The Automatic Radio Company
Tom Thumb Portable
Bringing Back an Early Suitcase Set
BY DICK PARKS
Sometimes a "fixer-upper" becomes more than just that. Dick Parks writes about an acquisition that turned out to be more interesting than he had expected. (Editor)
The Automatic Radio Manufacturing Company began operations in Boston about 1920, and before folding in about 1957, had produced a wide variety of interesting receivers. The "Tom Thumb" name was given to several portable sets and even to a cathedral model.
Some colorful examples of Tom Thumb Catalin table sets can be seen on Radiola Guy's web site at www.radiolaguy.com/Showcase/AutomaticRadio.htm. There was even a Tom Thumb bike radio in 1949, but the last model with that name was a tube/transistor hybrid portable.
The 1929 Tom Thumb Portable
I found this 1929 radio on eBay, listed as a fixer-upper, and it turned out to be what I think must be their first design. Bunis lists it from 1929, but with the tube lineup of Types UX-222, 199, 199, and 199, I wondered if it might be earlier, possibly back to 1924. [See Wally Worth's January 1989 article in which he describes this same set as a 1929 model.] When I received the set, it had three dud Type 199s and a good balloon Type 22 tetrode. There had been some wiring changes, but one audio transformer was still good, and the speaker driver was OK as well. The radio is shown in Figure 1.
Figure 1. A front view of the Automatic Radio Manufacturing Company Tom Thumb portable.
This is an interesting radio. It's a good-sized, leatherette-covered, wooden box, about a foot square by eight inches deep. Both the front and back swing open. The front cover has a loop antenna built in, and as you might suspect, despite the metal front panel, there can be unwanted feedback between the antenna and the crudely-wound RF coil at the detector tank.
When you open the back, there are the four tubes and the battery compartment, as shown in Figure 2. A cute wooden box at the bottom has a horn-shaped interior passage between a magnetic driver and a grille at the bottom of the front panel. The tuning cap is a 2-gang part, with both rotors insulated from each other. A small balancing cap acts to even out any tracking errors across the broadcast band. The tuning range is roughly from 530 up to 1450 kHz.
Although Its schematic diagram refers to the radio as a "screen grid four," only one of the tubes is a screen grid type. The diagram appears in Figure 3 (see print version).
Using bench power supplies, I brought the set to life, fixing things as I went along -- a bad potted bypass cap, the replacement interstage transformer, the open "volume control," a wirewound part that changes the grid bias on the RF tube. The set depends on the gain of the screen grid tube for its sensitivity, since the design does not use regeneration.
Lacking a set of Type 199 tubes, I substituted 2-volt tubes: a Type 32 screen-grid tube for the 222, Type 30s for the detector and first audio, and a Type 31 for the output tube. After I was done, the schematic followed the original closely. The Figure 4 (see print version) schematic diagram shows the changes I made.
Figure 2. An interior view of the Tom Thumb portable after repairs were made. The battery eliminator is located on the bottom shelf at left. The built-in speaker is at right.
Finally I made a battery eliminator for the set that fits inside, where there is space for A, B and C batteries. If I wanted to use this as a true portable, I'd probably use a single 2-volt lead-acid D cell and eleven 9-volt batteries in a package, but it would still sound the same -- not quite as good as an Edison cylinder machine, with its simple horn speaker.
Nostalgia Air website, www.nostalgiaair.org, source for radio schematics
Radiola Guy website, http://www. radiolaguy.com/Showcase/AutomaticRadio.htm
Worth, Wally, "The Automatic Radio Mfg. Co. and the Tom Thumb Radios," Antique Radio Classified, January, 1989.
Dick Parks's interest in old radios began in the junkyards behind the Westinghouse plant, East Springfield, Massachusetts, where RA-DA radios were made. A design and consulting engineer on electronic radar identification systems for the U.S. Navy for over 30 years, he is also a sometime musician, performing early jazz and jugband music.