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The saga of the Crystal Set Recently I was taken to task because, in its six years, this 'Vintage Radio' column has not yet featured crystal sets. The point has been taken, and this month we cover some aspects of this classic and in its time, most important receiver. The basic crystal receiver comprises laity. Crystal sets were inexpensive, . Braun reported that only three components: an , a simple to make, cost nothing to run and various combinations of metals with ox- and a headphone -- although to be what is more, they worked. ides or sulphides, with small contact ar- completely accurate, there are also two There is no receiver simpler than the eas, displayed unilateral electrical virtual formed by stray ca- basic crystal set, yet an endless series of Conduction characteristics. pacitance of the aerial and the headphone circuit variations appeared in radio and As with many early discoveries, here leads. It would be impossible to create a hobby publications and were eagerly as- was a solution waiting for a problem. simpler receiver than the elementary similated — previous models torn apart Braun went on to other work, including crystal set, yet entire books have been and the new variations tried. Many cathode rays, and it was a further 25 written about it. What other type of re- young experimenters got 'bitten with the years, in 1901, before he conducted ex- ceiver has remained basically unchanged radio virus' as a result, and went on to periments into the use of crystals for ra- for 80 years? make radio their careers. dio detection. He worked with galena In the period from the advent of broad- Like much electronics technology, the (lead sulphide) and iron pyrites, but it casting to the arrival of the cheap transis- origins of research in the electrical char- was psilomelane, a mineral containing tor receiver, making crystal sets was one acteristics of crystals go back to the 19th manganese, with a sensitivity equal to of the traditional childhood activities. century. The earliest work of which I can that of the electrolytic , that he There were several reasons for its popu- find any record was by Ferdinand Braun considered showed the greatest promise. in Germany during 1874, in what could Braun reported his findings to Tele- be regarded as the first research into funken, but there seems to be no record Apparatus From a Contact Stud to a 16" Spark Coil can be obtained from us. eit vs r i. oErEC "r0

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• : : :...... •15,qr. I • • - - ANTIQUE. E LEL, 1-IONIC P17 ,G E 4 A - loco - 26vo -,,-,.. Fig.3: The British GEC variometer • tuned Gecophone crystal set Nol of C; ... Fig.4: In 1919, US firm Quaker Oats f 1922, in a dovetailed mahogany case. .., o ri, ., - a- offered this novel , based Ga, ... • Many sets of this era were similar in • on one of its cardboard oats cartons, appearance to scientific instruments. for $1 plus two carton labels. By 1921, Fig.2(b): The various configurations of they had issued 250,000 sets. It was the versatile Marconi multiple tuner — featured on the cover of Antique much more complex than the domestic Electronic Supply's 1991 catalog, crystal sets of a decade later. reproduced here. of that company or anyone else making encouraged widespread research into practical application of his work. crystal rectification, and in the period up The noted Indian scientist Sir Jagadis to World War I a large variety of exam- Chunder Bose, also in 1901, patented a ples appeared. The more popular detec- detector of electromagnetic radiations, tors incorporated molybdenite, galena, including Hertzian waves, which used a bornite, zincite, iron sulphide and tellu- pair of galena crystals in contact. Bose rium. Some of these depended on metal- considered that his detector worked as a lic contact, whereas others used two variable resistance self-resetting , different crystals in contact. but no further development seems to Some combinations were sold under have followed. trade names. These included the Perikon (chalcopyrites and zincite), Pyron (iron Various detectors pyrites and silicon) and Bronc Cell (tel- Meanwhile, radio communication had lurium and graphite). Fig.1 illustrates a developed into a commercial enterprise detector assembly using a zin- depending for reception on magnetic and cite/bornite combination. electrolytic detectors, and . Often, these crystal detector assem- Fleming's , an adaptation of the blies were switchable. One gets the im- 1884 patented Edison Effect lamp, were pression that there was a suspicion of used by the Marconi Company as detec- reliability about crystals. It was common tors from 1905. to mount two or more crystals in a turret, The first practical application of a so that rapid changes could be made. In crystal as a detector has been credited to other cases, the tuner could be switched one of Lee De Forest's employees, a re- to either a magnetic detector or a crystal. tired army general, H.H. Dunwoody. Fig.&: This 1924 novelty model, known A complete 1915 shipboard receiver cir- This was in 1906, the same year that De as 'Uncle Tom, was made of cuit with this feature is shown in Fig.2. Forest himself first experimented with Staffordshire china with the coil Rather than rectifying more or less the valve. wound around the top hat. evenly over their entire surface, a few Dunwoody had been experimenting crystals were found to have sensitive with silicon carbide, or carborundum, vice; he found too that a polarising bat- spots that worked best when touched which is made by heating a mixture of tery increased sensitivity considerably. lightly with a piece of fine . This carbon and silica with an electric arc, and By applying a bias , the forward piece of wire came, of course, to be whose crystals approach diamond in conduction point of the crystal was called a `catswhisker' and one of the hardness. He developed his detector to brought nearer to zero voltage. We see most sensitive crystals was a piece of the point where the crystal could be this same characteristic today in the for- galena, a common lead ore. One source clamped between steel electrodes, creat- ward voltage drop of silicon diodes. of high grade crystals was the American ing an extremely rugged and stable de- Carborundum's success seems to have town in Northwestern Illinois actually

ELECTRONICS Australia, July 1994 99 Antenna The situation was different here and in VINTAGE RADIO America. Income was higher, and a fair Detector percentage of the community was remote called Galena, where at one time, a thriv- from transmitters. Even so, both com- ing mail order business was carried on mercially made and homebuilt crystal, with radio enthusiasts. sets were popular, a remarkable Ameri- Unlike carborundum, the galena crys- can example being the Quaker Oats pro- tal had the advantage of not requiring a motional model featured in Fig.4. biasing voltage to increase its sensitivity. Locally, firms like Emmco and Radiokes Biasing added the complication of a bat- manufactured components to meet the tery and rheostat, and the operator hav- needs of enthusiasts, and Wireless Weekly ing to remember to switch the current off provided them with plenty of projects. after use. Only so much could be done to im- The modern diode is a Fig.6: The fundamental circuit used in prove a simple crystal set, but one way of direct descendent of these detectors, and the five crystal sets illustrated. improving reception was to add valves. the point contact germanium diode can Although as simple as you could get, For the Brownie user, a matching two- be seen through a magnifying glass to be its weakness is lack of selectivity due stage could be attached to pro- fundamentally a catswhisker in contact to heavy damping of the tuned circuit. vide loudspeaker reception. In many with a tiny piece of crystal. cases, a crystal detector was used in con- A crystal set's performance is vitally junction with valve RF and AF amplifi- Unsuitable at sea dependent on its tuning system. The ers, and reflexing was common. Despite its good performance with complex Marconi tuner in Fig.2 is capa- A different approach was the 'electro- weak signals, the galena crystal was not ble of superior results, but would have mechanical ', in which a headphone popular in marine service. As anyone been quite out of the question for a do- receiver mechanism was coupled to a mi- who has. 'tickled' a crystal will know, a mestic receiver. It required considerable crophone. Energised by dry cells, there slight bump is sufficient to dislodge the operational skill and the cost would have was sufficient power for a horn speaker. catswhisker; obviously, on a ship in a been quite prohibitive. In fact, in the hey- One well known model came from the storm or with gunfire, this fragility day of the crystal set, the listener who English firm of S.G. Brown, who would be hopeless. could have afforded a Marconi tuner or adapted their telephone repeater relay de- Another weakness of the galena crystal similar would have been able to buy a veloped about 10 years earlier. is that it can be paralysed by strong valve receiver anyway. American practice generally was not to nearby transmissions and atmospherics. bother with crystals in valved receivers. However, its simplicity and relative Affordable radios To increase performance of simple re- sensitivity made the galena crystal popu- Manufacture of crystal sets for radio ceivers, regeneration and reflexing were lar with amateurs, who were quite pre- listeners boomed in the early and mid used extensively, but by 1925, the classic pared to fiddle with catswhiskers, 1920's, especially in England. The book TRF with two RF stages, a grid-leak de- forever looking for more sensitive spots. Vintage Crystal Sets 1922-1927 by Gor- tector and two audio stages had become When broadcasting commenced in the don Bussey lists more than 200 different practically universal. In our part of the early 1920's, the galena crystal receiver models and their makers. world, there was the whole range of re- played a significant part, especially in The remarkable popularity of the crys- ceivers from the superheterodynes right areas within a few miles of a transmitter. tal set in England was due to two factors. down to the home built crystal set. The galena crystal did not have the One was the proximity of a large number Contrasting with the Marconi installa- monopoly in home receivers, though. of listeners to a transmitter; the other was tion in Fig.2, amateur and broadcast Some proprietary crystal assemblies, economic. Wages in Britain were low, crystal sets were not very complex. In- such as the popular Red Diamond, used and a crystal set was all that many could deed, although they were made at very other crystal combinations and were afford. Furthermore, anyone with a small different times and places, the receivers somewhat more stable. degree of skill could make one. in Figs.3 - 8 have the same simple unse-

Fig.7: The Brownie Wireless Co. produced several crystal Fig.9: Variometers were frequently used as variable sets, their No.2 having an ebonite case. The coil is inside, for tuning in the better grade sets, such as the tuned by a slider between the earphone terminals. For long GEC model in Fig.3. They could also be used to provide wave coverage, an additional coil plugged in at the top. variable coupling in double-tuned sets.

100 ELECTRONICS Australia, July 1994 found that there was about a 3dB im- provement, which is just discernable, in rectified voltage by applying about 0.75 volts of forward bias. For extracting the last fraction of performance this is prob- ably worthwhile. For someone who wants a bit of variety, and a challenge, there is still scope for experimentation with the humble crystal set. As I indicated at the beginning, al- though the crystal set is a deceptively simple device, there is a lot that can be written about it. But I suspect that if I carry on much longer at this stage, the Editor may reach for the delete key! (Editor's Note: For those who do want to experiment with a crystal set, I described Fig.8: Plastic may have been substituted for ebonite and wood, and the crystal a very flexible model back in June 1988 diode is non-adjustable, but today's basic crystal set has not otherwise changed — the 1988 Deluxe Crystal Set. This in- in 70 years. This one comes as a kit from Dick Smith Electronics.

lective circuit, and this was to be their downfall. While there was only one transmission in the area, there was no problem; but if more stations opened, the Fig.10: The expert's inherent lack of selectivity became all crystal set was often built around a loose coupler, too apparent. which could be up to a I confirmed this by tests with the metre in length. Both Brownie and the Dick Smith Electron- coils were of variable ics receivers. With five local transmis- inductance, via switched sions of similar strength spaced fairly taps or a slider. The evenly over the broadcast band, it was position of the inner coil not possible to separate the stations allowed adjustment of the completely, the best compromise being coupling — not a thing of an annoying background from at least beauty, but very effective. one interfering transmission. Huge variety The solution was of course, improved quency band. Too close a coupling cludes two tuned circuits with 'preset' selectivity; but the problem was to passes a broad range of frequencies, adjustable coupling, and also adjustable achieve it without sacrificing precious whereas minimal coupling causes ex- DC bias for the diode. Photocopies of the signal strength. Tapping the aerial and cessive attenuation. article are available from our Reader In- crystal down the coil would certainly re- We are familiar with this charac- formation Service.) • duce loading, but at the cost of volume. teristic in superheterodyne IF trans- For the experimenter, an endless vari- formers. As these operate at one ety of clever circuits now followed, each, frequency, the spacing of the coils can one claiming to be the answer. It is amaz- be fixed, but for a crystal set which has ing the remarkable number of variations to tune a range of frequencies, the cou- that were created around the simple crys- pling has to be adjustable. tal set. Virtually every issue of the nu- Two popular methods of controlling merous radio magazines that came on the coupling were the variometer and adjust- scene had some new design or circuit able coils mounted on swinging arms. configuration. Few, however, ap- Very impressive crystal sets were also proached the Marconi Multiple Tuner in built around the loose coupler. With complexity and versatility. skilled operation, the loose coupler can In fact, there is a way of combining produce quite remarkable results, but selectivity and sensitivity, and is the prin these are rather large technical looking ciple of operation of the Marconi tuner. devices, some approaching a metre in This is to have two tuned circuits, criti- length — although the one illustrated in cally coupled with one tuning the aerial Fig.10 is only half this. system and the other feeding the detector. Modern germanium or diodes are very Given large, high Q coils each tuned efficient, but theoretically they could to the operating frequency, and accu- benefit from a small bias voltage. Re- rately coupled to each other electrically cently R.G. Newlands of the NZVRS or physically, it is possible to have an carried out some quantitative research efficient transfer of one narrow fre- into biasing germanium diodes and

ELECTRONICS Australia, July 1994 101