Special Operations Executive A Mk I koffer zend/ontvanger.
© John Hearfield, www.johnhearfield.com
MANUAL                                                                     transcription by John Hearfield

Set Serial No ........


Case containing:

* 1 Transmitter
* 1 Receiver
* 1 AC Power Pack suitable for use on 100-150V or 200-250V c.p.s.
* 1 6 Volt Battery Power Pack
* 1 60 ft. Aerial wire
* 1 10 ft. Earth wire
* 1 Transmitting key
* 1 Pair of headphones (low impedance)
* 2 Brass Pins to convert mains plug to Continental fitting
* 2 Crystals (between 3 and 4 Mc/s

The following spares are provided:

* 12 Fuses
* 6 Bulbs
* 1 Transmitting valve type ATS25 (807) (V5)
* 2 Receiving valves type ARP34 (EF39) (V1 & V3)
* 1 Reveiving valve type ARTH2 (ECH35) (V2)
* 1 Receiving valve type AR21 (EBC33) (V4)
* 1 Rectifying vallve type 5Z4G (AU13) (V6)
* 1 Vibrator (synchronous type)

N.B. For battery operation a 6 volt car type accumulator must be obtained. This is not supplied.


  60 ft. of aerial wire is provided and it is desirable that as much of this should be used, and in any case 25 feet must be regarded as the absolute minimum. It is also important to arrange the wire as high as possible, while keeping it away from earthed objects. In general, outdoor aerials will be better than the indoor variety, while of the latter one, erected in an upper room or loft is preferable to one on the ground floor. Plug one end into the insulated socket, marked AE, mounted on the transmitter.

Earth  A good electrical connection must be made to an to an existing earth tube, a main water pipe or central heating system. If these are not available, a wire of the same lenght as the aerial should be suspended underneath it, preferable two or three feet above the ground. The earth wire should be connected to the terminal on the transmitter marked E.

A.C. Power Pack  100 - 150V, 200 - 250V; 40-100 c/s. Valve 5Z4G. Ascertain whether the mains are AC or DC. The mains power pack must be used on AC mains only. Ascertain the voltage by reference to electrical apparatus in use; the electric light meter or the markings on electric light bulbs.

Adjustment of the Mains Power Pack to suit the supply voltage: The waterproof strip on the power pack is unfastened, disclosing a five-pin plug with an arrow which indicates the range for which the plug is set and a flexible lead with a screw connector for the exact voltage. For example for operation on 220 mains the arrow on the plug would point to 200-250, and the lead would be fixed by the screw to the socket marked

* 215-235
* --------
* 115-135

The waterproof cover must be refastened before the mains connection is made. The power for the transmitter and receiver is fed from the power pack through a five-pin socket on a short cable from the power pack and a five-pin plug on the cable connected to the transmitter. When the aerial and earth have been fixed up and the AC mains unit adjusted as described above, withdraw the mains lead from the suitcase and adjust the plug to suit any available socket or lamp-holder, but do not switch on. Plug the key and telephones (
headphones) into the sockets on the transmitter and receiver respectively. If the supply voltage cannot be discovered, adjust the power pack, as described above, for 250 Volt operation. Set the SEND.REC switch to SEND and the meter switch to SEN H.T. VOLTS. Plug the mains connector in and switch on. The meter reading should now be approximately 475 volts, the mains adjustment should next be set on the 215-235 volt tap, and the H.T. volts again read. This must be repeated on progressively lower settings if necessary, until the H.T. voltage reading lies between 450 and 500 volts. Switch off the current and do not finally connect up the power supply until ready to operate. The consumption is 60 watts when transmitting and 40 watts when receiving.
With an internal dissipation of 40 W and no ventilation, I suspect the suitcase would have warmed up nicely.  J.H.

Battery Power Pack  For battery operation a large capacity 6 volt accumulator is required. This is used to drive the battery power pack which will then supply the set when mains are not available. The current drain is 7 amperes when transmitting and 4½ amperes when receiving.

To change to battery operation:
(a) Switch off the AC power supply.
(b) Remove the 5-pin plug from the AC power pack and insert it in the 5-pin socket on the battery pack.
(c) Connect the spring battery clips to the accumulator terminals: red clip to positive (+) and black to negative (-)
(d) Put the switch on the battery pack to "ON". A faint hum will be heard from the pack if everything is correct.

NOTE. If the battery pack is set up like this before operating on the AC power pack, it will be possible to change over in a few seconds should need arise.

The original circuit was apperently designed by SOE's Station X in 1942 (according to "SOE - the scientific secrets",
Boys and Everett, Sutton 2003)
The circuit diagram I have is however dated "25-1-43, so the Mark 1 was evidently a revised design, and capable of 8 watts output as against the original 5 watts.  J.H.


* Valves ARP34 & ARTH2
* Frequency coverage: 3 - 8 Mc/s
* Intermediate frequency: 470 Kc/s
* Power supply: 250V, 25 mA; 6,3V, 1,2 A

Tuning Procedure

(a) Turn the knob marked SEND.REC on the transmitter to REC and switch on the current. After a few seconds a faint
     hum will be heard in the telephones.
(b) Set the CW-RT switch to suit the type of signal to be received.
(c) Set the TUNING dial to the frequency it is desired to receive and adjust the volume control unit until signal
     strenght is comfortable.


(a) Select a crystal of the desired frequency, or half the desired frequency should it lie between 6 and 8 Mc/s, and
     plug it in. (
N.B. Operation in the range 6-8 Mc/s is only possible by doubling  J.H.)
(b) Set the band-switch marked "3-4 Mc/s" - "6-8 Mc/s" to the position indicating the band in which the desired
     frequency lies.
(c) Set the CRYSTAL Tuning scale to 5.
(d) Set the AERIAL MATCHING switch to position 8.
(e) Turn the SENS.REC switch to SEND.
(f) Set the meter switch to GRID M/A and depress the key when the meter should give a small reading. This shows
     that the valve is oscillating.
(g) Turn the meter switch to AE M/A and keep the key depressed while making the following adjustments. (The key
     should, however, be released while the AERIAL MATCHING switch is being moved).
(h) Set the ANODE TUNING control for a maximum reading on the meter.
(i) The AERIAL MATCHING switch is now moved progressively step by step, while adjusting the ANODE TUNING
     controle to give a maximum reading at each step, until the position is reached beyond which no further increase
     in meter reading can be obtained.
(k) The CRYSTAL TUNING control is now adjusted for maximum reading. (The setting will, in general be found to be
     nearly correct at this stage if the previous adjustments have been carried out correctly. Large adjustments
     should only be necessary when using crystals lying close to 3 Mc/s or 4 Mc/s).
(l) The key is now released and transmission may commence.


With certain combinations of frequency and aerial it may be found that the method of tuning detailed above in sections (g), (h) and (k) gives meter readings which are inconveniently small. The following procedure should then be adopted:

(m) Set the meter switch to M/A (the meter now reads anode current) and keep the key depressed while making the following adjustments.
(n) Rotate the ANODE TUNING control until the meter reading "dips" to a minimum.
(o) The AERIAL MATCHING switch is now moved progressively step by step. At each step the ANODE TUNING control
     is adjusted to give a minimum reading on the meter until a setting of the switch is found at which the value of
     this minimum is as large as possible.
(p) Leaving the ANODE TUNING and AERIAL MATCHING switch set, adjust CRYSTAL TUNING for a maximum meter
(q) The key is now released and transmission commenced.


(i) The transmitter H.T. voltage, measured by the meter when the switch is in the SEN-HT position and with the key
    up, is proportional to the input voltage. The readings can therfore be used as an indication of the state of the
    battery when an accumulator is being used as the source of power. The grid current (GRID M/A) gives a measure
    of crystal current.

(ii) It is advisable to switch to receive at all times when not actually transmitting, and in particular not to keep the
     key depressed any longer than can be avoided.


The people operating this equipment would not have had much training in setting up readio transmitters, and it's interesting that it was felt necessary to include a section on aerials.   J.H.

The most usual type of aerial is the so called Marconi aerial which consists of a length of wire, one end insulated and the other end attached to the aerial terminal of the transmitter. A second length of wire joins the earth terminal of the transmitter to a conduct entering the ground. In this type of aerial the main losses are due to resistance of the earth connection and every effort must be made to obtain an earth connection as possible.
The rated power of a transmitter is the power it will deliver to a suitable aerial but the power delivered to the aerial is not the power raqdiated by the aerial, which is always less and unless the earial-earth system is efficient may be very considerably less. The remainder of the power is dissapated as heat in neighbouring objects such as walls, etc and in the ground.


If a long wire, insulated at one end, has the other end attached to the aerial terminal of the transmitter, an alternating current is produced in the wire, the amplitude of which varies along the wire. For a long wire the current reaches a maximum at a distance of a ¼ wavelength along the wire and then decreases.
Since the power radiated is proportional to the square of the current, it is clearly desirable to have at least one current maximum occur somewhere along the aerial. The shortest aerial which can be considered reasonably efficient is a quarter-wave aerial.

FREQUENCY            3Mc/c         6 Mc/s       12 Mc/s    16 Mc/s
WAVE LENGTH      100 metres   50 metres   25 metres  20 metres
¼ WAVE LENGTH      25m           12½m          6¼m         5m


The resistance of the earth connection usually varies from about 10 ohms, obtained when the earth wire is soldered to a main water pipe near to the ground to about 100 ohms, obtained from a moderate earth connection. The amount of power that can be afforded to be wasted when using suitcase sets is small, it is therefore never any use trying to use a shoter aerial than 1/8 wave and this only in conjuction with a very good earth.

Earth resistance in ohms                                                              10       50     100

Radiated power as a percentage of the power in the aerial   ¼ wave   80%    44%   29%
                                                                               1/8 wave   50%    16%    9%
                                                                             1/16 wave    20%     5%  2½%

It is not usual practical to erect a vertical ¼ wave aerial although this would be very efficient, but at least this length of wire and more if possble should be erected with a long vertical or rising proportion and the top bent in one way towards the horizon as in an inverted L. The exact length of wire is not critical as the transmitter is matched to the aerial in use the tuning operations.The whole should be left well awat from aerthed objects such as buildings, cliff sides, surrounding trees, etc and the end not attached to the transmitter should be insulated. In dry weather the rubber covering of the wire will be sufficient insulation but in wet weather it would be better to use an insulator. An old bottle neck may be used for this purpose.


This part of the ducument has been damaged at some time in the laqst 75 years, and the words in square brackets represent my best guesses of what the writer intended.   J.H.
[A good earth] is most important. The ideal would be to solder a short length of wire to a [copper pipe] buried in moist earth near to the transmitter and to attach the free end of the [wire to the earth] terminal of the transmitter. Falling this, a copper earth tube, a large coil of barbed [wire that has been] well scraped, or some such metal receptical could be buried instead, but it is most [important that the object] attached to the earthwire should be clean metal, a good electrical con[nection] should be made and that the ground should be moist. [Using] a water pipe may be convinient. Choose a cold water pipe near to the ground if [possible, rather] than a hot pipe which may be loosely attached to dry walls in several places before [making] a good earth connection. Scrape the pipe clean before attaching the earth wire. [When no water] pipes are available a length of wire arranged in a zig-zag fashion or a piece of wire netting [may] be placed underneath the floor covering and attached to the transmitter by a short eart wire. An efficient counterpoise earth may be made by arranging a wire of about the same length as the aerial wire, and insulated from earthed object underneath the aerial wire and 2 or 3 feet above the ground. If indoors the counterpoise earth should be on the floor-perhaps under the carpet and well separated from the indoor aerial wire.

                                                               © John Hearfield, 2011

The wireless Set A was developed by SOE in 1942 for use by British agents operating in wartime Europe. This page gives the circuit diagrams and part values listed in the used manual. I have talen the opportunity to highlight some obvious mistakes in the circuits - the draughtsman (
John Brown) was probably working under some pressure      J.H.

The receiver consisted of an RF stage (EF39) followed by a mixer/oscillator (ECH35), an IF stage (EF39) and a detector (EBC33) driving headphones. RF and oscillator tuning was by a 3-gang 500pF vaiable capacior. The IF stage was the usual 470 kHz. The CW/RT switch S1 presumably narrowed the bandwidth when receiving morse transmissions. The pot R13 must have been the front-panel GAIN control, and it seems to do this by varying the screen-grid voltage for the valves V1 and V3. Construction details for L1 and L3 are not given. It seems clear that R19 should be connected to earth- the draughtsman omitted a blob.

The transmitter was designed soley for use with the morse key. It used an 807 as a crystal-controlled oscillator running between 3 and 4 MHz, feeding the antenna through a tuneable tank circuit. The 807 only drew cathode current when the morse key was pressed.
This circuit is a little difficult to follow (and I think includes mistakes around the switch S5  J.H.) so I've redrawn it here. I think the short-circuit across the bridge used in measuring antenna current is probably also a mistake. No details are given of coil construction, nor of the values of the tuning capacitors C30 and C31. The
'TX wave-change' switch S4 allowed the oscillator to run as frequency doubler, giving an extended frequency range of 6 to 8 MHz. The 550Ω pot R29 was presumably factory-set to suit the particular antenna-current meaqsuring block (T2) installed. The full-scale deflection of the milli-ammeter connected to S5 is not specified, but R30 is 1watt, which suggests it was intended to carry around 20mA (at 0,6 watt dissipation); but the voltage drop across R30 would then be only 30 Volt, which makes no sense. If HT voltage measurement was done via a dropper resistor R30 and a 25 mA meter (say) then R30 should have been closer to 18k 7W.It's not obvious to me hoe HT voltage was measured. Perhaps the milliammeter had an unusually high resistance, or the values given for R24 and R30 are incorrect. It would even have been possible to use the 807 screen-grid dropper resistors to derive a current in the meter proportional to HT voltage (with the key not pressed) but the designer didn't do this. It's a mystery.

The transceiver could be powered from an AC mains voltage of 100 - 250 Volt, 40 -60 Hz by selecting the correct tap on the mains transformer primary. A reversible plug P1 allowed range selection of 100 -150 oe
200 - 250 Volt. The 5Z4G rectifier valve V6 provided the HT+feeds for the transmitter and receiver through fuse F1 (value not specified, probably 100 mA). Filtering was provided for the transmitter HT by a single 8µF electrolytic, and for the lower receiver HT by an additional 5k resistor and 8µF electrolytic. The dropper resistor was rated at 6 W, suggesting it was intended to carry a current of 20-30 mA. The user manual confirms that the receiver drew 25 mA at 250 Volt. The transmitter drew 55 mA at 360 Volt. Heaters were fed from a single winding on the transformer. The total load was 2,3 A at 6,3 Volt, according to the user manual.

The system could alternatively be powered from a 6 Volt battery, using a vibrator to generate HT. The RF chokes are presumably sized to carry the currents required - around 3A for the type 1 and 100mA for type 2. The 3 Watt rating for R33 seems too low -25 mA receiver current would give a dissipation of 3,1 Watt.

                                                           © John Hearfield 2014

DATA SUMMARY by Louis Meulstee & Rudolf F. Staritz uit 'Wireless for the Warrior Vol. 4 Clandestine Radio.

Organisation: SOE
Design: John I. Brown
Year of introduction: Late 1941 - Early 1942
Additiional Data: Mechanical design based on Army No. 18 Set transmitter unit.
Weight: in suitcase 13,2 kg


Wireless set 'A' Mk.1 is one of the very first agents suitcase sets produced by the SOE organisation. The set is self-contained and consists of four main units: Receiver, Transmitter, Mains Power Unit, and Battery Power Unit. The units are secured in the suitcase by a metal framework.
The SOE 'A' series were intended for ranges under 800km. Although much better known by the operational name, their designworkshop designation was Type-21, thus the A Mk.1 and the A Mk.1*; could also be designated Type 21 Mk.1 and Type 21 Mk.2, etc.
The construction and design of the transmitter and receiver is based on the British Army Wireless Set No.18 Mk.3 which was at that time in full production and using readily-available components. The changes to the original No.18 Set units are numerous, for example the replacement of the 2 Volt battery type valves by 6,3 Volt filament types. In addition, the receiver unit has a tuned RF stage and the fitting of a separate CW-RT switch. The transmitter is completely rebuilt rebuilt using some of the original components such as the meter, variable capacitors, etc and has some external resemblance to the original No.18 Set transmitter.
A later produced version is the Wireless Set A Mk.1* which has minor circuit changes.