Old Time Radio Construction Notes.

 

Just about all of the projects in Alfred Morgan's "Boy's First Book of Radio and Electronics" employ "breadboard" construction, which is simplicity itself--you just screw down tie points (tube sockets, solder lugs, terminal strips, etc.) to a wooden board and then run components and wires between them. You can attach panels and mounting brackets to any surface of the board.

 

When planning a breadboard, it's a good idea to "lay out" the actual components on top of it so that you can get spacing and distances just right. I prepared life-size paper templates for locating all of the holes. I secured all of my components using little round-headed brass screws, having pre-drilled all of the screw holes before finishing the boards with polyurethane varnish. I mounted rubber feet on the bottom of each board.

 

In the circuitry itself, I used carbon-composition resistors and fabric-insulated wire to give everything a suitable "retro" appearance.

 

Getting components is easier than you might expect. Back before transistors and integrated circuits made it obsolete, the electron tube was the essential engine of electronics, and manufacturers such as RCA, GE, and Westinghouse made hundreds of millions of them. They are still quite plentiful, and except for certain highly-sought "audiophile" tubes, remarkably inexpensive. NOS (New Old Stock) tubes are now often cheaper in constant dollars than when they were current; I bought my 6BF6 tubes for all of 99 each!

 

Three suppliers of "obsolete" radio parts are:

 

Radio Daze, LLC

7620 Omnitech Place

Victor, NY 14564

 

http://www.radiodaze.com

 

and

 

Antique Electronic Supply

6221 S. Maple Ave.

Tempe, AZ 85283

 

http://www.tubesandmore.com/

 

and

 

Ocean State Electronics

6 Industrial Drive

Westerly, RI 02891

 

http://www.oselectronics.com/

 

Ocean State carries a broad line of parts, including unusual items such as variable capacitors and plug-in coil forms. Radio Daze and Antique Electronic Supply have vast stocks of electron tubes and just about everything else!

 

I mentioned on my Radio Page that Morgan's two-tube resistance-coupled amplifier has a fatal defect. If you look at the schematic for the receiver itself (not included here, for reasons of copyright), you will observe that the headphones are the actual plate load for the tube--there must be continuity at DC. Should you connect the receiver to the two-tube amp as shown in the book, it will fail to work, because the amp input is capacitively coupled! A cure is simply to put a 2k ohm resistor across the amplifier input terminals. Electrically, this makes the amp "look" just like the headphones (which themselves have an impedance of 2k ohms), and allows the receiver to function.

 

In my version of the two-tube amplifier, I capacitively coupled both inputs, to prevent the B+ supply from being short-circuited should the connections between the amp and the receiver be swapped. As designed, the rightmost receiver 'phone terminal must never be connected to the non-capacitively-coupled amp input terminal (the book depicts the correct arrangement).

 

Regenerative receivers such as the one shown in Morgan's book are quite primitive compared to modern superheterodyne radio sets. They are only a step or two beyond the crystal set, so you will need to arrange a good antenna and ground in order to get satisfactory performance. Morgan's book discusses a suitable "long wire" antenna in detail.

 

Modern electronic parts catalogues are full of anachronisms--believe it or not, you may still buy electron tube filament transformers and radio B batteries from all of the major distributors! Typical B (electron tube plate supply) batteries include the Eveready types 412 (22.5V), 455 (45V), and 467 (67.5V). The problem is that general demand for these batteries has declined severely with the obsolescence of tube gear; the old economies of scale are no longer working, and so prices have gone way up. For instance, the Eveready 467 now costs over thirty dollars; if you plan to do more than just a little "old time radio" experimenting, a line-operated "battery eliminator" makes good economic sense.

 

The eliminator is nothing more than a line-operated regulated power supply having B-battery-level outputs. I packaged my unit in an aluminum cabinet, with panel-mounted binding-posts for connecting it to the breadboard radio components using ordinary hook-up wire. Its outputs are fused so that they may withstand accidental short-circuits.

 

I prepared the front panels for my regen receiver and battery eliminator using a commercial laser engraving and cutting machine, in the same way as described in the Design and Construction Notes for the D16/M minicomputer.

 

References

 

Alfred Morgan, The Boy's First Book of Radio and Electronics, Revised Edition. New York, NY: Charles Scribner's Sons (1966).

 

First published in 1954, this book has been reprinted many times and revised once (in 1961). This one is only the first; there are Boy's Second, Third, and Fourth books as well!

 

 C. F. Rockey, Secrets of Homebuilt Regenerative Receivers. Bradley, IL: Lindsay Publications (1996).

 

This more advanced book, one of many fine editions from Lindsay Publications, goes into considerably more detail on practical issues associated with regenerative radios. There are lots of tips and techniques, as well as a theory of operation and detailed discussions of components.

 

T. J. Lindsay, The Impoverished Radio Experimenter, Vol. 1-6. Bradley, IL: Lindsay Publications (2000-2006).

 

These smaller (~48 pp) booklets are *packed* with interesting projects, including test instruments, together with much general information and lore. A significant emphasis seems to be on doing the job with as little cash outlay as possible, so Lindsay presents clever techniques for making essential components (variable capacitors, radio dials, etc.) out of commonly-available materials and scrap!