At Glance
Since I am Amateur Radio, i want to build the Paraset but you can imagine located here in Borneo i am far away from everything and to find parts and components here is simply impossible . I can order online but the shipping cost to Brunei is simply crazy and in addition the Postal services are very poor . Most of the time, mails are simply lost and the only way to ensure you will receive it on time is to use DHL or FEDEX but the shipping cost is even higher and totally insane.
The Paraset Project had started about 2 years ago and from time to time i manage to collect all missing components . The initial idea was to build it as the original but this task was by far too tedious and i will say almost impossible when you are located in the middle of the Borneo Jungle hahahahahahahaha.
50% of the build was using genuine vintage components but the rest was with moderns one (resistors, Capacitor, variable capacitor etc etc…..)
The build itself took about a month. According the initial tests i realize very quickly i will say poor RX performances and i notice a lot of builders have the same issue. The TX part no issue at all and currently i manage to have about 10w output key down on 40m band.
According the RX issue and poor performances i did many tests with some tiny modifications but without a tangible success and according the final RX modification in fact it is simple.
According the coupling capacitor which is 100pf in the original design i have tried with lower value likely 50 and 30pf with really good result for strong signal and the 100pf when the signal is weaker , I have simply implemented a mod on the rotary switch , pretty much like the Bandspread mod and now for RX i have 4 positions using alternatively the 100pf , the 30pf,50pf or the 150pf as a coupling capacitor.
The other modification i did is regarding the TX and for the keyer i remove the 220 ohm resistor and instead i am using a choke inductance about 2.5mh .i also replace the 20k ohm grid resistor (the one close to the Xtal) with a 4.7K ohm resistor. During the TX no chirp at all and output power is really great (8W key down and about 10w peak during key swing).
Currently the power supply is providing about 360vdc and i am using also DC voltage for the heaters/filaments.
For the 20K regen potentiometer i am using a multiturns one and make the RX tuning very easy.
The Paraset is now fully operational and manage to contact easily Asian countries. i can ear European countries with a really good signal but not contacted yet .
History
The Paraset (Whaddon Mark VII) was a vital, compact, low-power Morse code spy radio used by British WWII agents (SOE, MI6) dropped behind enemy lines in Europe for clandestine communication with the UK, known for its simplicity (few valves, no meters) and efficiency, allowing agents to send vital intelligence despite German detection efforts, becoming a legend and inspiring modern amateur radio builders.
The Legacy
The first reference to a Paraset appears in a Second World War BCRAM report about the state of the nascent radio communication networks in France during the second year
of the German occupation.
The Bureau Central de Renseignements et d’Action Militaire (BCRAM) was the intelligence wing of the Free French Forces, then exiled in London, and this report was written for them towards the end of February 1942 by Captain Pierre Julitte.
It was published in March 1942. The author has not been able to find a copy of the original report in French but a few British authors, including Pat Hawker G3VA, have commented on
it so often over the years that enough of the translated version has been published to get the gist of the report.
It included some fairly strong criticism of the agents’ sets
supplied to the BCRAM by the British Secret Intelligence Service (SIS) Section VIII at Whaddon Hall [The BCRAM dropped ‘Militaire’ from its title in September 1942 and is
generally known these days as the BCRA]. These same agents’ sets were being supplied to the Special Operations Executive (SOE) at the time, as they were also dependent
on SIS Section VIII at Whaddon Hall for wireless communications and radio equipment for the first two years of their existence.
Origins & Purpose (WWII)
- Development: Created in secret workshops at Whaddon Hall and Little Horwood, England, for Britain’s Special Operations Executive (SOE) and MI6.
- Name: Called “Paraset” because it was dropped by parachute (Para) for field agents.
- Function: To establish two-way, low-power (4-5W) Morse code communication, vital for espionage in occupied territories like France, Norway, and the Netherlands.
- Design: Extremely simple, featuring just three valves (tubes) and minimal components (no meters, no on/off switch) for reliability and to avoid detection.
Key Features
- Valves: Used a 6SK7 for receiving and a 6V6 as a power oscillator for transmitting.
- Frequency: Covered 3-7.6 MHz.
- Portability: Initially housed in a suitcase with accessories, later in wooden or metal boxes to disguise it.
- Performance: Capable of reliable communication over hundreds of miles.
Legacy
- Rarity: Most original sets were destroyed post-war, but a few survive in museums.
- Inspiration: Its design has made the Paraset a popular project for amateur radio enthusiasts, who build replica kits to experience wartime radio operation.
The Paraset and Amateur Radio
A number of Amateur Radio operators build and operate replicas of the Paraset.
Therefore , the use of the Paraset during war time and the way Amateur Radio are using it is very different. Per design the Paraset is not offering the best performance for Receive and few adjustments are made by the Amateur Radio community in order to adapt the TRX for HAM radio bands.
You will be able to find a lot of website describing the build and hereafeter you can find a non exhaustive list of websites where you can grab some useful information for your build , thanks to the HAM Community sharing some precious information with us.
CREDITS
With the following websites you will find all useful information regarding the Paraset
https://www.sm7ucz.se/Paraset/Paraset_e.htm
https://www.wa8ywo.com/paraset.html
https://www.sm7ucz.se/Paraset_F6EJU/Paraset_F6EJU.htm
https://www.cryptomuseum.com/spy/mk7/
https://qsl.net/ve7sl/paraset.html
Paraset Warehouse
You will able to find a lot of parts to build your Paraset here :
Facebook and Google group
https://www.facebook.com/groups/763205137418553
https://groups.io/g/ParasetBuilders
The build
The Paraset is not a kit you can build following the step by step instruction. You will need to find all the component and will need to put together on your own all the parts.
Some genuine parts will be impossible to find and you will need to build yourself some parts since it impossible to find nowadays. For some you will need to substitute some parts with modern one.
Building the Paraset you are on your own with your components , parts and soldering station.
Building a Paraset is a long way to go , it is not a kit and patience is required in order to achieve a good performing TRX.
Therefore building this Iconic Transceiver is rewarding and achieving a QSO using it will bring a really nice pleasure and you really feel operating a Radio Transceiver during your QSO.
Homebrewing is not an activity from the past for sure!
Some Paraset TRX from Amateur Radio:
VE7SL Paraset
W8PU Paraset
Mk-VII – 2357 JC Buffet F6EJU
Presentation
Schematics
Original Schematic
Alternative Schematic
Modified Schematic
The main issue regarding the Paraset is this TRX was created for a totally different purpose and not really performed according the Amateur Radio Needs.
Hereafter some modification in order to enhance the following
- Output Power up to 10W and i can use all sort of Xtal without any chirp.
- RX selectivity and sensitivity using 4 capacitors value you can link to the rotary switch the Paraset is using to switch from TX to RX
Wiring
Credits to SM7UCZ
The Coils
Credits to F6EJU
Receiver Coil
My coils
GRID Resistor
The original Paraset Grid resistor is about 20k
The Main issue is have since this TRX is using 2 kind of Xtal mean HC49C and genuine FT243 is implying some tiny chirp and piew piew effect. The Chirps are not severe therefore in need to do something about that. With some Xtal , i have some IMD and the carrier was not perfectly clean .
I install a 20k pot in the grid resistor in the front panel and adjust for peak power output. The peak power resistance will vary with the tube power output, so first tune the tube for the desired power output. It is assisting to remove the chirp all bands and whatever the Xtal i am using.
In addition in order to provide a perfectly clean carrier i have added a RFC of 2.5mH in place of 200R Resistor and this really assisting to provide a pure and clean carrier.
Now the TX is clean and without chirps or Piew Piew all bands.
Notes
- Grid resistor. Traditional values were usually in the range of 20k to ‘èk, but some have reported that lower values (down to 4.7k) give better notes and are easier on crystals. Experiment! For lower values (maybe 22k and lower), add an RF choke (RFC1) of around 1mH. If RFC1 is not added it must be jumpered. In case just install a 25k pot in the grid resistor position and adjust for peak power output, then replace with a fixed resistor of equal or higher resistance. The peak power resistance will vary with the tube power output, so first tune the tube for the desired power output. After many testing all bands , the best value i found is from 4.7k up to 10k.
- Screen dropping resistor. Screen voltage must be within rated values at the screen current listed in the manuals. Use Ohm’s Law to determine resistor value and power rating.Adding a Choke in place of the 20k Resistor for about 350vdc common voltage is able to drop a bit the Screen voltage. Always remember that the key-up B+ voltage will be higher than the key-down voltage, and make calculations and choose resistor values accordingly. Use the minimum screen voltage that will deliver the desired power output. This is usually much lower than the voltage called for in most designs – very often 100V or less, very rarely over 300V. You can use 5W zeners to set regulated screen voltage. With an overall voltage of 355v using a chocke , I have about 290v for the screen voltage. With this mod , the carrier is by far cleaner, no chirp and assisting to provide about 10w output key down. I use this design for all my 6V6 Vintage style TRX.
- Adding a 220pf Capacitor to pin 8 Cathode & Beam-Forming Plates is assisting to stabilize the tube , make the oscillation great without chirps and incresing the output power. With this capacitor , the output power was increased to almost 2w. Since the Pin 1 is grounded i have installed to 220pf capacitor between Pin 1 and 8.
- Cathode RFC. Values of 1mH or 2.5mH are common. A large pi-wound RFC is not required here. It can be a physically smaller RFC because not much RF voltage is developed across it. Just make sure it can handle the tube cathode current.
- Plate RFC. Values of 1mH or 2.5mH are common. This should not be a small device because significant RF voltage is developed across it. Use a traditional pi-wound RFC.
Sidetone : Piezo Buzzer
Since i am using mostly the Paraset with a BUG , I have implemented a Piezo Buzzer in order to have a kind of sidetone
Credits to ON7WJ
Bandspread mod
I am located currently in Brunei and unfortunately the 80m band is not usable since i have a really huge noise floor. I never use the 80m band.
So in order to enhance the RX performances i have implemented the Bandspread mod proposed by VE7SL but only for the 40m band.
My Bandspread mod only for 40M band
POWER SUPPLY
Spectral Transmit purity
Homebrewing is nice but at the end when you achieve a TRX or Amplifier , Amateur Radio we are deemed to ensure the TRX is transmitting nicely without generating spurious and nasty harmonics
As i said previously , i put a lot of effort on the Transmit part in particular regarding the power supply purity , the PI network and tube polarisation.
For a maximum output power of 11w, the H2 is about -54dB which is really good .
PICTURES AND VIDEOS
CONCLUSION
After almost a year collecting all the parts and as you can imagine it was not easy when you are staying in Borneo , it is not easy to collect and to find the parts .
The main dilemma was : build a really genuine one but not really adapted for HAM purpose or to build one i can use on Ham frequencies in particular in 40m I have decided to build one i can use and made some adjustments and tiny modifications .
The power supply is able to provide 360VDC and with some adjustments i manage to achieve about 10w output .
The main difficulty was also to adjust the TX regarding all sort of Xtal especially the moderne one and now the output is really clean , no chirps .
For the RX i have added the the Bandspread mod and the coil is tailored to provide the best efficiency on 40m band .
In addition I am using 4 coupling capacitors from 30pf up to 150pf link with the 4 positions 2 poles rotary switch . according the band conditions and the signsl strength, i can use the suitable RX coupling capacitor .
I add few decoupling capacitors and a an 100uf Electrolytic capacitor on the High voltage input .
I have removed to the gimmick mod since for modern Xtals i have few issues and a huge chirps . instead i add a 220pf capacitor on the grid .
For the Keying i am not using a 200R resitor and replace it with a 2.5mh choke .
On 40m i am using my Vintage style homebrew TRX only and obviously the Paraset almost every evening .
Homebrewing is not an activity from the past for sure !
F5NPV - V85NPV 