The G3XRT Crystal Set

A classic 30's design

We set out on this project with the aim of building a crystal set that closely matched the designs of the 1920's and 30's. We've all built crystal sets at some time that have used a modern semiconductor diode, but we wanted to build a set using an original style of "crystal" detector. A Germanium diode just wouldn't do in this project! Our receiver is built up on a wooden base with a hardboard front panel. This "breadboard" method of construction was popular at that time.

Building the detector

The vital part of the project! The detector was assembled from materials that were to hand. Any worthy amateur junkbox should hold the parts needed. The "catswhisker" is a bristle from a brass brush. Make sure that it's a true brass bristle, and not just brass plated iron! This is soldered to a length of brass studding. The size of the studding and nuts you use is up to you, anything is fine as long as it's solderable and fits! The size of the spring is nothing special either, as long as it slides over the studding and meets the locknut/spacer at either end. In fact, the Pyrite is the only important piece in the puzzle. The construction is really up to you - As radio amateurs we should feel free to experiment! The knob is there to provide "tickling" of the catswhisker for best audio output. Again, anything that fits will do!

The crystal

The heart of the detector is the crystal, for which we used a Pyrite. Pyrites are popular with crystal healers and should be easy to come by. Metaphysical, New Age and Occult stores are ideal places to obtain one (Check your Yellow Pages). It shouldn't cost more than about £1.50 ($2.40). The Pyrite we bought would supply several crystal sets! Pyrites are generally heavy for their size, and have a brassy colour and a metallic lusture.

Once you've got your Pyrite, you'll need to carefully break it. Ours was broken with a pair of heavy pliers. EYE PROTECTION IS RECOMMENDED - TAKE CARE WHEN BREAKING THE PYRITE! Note too that Pyrites can spark when struck with other ferrous materials. You'll need a piece of Pyrite about a 1/4" all around, though any reasonable size works okay. This needs to be fitted into a copper "cup". The cup is really just a square box made from flat copper or brass strip. Just make and solder up a box big enough to hold the Pyrite comfortably, then drill a hole for a bolt in the end. The bolt will need to be long enough to pass through your bracket. Fix the bolt with a nut and washer, as you'll be filling the cup with solder and won't want to mess up the thread. The next stage is to fill the cup with solder. We used a high temprature iron for this and a large amount of 20 gauge solder! The trick is to fill it enough to fix the Pyrite in place and make a good connection. When you've got enough solder in there, carefully place the Pyrite in the cup and allow it to cool. Don't forget to fit a couple of solder tags onto the brass angle pieces to connect the detector up.

Another way of fixing the crystal in place is to use a brass holder. A brass boss from a knob could be useful. Another version of the detector used a brass bush as a holder. This was tapped and had three grub screws fitted to secure the crystal.

The coil

The coil that we used in our receiver is a "basket-weave" coil. These are easy to make, self-supporting and work well. To start, get a thick piece of wood and mark a 50mm (2") diameter circle on it. Next, mark out the points for 11 nails that will need to go into the block for winding the coil on. Using a protractor, draw angles of 33 degrees all around the circle. This will ensure your nails are correctly spaced. Tap the nails in far enough so they don't bend when you wind the coil, but not so far in that you can't remove them afterwards. A coil of 75 turns will cover most of the AM band. You'll also need to provide tapping points at 25 and 50 turns along the way. Wire gauge isn't too critical, we used 26 SWG enamelled copper wire for our coil.

To start the coil, take and twist the wire around any nail. Next, take the wire inside the second nail, outside the third and then inside the fourth, and so on. When you've made the 75 turns, carefully bind the coil in six places with some cotton, remove the nails and that's it. You now have an authentic basket-weave coil!

It's ready to go!

The set we built works very well. Don't forget that a good antenna and ground connection are essential. A long piece of wire will be fine for the antenna and a copper tube or better still, a proper earth rod buried in the soil will be fine for a ground. Don't attempt to connect your ground to the AC mains supply earth/ground connection. There are plenty of high powered AM transmitters in our area, and we had no problems tuning in five stations. We even had a German and Italian station coming in clearly at one time. High impeadance headphones will be needed for this receiver, but we used a seperately built audio amplifier which worked fine. The detector will need a little "tweaking" to get the highest audio output from the receiver, and with care you'll find a good spot on the crystal. The circuit capacitance tends to change when the detector is altered, so expect some re-tuning! This crystal set turned into a really entertaining club project with some other members having a go at building their own. We hope that you find it fun too - Happy listening!

Some feedback!

We were recently contacted by Ray Andrade, who has built one of our receivers and has made some suggestions to improve it. Ray used a Galena for the crystal in his detector and suggested connecting the antenna to the 75 turn tap on the coil to get the tuning range lower (Ray's set tuned down to 640KHz afterwards). He also fitted a 470K shunt resistor across the 1000pF capacitor to discharge it as the Galena and some other crystals may not provide significant leakage. Ray says that any value between 70K and 500K could be used. Ray's set also tuned in five stations, so now we can say with confidence that our design is repeatable! Our thanks go to Ray Andrade for his suggestions.

Some further thoughts

We received this e-mail from Ian Johnson in the Antipodes after he read this page. We thought Ian's reminiscences were worth sharing with you, so here's a big 'Thank you!' Ian for letting us reproduce your thoughts here.

A perverse whim made me search for "crystal set" on the net and I was most intrigued to find your page dealing with the ancient version. Well, when I was a pup, my father told me of such beasts but it took a few years before I allowed myself to be lured into such a game in which said father was convinced to obtain the pre-made components for a crystal set. It all fitted together and worked without much effort or care on my part until a few months and the normal attendant changes in a boy made me more curious. After that I became interested in my father's recollections as a signal man in WWII, which led back to his descriptions of the crystal-set receivers which he contrived to make work in his adolescence.

Your description deals with pyrites as an agent of detection but my father's prescription was for Galena or a Gillette razor blade touched with a sharpened piece of copper wire (preferably tinned). Where the devil a poverty-stricken lad in the last years of the Depression was able to obtain galena crystals I was never able to satisfactorily ascertain; but I was able to achieve the same. The efficiency was nothing like a germanium diode but to make a radio receiver out of a coil of wire, a remnant variable capacitor and a high impedance ear piece coupled with a discarded razor blade and a sharpened, tinned wire, I still count that as one of my greatest technological achievements.

I am now paid large amounts of money for lesser things; or perhaps I am paid for the ability to see and make work what others can or may not. I would still recommend the humble crystal-set as a good place to start for a budding technologist; especially now that radio technique (wireless) is back in the high-tech sphere. Anyway, that's my two-bob's-worth.