We now come to the baby of the DX family, the smallest of a singularly successful series of gadgets and receivers which have made quite a name for themselves in the field of radio. If my test results are of any value as a guide, this little set will become the most famous of them all.
Since the Bon Ideal and Interstate crystal sets appeared in these columns, I have been the guest of quite a number of proud owners of these receivers, who demonstrated how much more efficient their instruments were then the original which gave rise to my claims.
You have noticed the number of letters of appreciation in these columns, so that when I set out to find a better crystal set, I was not sure that my efforts would meet with success. However, after a lot of trying, I claim to have a crystal set that is far exceeds all those which have appeared in these columns; indeed, it is the best crystal set I have heard.
Readers may remember that in describing previous crystal sets I have stressed that in order to obtain maximum signal strength it is necessary to tap the crystal across portion of the tuning coil; hence the centre tap in the Bon Ideal and the Ten Shilling, and the six-point tapping switch of the Interstate. The effect of this tapping is that the heavy damping exerted by the comparatively low resistance of the crystal (particularly modern synthetic ones), is considerably reduced, and greater volume is obtained than when the crystal is in the usual position, right across the whole tuning system.
The optimum tapping point is quite critical, and really if such a multitapped coil as in the Interstate is not employed, the system is not really practicable. But there is a scheme whereby the equivalent of multi-crystal tapping is achieved (this is the secret of the DX crystal) with a simple untapped coil and a midget variable condenser. Coil tappings introduce great loss due to their self-capacity, which is equivalent to small condensers connected across certain turns, which short-circuit portion of the small voltages across the tunning coil.
Our condenser arrangement is shown in the sketch. Instead of taking the crystal to a tap some turns down the coil we connect it through a variable condenser to the top of the coil. Variation of this condenser gives us all the benefits of a tapped coil without the attendance capacity losses across its turns. Now, since the variable condenser in the crystal circuit, it is necessary to provide a path for DC component of the retified current. A radio-frequency choke is, therefore, connected across the crystal, and phones in the position indicated in the sketch.
It will be found that on one particular setting of the 19 plate midget condenser, the signal attain a maximum strength, but fall off on either side of this position. This means a point is reached where the damping due to the crystal is reduced as far as existing conditions permit, and at this point of minimum damping signals at loudest. Readers will at once appreciate one advantage of this system of tapping-the fineness of crystal tap control is far more gradual in variation than jumpy coil taps usually used. The whole circuit only consists of a 38-turn Litz wire coil tuned with a .0005 condenser, across which are connected two series arrangements, one the midget variable condenser and the Radio-frequency choke, and the other the crystal detector and phones.
In order to build this eye-opening crystal set you will require the following components:
|1||1 panel 9 by 6 by 3.16 Bakelite busbar, solder and screws|
|2||One Formo .0005 de luxe condenser|
|3||One Millgate art dial|
|4||One wetliss 19-plate midget condenser|
|5||One Omniaphone crystal detector|
|6||One Neutron crystal|
|7||One AGN or AWA RF choke|
|8||One 3in length of 3in diameter Bakelite tubing|
|9||Ten yards 9-38 Litz wire|
|10||Four ebonite topped terminals|
|11||One baseboard 9 by 7 by 1/2in|
Our first step is the winding of the coil, which consists of 38 turns of 9-38 Litz wire. Commence and end the winding by punching two holes in the tubing, and threading the ends of the wire through. This untapped coil will be a joy to many who found Litz wire so hard to tap in the Interstate set.
The assembly is very simple. The .0005 condenser is mounted in the centre of the panel with the 19-plate condenser on the left-hand side, and the crystal detector on the right. The panel is screwed to the baseboard and the coil is mounted right behind the .0005 condenser, while choke is mounted on the left of the panel, and the two phone terminals on the right.
All mounted: right; we are ready to wire up. If you use glazite busbar for this job a really neat set will result. For actually connecting up the various components I refer you to the sketch, but a few pointers will help. See that the tuning condenser and the tapping condenser are connected as shown in the diagram, where M means moving plates and F fixed plates; otherwise you will suffer from severe hand capacity, which will prevent you from getting most out of the set.
If when completed the set does not seem to give all it should, I suggest you try RF chokes in series, instead of one only. The crystal detector is of the cat's whisker type, but if you would prefer one of the semipermanent type use a Lion micro.
On test this receiver gave really good loudspeaker results on a 75ft aerial 30ft high on 4QG every night; indeed, on some nights the volume was audible two rooms removed, and on Sunday last two amateurs also worked the speaker. As for inter-state reception, I have tried the set every night for a week, and never failed to tune in 2FC, 2BL, 3LO, and 3AR, while on some nights 2GB, 2UE, and 3UZ could be heard on the phones. On Wednesday, rather as a lark, I tried the little set in conjunction with a wave trap, and to my surprise 2FC could be tuned in while 4QG was on air, but none of the others could be heard.
Well, you now have the DX crystal set, which will be the gem of your evergreen and ever popular crystal set experiments. If you only try it you will agree with me that it is the best of all.
22 June 1930 update - RF choke 250 turns 38 Gauge DSC (double silk covered) wire on a 1.5 inch bobbin, 1/4 inch in diameter
24 Aug 1930 update - revised DX coil now 30 turns Litz wire 9-38 litz wire