or EL12)
StagesOscillator, Power Amplifier (PA)
Frequency4.5 - 7 MHz and 8 - 12 MHz
TuningVariable Frequency Oscillator (VFO)
AntennaSymmetrical dipole (wire)
Obtained from Frank's Electron Tube Pages .
- Louis Meulstee, S 87/20
Wireless for the Warrior, Volume 4, Supplement, Chapter 117.
Retrieved March 2018.
- Rudolf F. Staritz, Original S-87/20 circuit diagram
Obtained via .
- Radio Museum and contributors, Siemens Sirutor
Retrieved September 2016.
- Frank Philipse, Valve datasheets
Website: Frank's Electron tube Pages.
- Various collectors, Personal correspondence
The following collectors have contributed to this page:
Gerd Reinel (DH2FAI), Günter König (DJ8CY), Manfred Bauriedel (DK4NQ),
Rudolf Staritz (DL3CS).