Antique Philco Radio replica speaker plug

thingiverse

This is a replica of the 4-pin plug that is used by many Philco radios of the 1930's to connect the speaker cable to the chassis. Examples include the Philco 1931 models 20, 70, and 90. Use two half-inch 4-40 screws with nuts to hold the parts together. For the pins, cut three lengths of 4mm OD brass tube and one of 1/8" brass tube, each 0.65" long. I used a carborundum wheel on a Dremel tool. To flare one end of each pin, place each one in a pre-drilled hole of an aluminum block so that the back end sticks out, and hammer a steel punch into the exposed open end. I've provided a photo of the tubing stock, aluminum block, punch, and hammer that I used. Antique-replica cloth-covered 3-conductor wire can be purchased from Radio Daze LLC. Insert wires into the tubes and solder before installing in the plastic parts. Use a small, narrow cable tie to secure the cable within the assembled plug. Print using brown ABS. For the rear piece (speaker plug 3), don't use a brim (I did use a skirt). Use only about 10% support to fill in the interior and around the rim. Use a small screwdriver to scrape around the rim to remove the support and allow the pieces to nest together. Needle nose pliers work well to clear the interior support. The front piece (speaker plug 1) also needs support for the screw insets, but that's easy to remove. A brim is OK for that part, and for the middle part. It should be clear from the exploded view and the photos how it all goes together. I designed this because I purchased a Philco 20 that was missing its speaker. Original speakers of that era are expensive, and can be hard to find. Anyway, adapting a modern speaker is much more fun, even if the the cost is nearly the same and much more time is required. A modern (permanent magnet) speaker of the correct size can be adapted by adding a small impedance matching transformer (Bogen model T-725 from Amazon works well) and a coil of the correct inductance and resistance to mimic the original field coil. I plan to design and 3D print a coil form and coil winder. I'll post those if and when they are done and working.