What is a Paraset?

A Paraset is a small low powered CW transceiver that was developed during World War II by the British SOE (Special Operations Executive similar to the our OSS now CIA). The unit was about the size of a shoe box and was frequently air dropped thus the name Paraset. The transceivers were used mostly in France, Holland and Norway by Allied agents behind the enemy lines.

The Paraset consisted of a two tube regenerative receiver employing two 6SK7 vacuum tubes and for the transmitter a single 6V6 tube used as an oscillator. The frequency range on the receiver was roughly 3 to 8 MHz in a single band and the transmitter tuned approximately the same range in two bands. The "bandswitch" on the Paraset worked only on the transmitter portion and essentially added a 100PF capacitor across the tank circuit.

The antenna consisted of a hunk of wire plugged into the set and the transmitter tuning was rather unique in that two low voltage light bulb indicators glowed to indicate tank resonance and that RF was flowing into the antenna. It appears that optimum tuning is achieved when they both simultaneously glow at a maximum brightness. Typically 5 to 7 watts of RF would be generated using a 350VDC supply. Some of the Parasets came with vibrator power packs so that the radio could be operated from a 6VDC auto battery.

My project was to build only the transmitter portion and to see how many contacts could be made. There were two versions of the transmitter schematics and the first version used a classic Colpitts Oscillator that did not perform very well. Now I can see why the second version as that has proven superior relative to chirp and stability. With a 300VDC supply and a 6V6, I got about 2 watts out of the radio, including lots of key clicks and a somewhat touchy load control. With a 6L6GC in place of the 6V6, I got about 4+ watts; but the signal is not of the quality that I am used to with a well designed modern solid state radio. The transmitter is of course crystal controlled and for the spy's that is probably OK. But with only two or so crystals that limits the QSO opportunities.

While there are many references on the Internet to the Paraset and in fact many clubs dedicated to faithfully replicating the unit, one piece of information that is lacking is specifics about the tank coil. In one reference there is a drawing (in French) but the coil diameter is not given. With a little reverse engineering I determined the coil to be 1.5 inches in diameter and the original tank was wound with #15 wire which is not common. My final coil is 18T of #16 for the Tank and 20T #18 plastic covered bell wire for the aerial coil. The light bulbs are #52. The coil form is a piece of 1.5 Inch diameter plastic sink drain cut to about 4 inches long.

I did get some experience in building an aluminum chassis (6"X5.5"X2") using my Harbor Freight $28 metal brake and that turned out pretty nice.

As a critical life line for the spy behind enemy lines, the Paraset was an important piece of equipment. I suspect the simple circuits that were employed also enabled field repairs should that be necessary.

I did make one contact so far on 40M with a station in California at a distance well over 600 miles (that is with the 6L6GC in the radio). Would I regularly use this radio --probably not! But I did have all of the parts in the Junk Box and so there was no cash outlay. Those contemplating building a paraset would do well to realize that this is 1938 technology and there are limitations. But that is part of the buzz --making contacts with minimalist radios! Enjoy, de W6JFR!