Tue May 13, 2014
A Radio for the Roaring Twenties
Last month we looked at radio cabinetry. This month we take a look at one of its examples, a 1929 RCA Radiola 46 in a high boy console. David Sarnoff (*1), a Belarusian born immigrant, led the Radio Corporation of America for an incredible span of 51 years, from 1919 until his retirement in 1970. He passed away a year later at age 80. By 1929 RCA, under Sarnoff's leadership, had established itself as a giant of the radio industry. It’s chief (and at this time larger) competitor was the Atwater Kent company, named after its owner. (*2), Atwater Kent was a privately held corporation whose radio factory covered more than 15 acres. I’ll spend more time on Atwater Kent and his company in the future.
The RCA Model 46 was a prime example of where radio was prior to the market crash of 1929 and the great depression that followed. This radio was grand, it was a reflection of the times. It represents a high end set that featured new technology and an elaborate cabinet. It sold for $179 (*3) which is the equivalent of $2,430 in 2014 dollars. (*4). For the upper class, primarily business owners and those invested in the stock market, it was a time of affluence. The average per capita income nationally however was $750 per year, with that of rural and farm income being $273 per year (*5), so this radio would have been out of reach for most. It should be noted though, that when compared to high end sets made between 1925 and 1929, that the average selling price of radios was dropping. As with almost all technology, this was due to higher volumes, and better manufacturing techniques. The upcoming economic depression would force this trend to a whole new level.
Although console radios go back to the early 1920s, their popularity grew in the latter half of the decade. The Model 46 was simple to operate. The volume control and tuning knob are placed one inside of the other, with the power switch on one side of the cabinet, and a local/distant switch on the other. The dial scale is internal and made of a stamped piece of steel. A lamp was placed behind it and projected the frequency onto an opaque piece of glass located just above the tuning knob. A new innovation was the dial nomenclature, which listed the actual frequency, 550 to 1500 kHz, the way a modern radio does. For familiarity, the old 0-100 scale was displayed to the right of the frequency. (note 1) A large 12 inch diameter paper cone speaker provided ample volume and decent quality sound. This was important as music programs, including the broadcast of live performances, had become quite popular by this time.
The cabinet of the Model 46 is exceptionally well made. It is constructed of mahogany with a burl maple veneer border just below the top, and maple scroll work below the controls. The back, also mahogany, is easily removed to service the set or replace a failed tube. Craftsmanship and finish is consistent with other fine furniture from the same period. The combination of chassis and cabinet resulted in a radio with a modern feel built into a cabinet that reminds one of early American furniture.
The radio utilized the ever popular single tuning knob three stage tuned radio frequency (TRF) design, (note 2) but changes were on the horizon.
There were two major factors driving vacuum tube design in the late 1920s. Foremost was the growing popularity of AC powered sets such as the RCA Model 46. Early radios were almost all powered by batteries. A radio would use several batteries. One would be a large rechargeable lead acid battery, similar to a modern car battery, which would often emit a sulfurous smell when in use. This battery provided low voltage high current to heat the tube filaments. It was bulky and heavy. The radio would also need one or more high voltage, low current batteries to power the vacuum tube circuitry that made the radio work. These were typically zinc carbon dry cells and being of early battery technology, were short lived, expensive, and not rechargeable. Consumers welcomed the introduction of AC powered sets.
AC powered sets however, required new tube technology. The battery powered tubes used the actual hot filament as the electron emitting surface known as a cathode (fig. B left). In this case technicians refer to such a tube as having a "directly heated cathode." If such a filament was heated with AC in a high gain amplifier, it could induce an undesirable hum. The problem was solved by coating the filament with an insulating porcelain material and placing it inside a small metal cylinder (fig. A left). The cylinder, not the filament, became the electron emitting surface, free from the AC current used to heat it. Tubes of this type are said to have "indirectly heated cathodes" and technicians refer their filament as a "heater."
The first AC sets emerged mid decade and used the battery type tubes as that was all that was available at the time. The manufacturer had to take extra steps and use extra components to suppress the hum that otherwise would have resulted. This added additional cost to the radio. This brings us to the second factor driving improvements in tube technology, cost.
More efficient tube designs offered more gain and stability. This meant that the manufacturer could produce a radio offering good performance with fewer tubes and their associated components. The 1927 Radiola 18 used seven tubes. The 1929 Radiola 46 offered the same performance and louder volume with only five tubes.
RCA introduced two new tubes with the Model 46. One was the type 45, (*6) an improved audio triode (note 3) capable of delivering several watts of audio power. This allowed the use of the large 12 inch diameter loudspeaker.
The second and more important was the type 24 (and later that year, an improved type 35) tetrode. (*7) As the name implies, the tube has four instead of three active elements. An additional grid, known as a screen grid, was added and it was a substantial improvement in tube design. The added grid increased the gain of the tube (its ability to amplify) while also increasing stability. Increase the gain of a triode too far and the tube can oscillate (produce unwanted frequencies), the screen grid served to suppress undesirable oscillation. The tube also had an indirectly heated cathode, which made it ideal for AC operation. High gain circuits were now less prone to unwanted oscillation, drift, hum, and noise and this allowed the use of the calibrated dial while increasing the sensitivity of the radio significantly.
Extra steps were also taken to attempt to keep the dial accurate. To prevent drifting due to temperature change, the 3 gang tuning capacitor was mounted on a thick cast aluminum chassis and two of the sections were enclosed for shielding and temperature stability. These improvements allowed accurate displaying of the frequency on the dial, which made it easy for the listener to find their favorite stations.
The volume control also changed its position in the circuit. It now controlled the gain of the audio amplifier as it would be in a modern radio. Previous designs placed it in line with the antenna and attempted to vary the strength of the radio signal coming from the antenna. This worked fine in battery radios or in the the electrically quiet environment of the suburbs or rural areas, but not so well in the cities. Electrical interference such as motor noise from a vacuum cleaner, trains, elevators, and static from neon signs or other sources could enter the radio via the AC power cord, and be greatly amplified. The new design kept the radio signal strong right up to the audio stage, helping to overpower the interference.
The screen grid tube along with its isolated cathode, brought the TRF radio circuit to its pinnacle. It also brought about its demise. The tube made it easy to mix two signals together for the purpose of producing a third. This would be the basis for the heterodyne circuit that would replace the TRF design in the early 1930s. RCA held the patent for the heterodyne circuit since the early 1920s, but using early tube technology it would have been too expensive to produce sets using it. More on how the heterodyne circuit was about to turn radio design on its head, and the unique way that RCA handled the patent, and why they chose to introduce the circuit after the market crash of 1929, next month.
The particular radio pictured here is completely original except for a few frayed wires which were replaced. It still operates and is a good performer.
COLLECTOR'S TIP: Console radios were originally shunned by collectors because of the space that they consume. As vintage table radios became harder to find, and on line auction sites drove prices up, many collectors revisited the console radio. Today’s larger homes allow more room to display these classic sets. It is not uncommon for a console such as the model 46 to fetch $300 to $1000 or more, depending on condition. Many were stored in the basement or attic, both being rough environments for any furniture. This does however, offer the opportunity for someone who enjoys woodworking or furniture restoration to occasionally find one in need of cabinet work at a good price. The 1920s consoles utilized simpler circuits than those of 1930 and up, making them somewhat easier for a less experienced hobbyist to troubleshoot.
Note 1. 540 to 1500 kHz was the range of the AM broadcast band in 1929. The original 0-100 scale represented the percent that the interwoven plates of a tuning capacitor were fully meshed. This was counter intuitive to the average consumer though as it resulted in the highest frequency (1500 kHz) being found at the 0 end of the scale, and the lowest one (550 kHz) coinciding with 100 on the dial.
Note 2. TRF- Tuned Radio Frequency circuit, an amplifier circuit that can be tuned to amplify only a specific desired frequency, such as that utilized by a particular radio station.
Note 3. Triode: a basic vacuum tube having three active elements, a hot cathode, a control grid, and a plate. Read my November 2013 article for more details about vacuum tubes.
(*1) David Sarnoff: IEEE archives: http://www.ieeeghn.org/wiki/index.php/David_Sarnoff
(*2) Atwater Kent Company: http://www.atwaterkent.info/akHistory.html
(*3) Chart of RCA original prices: http://www.radiolaguy.com/RCAradiola.htm
(*4) calculated via: http://www.measuringworth.com
(*6) Type 45 aka UX-245 triode history: http://www.radiomuseum.org/tubes/tube_ux245.html
(*7) Type 24 aka UX-224 tetrode history: http://www.radiomuseum.org/tubes/tube_24.html