Grimes "Baby Grand"

David Grimes "Baby Grand"

Inverse Duplex, c. 1924

By Doug Criner

David Grimes, who established his own radio manufacturing company in 1922, championed the concept of what he termed the “inverse duplex” radio system.

Essentially, this is a type of reflex receiver. (A reflex circuit uses the same tubes once for amplifying the radio frequency and then again for the audio frequency.) Grimes’ wrinkle was to reflex the second audio with the lightly loaded first RF tube, and the first audio on the more heavily loaded second RF, thus helping equalize the loading on the reflexed stages.

Top two knobs are for tuning. Lower three are: antenna coil selector, stabilizer, and filament voltage.

The tops of the three UV-199 tubes are visible. The compartments at either side of the cabinet are for dry-cell batteries.

David Grimes manufactured a variety of inverse duplex sets, some with up to five tubes. The Baby Grand model has just three tubes, which saves initial cost and reduces battery power consumption. Other components include: two audio interstage transformers, two RF transformers, two variable tuning capacitors, five fixed capacitors, and a grid-leak resistor. Pretty simple, mechanically, but electrically complex. Using the reflex principle, there are five stages achieved from three tubes (two tuned-RF stages, two AF stages, and a detector).

This model uses three UV-199 tubes. These are vacuum tubes that were designed for use with dry-cell batteries, each drawing only 60mA at 3V. The cabinet has compartments on either end for placing batteries.

This particular set has its original finish in very nice condition and is complete with all original knobs. The finish shows very little wear, perhaps because the set became somewhat obsolete early in its life--as AC-powered radios and more powerful tuned-radio-frequency receivers, using more advanced tube types, became available. Also, I discovered a wiring error, probably introduced when an audio transformer was replaced long ago, which prevented the set from playing. So, it seems likely that the radio was in storage for many decades.

The radio is housed in an attractive two-tone, solid wood cabinet (no veneer) with a hinged lid. The front panel is copper, painted black, and etched to reveal the copper for the markings. The chassis is bakelite, as are the knobs.

There is a dealer-installed, placard on the front of the wooden cabinet which reads: Installed by Pacific Coast Radio Co, Ontario-Pomona. David Grimes’ organization was situated in New Jersey and New York City, and his business was relatively small and short-lived—so it’s interesting that his radios came to be marketed by at least one dealer in California.

By 1928, David Grimes, Inc., was out of the radio business, not even lasting to the start of Depression, which subsequently felled many of its competitors. It seems clear that Grimes was a technical genius. Did he fall short in the areas of management or finance? If my Baby Grand is any example, his sets were ingenious, although perhaps a little “touchy” to operate and troubleshoot—probably appealing most to those that we now call “techies.” Even so, this set seems more sensitive and selective than some regenerative receivers, such as the 2-tube Crosley 51 or even the 3-tube Crosley 52. Eventually, reflex designs, in spite of their elegant technical design, lost popularity as the advent of AC-powered sets made battery power consumption a non-issue. (Nowadays, battery-powered sets, such as the Baby Grand, are best operated with modern battery eliminators.)

When I obtained this radio, it was missing its UV-199 tubes, which are relatively rare. (UX-199s will not fit the sockets.) Tube dealers typically list NOS UV-199s for $70+ and used ones for about half that amount. Luckily, I snagged on eBay three NOS RCA tubes, in their original boxes, for $40, total.

Before powering up the set with tubes, I inspected the wiring carefully. Only one audio transformer seemed to be original, and it was open—which I replaced with a Hammond 124A. Attempting to hook up the radio (without tubes installed) to a battery eliminator revealed a wiring error that would have prevented the set from playing. It seems that the wiring error might have been introduced when one of the audio transformers was replaced, long ago. (I say “long ago” because the replacement transformer appeared to be a very old design.) So, it would appear that the set had not been operable for perhaps 75-80 years or so, during which time it was in storage. When the original tubes were removed is anybody’s guess. Except for the replaced audio transformer, everything was original.

Checking the wiring was made more difficult because I was unable to find a schematic. There is a schematic for the four-tube Model 4-DL on the Nostalgiaair website, which I used as a “go-by.”

It’s interesting to speculate how the wiring error might have occurred. In this set, all electrical terminals are nut-and-bolt, not soldered. I can imagine an amateur replacing the audio transformer, first disconnecting the screw-type connections from the four terminals, which would have freed seven or eight leads. Confusion might have ensued.

Another common problem with radios of this vintage is that the detector's grid-leak resistor has drifted up from its original value, in this case from 4 megohms to 9+ megohms. I "haywired" a replacement resistor underneath the the chassis and re-installed a dummy, gutted grid-leak for ornamental purposes.

Once the wiring error was corrected, I installed the three tubes. Initially, the radio didn’t play, but after cleaning the tube sockets and prying up the spring-loaded socket tabs to make better contact, it worked, driving a high-impedance RCA 100-A speaker!

The sensitivity is little limited—but, a longer antenna wire helps greatly. A ground wire is important. Selectivity is good (the ability to sharply tune stations).

Top view of bakelite chassis.

Bottom view of chassis.

David Grimes went on to work for RCA and then Philco. As Philco’s chief engineer, he died in a 1943 plane crash—while traveling in the UK, supporting the United States’ technical effort in WW2.