I built the radio according to Sleeper hook-up #18, a fairly simple circuit with the antenna and ground feeding the Primary via taps and the secondary tuned with the capacitor. As most of the components for this set were ready made, the construction planning felt a bit much like assembling a kit. I had originally intended to use modern tap-selector switches as well. In order to make the construction challenging, and more importantly, to maintain the vintage look and feel of the set, I quickly abandoned the modern switches and chose to fabricate instead classical wiper switches in the early style. Of course, I also include my own style cat’s whisker and SPDT knife switching to a more modern diode. The diode is my only nod to post war technology. In the crystal cup I have some lovely Utah steel galena potted in wood’s metal, finely crystalline, hot as hell all over its surface.
As one can clearly see, my obvious preference is for the breadboard style of early radio. Much of the fascination and beauty of these sets lies in the component hardware. To enclose them behind panels or put them in boxes seems such a waste. The base is a box with about ¼ inch of space inside to hide the bolts and wires. I did need a small panel to place the wiper switches on. This is made of 1/8 inch black Delrin plastic resin, a very hard and nice to work with material. In fact though, it is a bit too shiny, too “plasticy” for my taste. Well, that is what I had.
Fabricating the wiper switches, buttons, and leads to the inductor was an odyssey in its own right, amply satisfying my desire for more challenge. Attaching the leads to the primary, soldering without damaging the windings, and the added touch of shrink-wrap “mittens” was a labor of love. I do hope it is justified by the final presentation. The rest of the construction and wiring was a piece of cake by comparison. The wiper switch knobs are wood with a felt buffer against the panel. Behind the panel compression springs keep a steady pressure on each switch. The wood knobs have a nice easy action, the switch passing readily between contact buttons.
The radio works quite well, quite sensitive with a strong signal. Its selectivity on the other hand leaves much to be desired. This can be expected from the simple circuit layout with only the secondary circuit capacitively tuned. I was hoping that by varying the coupling I could achieve some increase in selectivity, but it de-tunes the station with an additional unacceptable loss of sensitivity. Take this outside the urban environment with such crowded airwaves and the set ought to perform admirably. Signals come in strong with the steel galena nearly as well as with the diode. I like this set.
De Forest circuit diagram (aka Sleeper #18).
Performance summary of my DeForest is so so, it has a low set Q (loaded) of 7 but my sensitivity calculation is 37%. This set clearly trades Q and bandwidth for sensitivity. Measurements made with the set farily optimized for a test frequency of about 1100 kHz.
From what I have read of De Forest’s writings, patents, and exchanges with Armstrong in the Proceedings of the Society of Radio Engineers, it is pretty clear that De Forest never really understood how his tubes worked. So, he was never in a position to develop the tubes to their full potential. Armstrong for his part seems to have treated De Forest with academic contempt. Armstrong went on to invent the super-heterodyne (1918) and super-regenerative (1922) amplifier circuits and created almost single-handedly FM radio (1933) as we know it today. His life ended in tragedy, representing the closing chapter in the age of great individual inventors, finally crushed by the greed of corporate America. Virtually all advances following his were the product of team efforts in company or government research laboratories.
If therefore you have misgivings as to my honoring De Forest with a radio, please understand I do so not for his Supreme Court victory but to honor the inventor of the triode tube, a debt which even Armstrong would always acknowledge. Should you still feel uncertain, then just enjoy the sweet irony in honoring the father of the triode tube with a crystal radio.
