The Brass Tack Crystal Radio

Tom's Designer Crystal Radio, The Brass Tack Main View

This crystal set is the first I have attempted in several years but the rediscovery of some convex dial lenses prompted this effort to use one for the tuning dial. I was fortunate to find a beautiful gallery of these old dials that I simply printed off and mounted on heavy stock. Machining the frame for the lens and dial proved to be a very demanding project. Not only did the inside diameter have to be exact but the beveled rim had to be exactly right to hold the lens flush to the radio without slack or stress.

The cabinet is on the large side for a crystal receiver but the necessary diameter of the lens and dial frame dictated everything else. The dial frame diameter is 4.75”, the set is 7” tall, 7” wide and 10” long. The cabinet is built from tiger maple with matching knob skirts under padauk wood grips. The knob for the detector stand is mahogany and everything was stained with a dark red mahogany shade that I mixed and then everything was finished with nitro lacquer that I flatten here in my shop.

A double tuned circuit was used for the radio with two 365pF variable capacitors and a honeycomb coil I wound to 240uh using a really attractive tattoo gun wire. This simple schematic always amazes me with great selectivity and volume. A switch on the back panel allows the choice of using a diode for detection or the brass detector stand on top loaded with a chunk of pyrite (which can be changed to argentiferous galena if desired). That detector stand has a very thick brass base and top with a plexiglass viewing column that weighs almost one third of a pound. The cat’s whisker shaft is held by a brass ball split in half and enclosed in the brass cap. Action of this shaft is adjusted by tightening the cap or loosening it up.

The main tuning is accomplished using a vernier drive on a vertical shaft from the top panel to the variable capacitor mounted on an aluminum plate on the floor of the cabinet. The horizontal dial shaft is connected to this vertical shaft via matching brass bevel gears. Additional support for these shafts is required for gear function and is provided by wood blocks bored exactly to the diameter of the shafts.

The set turned out to be very hefty and will be holding down several things here until I finally decided to auction if off. Until then it will provide many hours of entertaining listening.

Tom's Designer Crystal Radio, The Brass Tack Dial
Tom's Designer Crystal Radio, The Brass Tack Pyrite and Catwhisker Detector

The Insides

Tom's Designer Crystal Radio, The Brass Tack, the inside view