VOLUME 14             AUGUST 1997                NUMBER  8

The De Forest Reflex Radiophones

WEB EDITION

BY DAVE GONSHOR

Dave Gonshor's description of the De Forest reflex Radiophone models provides another glimpse of Lee de Forest's bumpy ride through radio history. (Editor)

Studying the evolution of early broadcast receivers often reveals a fascinating combination of technology development and business decisions, sometimes showing folly in both. The De Forest D-12 Radiophone is no exception. The development of the D-12 begins with the D-10 design.

THE DE FOREST D-10
The D-10 is a 4-tube reflex receiver, using a crystal detector. Either Frank Squire or William Priess (the self-proclaimed "father of reflex") developed the D-10, although Gernsback gives credit for the invention of reflex to Marius Latour.

A reflex design simply means that one or more tubes are used for both RF and audio amplification. Reflexing reduces the number of tubes needed an important factor in early radio when tubes were relatively expensive and battery depletion was a big headache.

In the D-10, two tubes have both RF and audio amplification functions. So, the receiver performs as a 6-tube radio (three stages of RF amplification and three stages of audio amplification) seven if the detector is considered. Reflexing is a rather sophisticated electrical design technique, since the interstage RF and audio transformers are arranged so that a single tube can perform two functions. For a photo of the D-10, see Alan Douglas' Radio Manufacturers of the 1920's, Volume 1, page 176.

The D-10 has an obsolete (at the time) crystal detector as well as poor selectivity because only the antenna circuit is tuned. On the other hand, the radio's reflex circuits produced high amplification of both RF and audio. The poor selectivity is offset by the use of a loop antenna. The use of dry cell tubes, De Forest Type DV-3, accounts for the set's small size.

The external loop could be rotated to "null out" a stronger station in favor of a weaker station, compensating for the poor selectivity. At the time, it was the only loop set that had been made in production quantities. The only external item needed to make the radio play was a loudspeaker. The D-10 is an early, nearly self-contained, portable radio.

THE DE FOREST D-12
Versions 1 and 2
William Priess, De Forest's chief engineer, recognized the D-10's limitations in selectivity. He added a tuned RF circuit to the design, but kept the rotatable loop antenna. A restyled larger cabinet (mahogany or leather-covered wood) and the addition of an internal horn speaker resulted in the first version of the D-12 radio. Figure 1 shows the D-12 with the detector hole uncovered.

Note how reflexing makes the circuit diagram, shown in Figure 2, appear complicated. The D-12 was designed to use either Type DV-2 storage battery tubes or Type DV-3 dry cell tubes, with a change in battery voltage and RF coils (according to the instruction book) necessary to switch from one tube type to the other.

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Figure 1. The De Forest D-12, showing the hole where the crystal detector has been removed.
(Photo of Dave Crocker's set by Alan Douglas.)

Figure 2. Schematic diagram of the first and second versions of the De Forest Model D-12.

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The instruction book for the D-12 touts the crystal detector as superior to an audion detector due to the "practically perfect" rectification by the crystal and the "complete absence of the local generated noises" of an audion detector. Instructions are given on how to adjust the "gold wire" catwhisker for best reception. The first version of the D-12 used a crystal detector assembly like that of the D-10, and is considered quite rare. In the second version, a "bat wing" type of crystal detector replaced the D-10 type. Most of the D-12s found have this type of detector.

THE MODEL D-12 VERSION 3
Roy Weagant, a new chief engineer hired by De Forest, must have recognized that using a crystal detector in an otherwise sophisticated radio design was antiquated. So, he upgraded the D-12 by replacing the crystal detector with an audion detector, and restyling the cabinet and panel. De Forest released this modified model as the D-17. One wonders whether the marketing implications of this new model were fully understood at the time.

Because of a temporary injunction obtained by RCA preventing de Forest from selling to jobbers and retailers, de Forest was forced to consign his radios to the selling agents. This meant that he, not the seller, owned a radio until it was sold to a consumer. Weagant's redesign of the detector in the D-12 and sold as the D-17 immediately made obsolete all the D-12s in the showrooms.

In order to remedy this apparent blunder, the distributed D-12s were brought back to the factory, where tube detectors were added, and the crystal detectors, as well as one stage of reflex, were removed. Thus, these D-12s are the same as the newer D-17.

An easily identifiable characteristic of this new third version of the D-12 is that the detector adjustment hole in the front panel is covered by a small plate. In addition, the "detector" label on the front panel was blackened to hide it. This tube detector version of the D-12 is fairly common.

Undoubtedly, these initial design deficiencies and the production changes, along with a complicated intertwining of patent rights and lawsuits, hurt De Forest's radio business. De Forest put his company into receivership in 1926, and the company produced no more radios after 1930.

It would be an interesting addition to a collection to have examples of all three models of the D-12s, but they're so scarce that the chances of finding more than one (at an affordable price) are very slim. Hopefully, this information will help you determine which version of the D-12 you have.

Information credit: Alan Douglas and Raymond Thompson

Reference:

Douglas, Alan. Radio Manufacturers of the 1920s, Vol. 1. Vestal, New York: Vestal Press, Ltd., 1988.

(Dave Gonshor, 7121 S. Jellison St., Littleton, CO 80123) Dave Gonshor, an electrical engineer, is interested in collecting battery sets and wood AC sets for the 1930s. He enjoys both electrical and wood restoration, as well as the history of the development of radio. His collection includes two De Forest sets.)