Variations on the  Slider-Tuned Crystal Set

    My first crystal set used a prewound coil, a slider for tuning, a catwhisker and galena crystal, and had a large square mica capacitor (remember those?) across the headphone terminals, feeding a single 1000 ohm headphone.  For a variety of reasons, which I don't remember 40 plus years later, it only got one or two stations in Atlanta, GA, but, even at that, it was still magic.  Back in an era when responsible parents believed children should remain at least one economic level below their parents, having a radio of any type in my room was doing pretty well, and life was good.  This type of set is tricky to use, since maintaining contact between the coil and the slider is touchy, and then you have to find a good spot on the galena crystal with the catwhisker, also easily knocked out of adjustment.  Still, it has its advantages; it's inexpensive and it works.  The headphone was a cost driver, and throwing in a variable capacitor could easily double the cost.  A step up on the ladder came when AM tuning coils with  slug-tuned ferrite cores came on the market,  which were used in the tank circuit with a fixed capacitor, and feeding a germanium diode.  I had a "pocket" receiver which had these as well as a pretty sensitive earphone which was built into the case.
    About a year and a half ago I had a nostalgia fit and built another slider, but used a crystal earplug, a 1N34, and even added a 365 pF variable capacitor, remembering the tricky slide contact problem, but wanting to be able to vary the L/C ratio over a range of frequencies.  The set worked pretty well, but lacked selectivity.  This is a drawing of the set:


HERE is a picture

    I found that the best place to connect the antenna and ground depended on the antenna used and probably on local conditions. Sometimes connecting the antenna to the slider worked well.  I tried the detector tap at the top of the coil, as some designs do, but selectivity really suffered.  Some selectivity improvement was seen when I wound a smaller coil for an antenna loading coil and put it up inside the tuning coil.  Again, you can still play with where the antenna and ground are connected; you can put the antenna or ground to the loading coil, and try the other on either end of the tuning coil or on the detector connection.  You can also make your earth ground to both the loading coil and the tuning coil.  The best arrangement is what works best for you.  An operating benefit of connecting the detector to the slider is that you don't hear anything unless the slider is making contact with the coil.
    My latest design attempt, which shows promise in the breadboard stage, goes back to a fixed capacitor in the tank circuit , but incorporates an antenna loading coil and separate detector taps, two features which I consider essential to selectivity in a single tuned set.  The drawing for this rig is shown below:

    My coil was 100 turns of #26 magnet wire close-wound on a toilet paper core, with detector taps at 10, 20, and 30 turns from the bottom (connected) end. The antenna coil, L2, is 15 turns of #26, also on a tp core.  I chose to separate the coils so I could move the antenna coil about to vary coupling, but could probably have done about as well winding both coils on the same core with about a 1/8 to 1/4 inch separation between them.  Note:  I found that the loading coil had to be placed at the bottom end of L1 as shown; placing it at the open end didn't work very well at all (for me).  For a slider, I just straightened out a large paper clip, soldered it with a stranded hookup wire into a soldering lug, and then used a wood screw to hold the slider to the base I used.  Just for grins I rolled my own fixed capacitor, C1, making it about 300pF.
On the air results were pretty good. The detector tap is selected by an alligator clip.  I got the separation I wanted between the two strong locals that are 50 kHz apart, sensitivity is ok, BUT, the rig design still needs some work before I offer it to my students.  Current problems and possible solutions:
    a.  Only using about half the coil, and it tunes pretty fast - use larger wire, maybe a smaller diameter coil  form, and maybe a smaller value capacitor. (probably do at least two of the three)
    b.  Not happy with the paper clip slider; too flexible so you get backlash, and doesn't ride smoothly - use a metal strip and maybe add another washer where it connects to the base.
     c.  With detector tapped directly to the coil, you always hear something, whether or not the slider is making contact.  I guess I can live with this.
 

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