This project was an endeavor to see if all the components for a 1625 AM radio including an amplifier and speaker could be successfully crammed into a cabinet only 4” in diameter and 3” tall. It was indeed possible but the mechanics for doing so required many alterations of components and building techniques. Things were progressing reasonably well after dismounting the on-off-volume potentiometer from the amplifier chassis and reconnecting it with long leads for assembly. Placements for all other components had been calculated and then I discovered that I had completely forgotten about the speaker. Normally it would have been mounted under the top panel in a round cabinet but there simply was not enough room in this one. The only possibility was to place it on a vertical wall but the tightly curved surfaces presented real problems. Work with guitar construction however had taught me techniques of fitting almost anything to irregular surfaces and a convex plate was made for mounting the speaker to the interior wall and a concave maple speaker grill was fashioned for the outer wall.
The 1625 tube is one of my favorites since it is not only handsome and big but it only requires one source of power instead of ‘A’, ‘B’ or ‘C’ voltage sources and certainly no high or dangerous voltages of any kind. A standard 12vdc wall wart is all that is required for both the filament and plate operations.
The circuits for the set were drawn up by Dave Schmarder and it is the standard I use for most of these 1625 projects. Due to space limitations the inductance coil was wound on an Amidon toroidal core #FT-114A-61 using 175/46 Litz wire together with a standard 365pF variable capacitor from Xtal Society comprised the tank. A very compact trimmer capacitor from Jameco Electronics couples the antenna to it. A very substancial structure of ‘brushed’ brass bars from Enco Machine serves as the tubes plate connection. I polish all brass screws that mount the top plate to the cabinet and these are purchased from Micro-Fastners Inc. Other brass hardware such a thumb nuts, hex nuts and bolts are purchased from McMaster Carr. Guitar wood comes from Luthiers Merchantile International.