More planning, engineering and hard work was involved in this radio endeavor than anything in recent memory and I am happy that it is done and that it turned out as hoped. All components used in the circuitry are displayed on 1/16” brass rods with the exception of the regeneration potentiometer, the tuning capacitor, the tank coil and the amp/speaker. The 3/16” and 5/32” brass tubes serving as a socket for the 1625 tube had to be secured where they enter the cabinet to prevent them from sliding down when installing the tube and from coming up when pulling it out. Precision holes were drilled into the ‘pillow block’ that sits on top of the top plate and through the top plate as well and at the same time. The brass rods were then indented with wire cutters about 1” from the bottom ends causing each to swell in diameter. Then a shallow cavity was created in the underside of the pillow block’s tube holes just deep enough for these indentations to fit into. This keeps the rods from pulling out when the pillow block is secured to the top plate with the 6 recessed brass screws. With each rod’s indention pushed firmly up into its cavity the assembly was then pushed down into the matching holes in the top plate and secured with the above mentioned screws. The fit is so close that the rods are then prevented from sliding down or pulling up at all and the mission was accomplished.
I wound a honeycomb coil using 175/46 litz wire to an inductance value of 244µh but this proved to be too much when coupled with the ‘365’ variable capacitor since its actual measured capacitance was 400pF instead of 365! Therefore I had to remove windings until the set would tune the top of the AM band where most of the stations reside these days. The trimmer capacitor was set to 30pF and mounted on its brass stilts. I wound up using both a 1,000uF electrolytic capacitor and a 2,200uF electrolytic capacitor to help quiet the wall wart 12vdc transformer. This has become standard practice with almost all of my radios that are powered with one of these transformers.
The schematic used was one devised by Dave Schmarder which I rely on exclusively. It is very straightforward in design, efficient and successful. In the early days I had to call on Dave’s expertise at least 3 or 4 times a day and were it not for these tutorials from Dave I would never mastered any of my projects.
The radio stands 17” tall and is almost 7” in diameter with the cabinet itself being 3 1/4” deep. And it is heavy! The ½” brass tube plate cap, the plate’s vertical 3/8” brass conductor bar, the 1” diameter brass tuning knob and the ½” brass regeneration knob and amplifier on-off knob certainly added considerable tonnage.
I used highly flamed big leaf maple (fiddle back) from my guitar shop for the side and top plates. The side was carefully steamed and bent to shape and after through drying it was mounted on the framework constructed just for it. The speaker ‘grill’ was also machined from this same fiddle back stock which required several hours of work on the metal lathe and the vertical mill to complete. The ‘pillow’ block for the main brass tubes holding the 1625 tube also required a lot of time on these same machines to fashion and complete. These two components are the most time consuming and difficult parts of any to construct but they are very handsome and add a great deal to the assembly.
After everything was sanded to 320 grit all wood was stained, sealed, shaded and finished with multiple coats of satin nitro cellulose lacquer.
My materials list;
Speaker – FUTULEC….Australia
Brass Stock – ENCO Machine
Power Jacks – ALLTRONICS
1625 Tubes – ANTIQUE ELECTRONICS
WallWarts – JAMECO ELECTRONICS
Trimmer Caps – JAMECO ELECTRONICS
Guitar Woods – LUTHIERS MERCHANTILE INTERNATIONAL
365pF Tuning Cap – XTAL SET SOCIETY
Solder Lugs – ALLTRONICS
Electrolytic Caps – MOUSER
Resistors, Caps – ALLTRONICS, MOUSER, JAMECO ELECTRONICS
Power Switches – RADIO SHACK
Brass Hardware – McMASTER CARR