Guide to the exhibits

Shown are some of the many radios that apppeared in this set of exhibits.

Magnavox Co. (Oakland, CA) Amplivox audio amplifier: ca. 1920 This unit amplifies signals from radio receivers or microphones. It uses three type 201 tubes, one for each of three stages of amplification. It has a switch to transfer the signal input between a radio and a microphone, making it useful as a public address system. An interesting message is affixed to the bottom: "This instrument to be used for Audio Frequency Amplification only where messages are not transmitted FOR PAY."

Crystal set: Lemco Equipment Co. (San Francisco), Model 340: 1922
This little unit, built into a small wooden box of the type used to file 3" x 5" cards, is radio at its simplest: The only variable element is the tuning inductor, which has two sets of taps -- one for the antenna to ground tuning circuit, and the second to match the crystal detector and headphone output. However, with a good antenna (and not too many stations within 50 mles), it gave quite satisfactory reception. As no batteries were required for operation, the simple and inexpensive crystal radio was a good choice for the beginning radiophile. The pictured unit was made and labeled for San Francisco's White House Department Store. Crystal radios are still being built today--you can sign up for a MOAH workshop and build your own.


Bodine Electric Co. (Chicago) Basketweave loop antenna: 1922-1925 Early radio receivers required an outdoor antenna of significant length -- 50 to 300 feet -- in order to pick up enough energy for satisfactory reception. This early loop antenna was an effort to cut down on the need for a long antenna, but did not prove particularly successful. External antennas have largely been replaced by the ferrite "loopstick" antenna in consumer radios, which came into wide useage in the 1950s.



Radiograph II TRF: 1923 Battery operated, five tubes. Three radio frequency stages bring the received signal up to a level satisfactory for the vacuum tube detector circuit. An audio amplifier stage raises the detector output to a level adequate to drive headphones, which connect to a jack at the rear of the cabinet. The ubiquitous #201A tube is used in all stages. This set illustrates the beginning of an effort to simplify tuning. The first TRF stage is seperately tuned, but the two following RF stages are gang-tuned. As a result, the set has only two tuning dials, rather than the one-per-stage arrangement used with earlier TRF sets.

Atwater Kent (Philadelphia) Model 10C Tuned Radio Frequency set: 1924 This six-tube, battery operated radio represents the period before radios were installed in cabinets. Almost all wiring is done on the underside of the board, with thin copper straps and individual wires soldered for good connections. (This type of construction is known as "breadboard" construction.) The type 201A tube is used in all positions. There are two taps to set for use with a long or short antenna, and the antenna feeds three individually tuned (three tuning knobs!) RF amplifier stages. These are followed by a detector stage and two stages of audio amplification to drive a set of headphones. Audio volume was controlled by varying the voltage on the amplifier tube filament -- more glow means more sound.

RCA Victor Co. Radiola III, Type R1: 1924 This is a two-tube, battery operated set that originally sold for $35.00, including headphones. An amplifier and a speaker could be purchased as separate options for an additional $65.00. Batteries and the antenna were the user's responsibility. Both tubes are low power-drain WD11 types. The first tube is fed directly by the antenna as a tuned detector -- the second is an audio amplifier driving a set of headphones or speaker. Three different dry-cell batteries were required: 1.5 volts for the filaments (A battery), 90 volts for the high voltage supply (B battery), and 4.5 volts ( C battery) for grid bias. This set has the sensitive regenerative detector invented by Armstrong (and contested in court by DeForest). For the most sensitive reception, the regeneration signal (a form of feedback -- think speaker howl on PA systems) is set just below the point of runaway oscillation. If set too high, the sound output became badly distorted and the radio created interference with other nearby receivers. For this reason, the regenerative detector was soon replaced by the neutrodyne circuit. The Battery Adjust knob allows compensation for changes in battery voltage. "Amplification" is actually the regeneration adjustment. The Station Selector is the tuning control. The set can receive signals in the 200 - 500 meter band, roughly equivalent to the current AM band.

Gilfillan Corp (Los Angeles) GN3 Neutrodyne: 1924

With four efficient #UX99 tubes and battery operation, this receiver had relatively low battery drain for its time. Antenna signals were directed through two stages of individually tuned amplifiers. A detector tube converted the modulated RF signal to audio, which was then amplified through one stage and fed to earphones. The "neutrodyne" circuit uses negative feedback to stabilize the high gain tubes, which otherwise had a tendancy to oscillate. Carefully controlling the amount of feedback permitted relatively high sensitivity. The high efficiency tubes permitted the use of relatively small dry cells (located on a shelf below the radio's electronics) to power the receiver.

Atwater Kent Model 30 TRF: ca. 1926 This six-tube, battery operated receiver originally cost $85.00. There are three tuned RF stages, but they are all tuned simultaneously (gang-tuned) with one knob, a considerable convenience. Audio volume was controlled by varying filament voltage. The radio is housed in a wood cabinet measuring 6.25" high by 20" wide by 6" deep. The top is split, with the front half hinged to allow access to the interior. The batteries are bulky, and reside on a separate shelf, connected to the set via a cable. A second cable connects a speaker. All of the tubes are #201A, except for the speaker amplifier, a #71A.

Magnavox Co. Model T (No. 1217D, TRF) with Eton cabinet: 1926 A five-tube, battery operated radio with three gang-tuned RF stages and a speaker, it covered the AM band from 200 kHz to 1.5 mHz -- almost the same as the standard AM band of today. The tuning inductors are unusual -- wound in a toroidal (doughnut-like) form factor that is relatively difficult to manufacture. Batteries are located on a separate shelf. The receiver used four #201A tubes and a #112A as a speaker driver. The cabinet is also unusual: The working parts of the radio are pulled out, as from a drawer, from the center-front of the cabinet. This would have appealed to the radio hobbyist and the service technician alike.

RCA Radiola Model 17: 1927

Seven tube, tuned radio frequency, AC operated receiver. The original price was $130.00. This radio was built by subcontractor R. Prescott & Sons, Keesville, NY, before RCA acquired its own manufacturing facilities. The radio uses the first isolated (uni-potential) cathode tubes, allowing tube operation from AC line power without introducing hum in the reproduced sound. A three-gang variable capacitor allows simultaneous tuning of three RF stages. Tube lineup: 3 #26 tuned RF amplifiers, #27 detector, #26 first audio amplifier, #71A loudspeaker amplifier, #80 dual rectifier for 150 volt DC power supply. Note the increasing number of specialized tube types used in this and later receivers.

Atwater Kent Model 46 TRF: 1928 AC operated with eight tubes, this set has three gang-tuned RF stages. The associated loudspeaker is the Atwater Kent type F-2, an electrodynamic unit with an electromagnet winding that also served as a filter element (choke) for the high voltage DC power supply. This was a standard arrangement before the availability of the high permeability (strong magnetic field) permanent magnets used in more modern speaker designs. It could also be a surprise for the unwary service technician! Tube lineup: 3 #326 RF amplifiers, #27 detector tube, #26 audio amplifier, 2 #71A speaker amplifiers, #80 dual diode rectifier tube for high voltage DC power supply.

Crosley Radio Corp (Cincinnati, OH) Model 168 Gothic cabinet receiver: 1933 This AC operated superheterodyne receiver has seven tubes and is one of the earliest examples of "modern" table-type radio circuit design. Typical of the period, the tubes operated at a filament voltage of 2.5 volts AC, except for the rectifier, which required 5 volts. The husky (and heavy) AC transformer provides these voltages, plus 500 volt AC power (center tapped, so each half of the output winding produces 250 volts) required to generate the 200 volt DC high voltage supply for the tubes. An outside antenna was required. The design featured a single tuned radio-frequency stage, two tubes for the IF frequency oscillator and mixer (It would be another year before the invention of the pentagrid converter, which combined both functions in one tube), and a short-wave band covering 1.7 to 5.5 MHz in addition to the AM band. Except for these three unusual features, the set exemplified the "standard" consumer radio design that persisted into the 1960s and the advent of the AM/FM radio receiver. Throughout this period, cabinet styles and tube changes came and went, but the block diagram of the receiver remained essentially unchanged. Tube lineup: #58 TRF, #58 mixer, #56 IF oscillator, #58 IF amplifier, #2A6 dual section detector and first audio amplifier, #2A5 speaker amplifier, #80 dual diode rectifier.

General Electric Corp Model (Bridgeport, CT) G-50: 1934 With AC operation, wood cabinet and five tubes, this set conforms to the "standard" superhet block diagram. The filament voltage for the tubes has now shifted to 6.3 volts AC (a new standard) for the receiver tubes, but the same heavy multi-winding power transformer is still present. A novel feature is the pushbutton station selector arranged around the tuning dial, recalling the dial of the recently introduced (and, therefor, thoroughly modern) rotary dial telephone. Tube lineup: #6A7 mixer-oscillator, 6D6 IF amplifier, #75 detector and first audio amplifier, #41 amplifier, #80 dual rectifier.

General Electric Corp Model G-50 with plastic cabinet: 1934 This receiver is identical to the original G-50, but has a plastic cabinet, rather than wood. This was something of an experiment, as there were concerns about the fidelity and resonances of sound resulting from the use of plastic, but plastic became the standard cabinet material for less expensive radios as time went on.

Mission Bell Radio Co. (Los Angeles, CA) Model 383: 1937 An AC operated set requiring an external antenna, this radio is similar to the GE model G-50 (except for the detector tube, which replaces the #75 with the more modern 6Q7). This radio retains the heavy AC transformer, which is a design disadvantage compared to less expensive competing models, which could operate directly from 120 volt AC line without the need for a transformer.

General Electric Model H-634 AC/DC operation: 1939 This receiver shows several evolutionary improvements: Metal cased tubes for reliability, direct operation from AC line power without a transformer, a maximum of 8 pins in the tube base for connections and a standardized socket, and construction that eliminated the need for the mechanically vulnerable grid cap (the little metal post protruding from the top of the tube) on older receiver tubes. Equipped with pushbutton tuning, a superior speaker and two shortwave bands, it was considered a "quality" receiver. The set contained a loop antenna, but also had provision for connection to an outside antenna. The filaments of the tubes were connected in series with a voltage dropping resistor (or "ballast tube") included in the string to permit operation from 120 volt AC line voltage. This created a service problem: If one tube burns out, it is all tubes must be tested until the defective tube is located. Tube lineup: 6SA7 mixer, 6J5 IF oscillator, 6SK7 IF amplifier, 6SQ7 second detector and first audio amplifier, 25L6 speaker amplifier and 25Z6 dual rectifier. (With this nomenclature for tubes, the leading numerals indicate the filament voltage required.)

Cavalcade (Philadelphia, PA) RS1A AC/DC operation: late 1940 Another "standard" table radio design, but incorporating miniature glass tubes, a type in common use throughout WWII and thereafter.The tube filaments operate in series and require 121 volts, so no ballast tube is needed.A ferrite "loopstick" antenna (which concentrates the electromagnetic signal needed by the antenna) is included, eliminating the need for an external antenna. Tube lineup: 12BE6 pentagrid converter, 12BA6 IF amplifier, 12AT6 second detector and first audio amplifier, 50B5 speaker amplifier, 35W4 half-wave rectifier.

Coronado Series 5D2-4A with Wells-Gardner & co. antenna: 1941 This five-tube radio can use an external antenna as well as an internal loop. It is similar to many other table radios of the period in overall design. Tube lineup: 12AS7 converter, 12SK7 IF amplifier, 12SQ7 second detector and first audio amplifier, 35L6 speaker amplifier and 35Z5 half-wave rectifier.

Majestic Radio Co. Model 4705: ca. 1945 Operating on battery or AC line power, this seven-tube portable receiver incorporates a tuned RF amplifier for signal selectivity and a low power consumption set of tubes, but is otherwise similar in overall design to other radios of the period. The 1.4 volt filament tubes consume only 60% of the power needed by tubes of the previous decade. Tube lineup: 1T4 RF amplifier, 1R5 pentagrid converter, 1U4 IF amplifier, 1S5 decond detector and first audio amplifier, 3Q4 speaker amplifier (for battery operation), 50B5 speaker amplifier (for AC operation), 35W4 half-wave rectifier for line power operation.

Zenith "Transoceanic" 8G-005YT: 1947

An eight-tube superheterodyne receiver operable from AC or internal battery power, this good quality receiver offered an AM band and five shortwave bands. Its "loctal" tubes were designed to remain in their sockets even under conditions of shock and vibration, appropriate for a portable receiver. The set included separate loop antennas for AM and shortwave bands. The shortwave antenna could be removed from the case and positioned for best reception. Tube lineup: 1LN5 tuned RF amplifier, 1LC6 pentagrid converter, 1LN5 IF amplifier, 1LD5 second detector and first audio amplifier, 1LE3 audio inverter, two 1LB4 speaker amplifiers, and a 117Z6 dual diode power rectifier. This was the second of the "Transoceanic" series and the first to appear in the post WWII period. It's design took advantage of technology developed during the war.

Philco Model 46-1201 AM radio and record player: 1946 This is an attractively packaged "standard" table radio, but with an unusual 78 RPM record player. A hinged door at the bottom of the cabinet gives access to the record player -- the user pushed a single record into an access slot. The mechanism automatically accepted the record, centered it on the turntable, positioned the pickup arm and played the record.

Hallicrafters S-38D AM/Amateur service: 1948 Priced originally at $49.00, this inexpensive AC/DC receiver can be tuned to the AM band and three shortwave bands. It was used by many amateur radio operators as a "starter" receiver. Other than the shortwave feature, this set is just another cookie cut from the standard AC receiver block diagram. Performance is marginal: It has no TRF stage and only one stage of intermediate frequency for minimizing extraneous signals, no calibrated bandspread (fine tuning) for separation of closely spaced signals, and the CW beat frequency oscillator (BFO) has no control for optimum tone adjustment. But it was inexpensive.

RCA Victor Model 68R3: ca. 1955 An eight tube superheterodyne, this receiver was a marginal performer despite the high tube count. There is no TRF stage and only one stage of intermediate frequency (IF) amplification. A single pentode audio output stage provides sound with noticeable distortion. It has internal loop antennas for AM and FM bands, but no loopstick antenna for high- efficiency pickup. Tube lineup: 6BE6 AM pentagrid converter, 6BE6 FM pentagrid converter, 6AU6 local oscillator, 6BA6 IF amplifier, 6SQ7 AM detector, AVC and AM/FM audio amplifier, 6AL5 FM ratio detector, 6K6 power pentode speaker driver and 5Y3 dual diode high voltage rectifier.

Grundig Werks AM/FM Stereo with record player Model SO 101/60PX: ca. 1960 This German-made superheterodyne receiver had eight German tubes and a selenium rectifier for its internal DC power supply. The record player operated at 16, 33-1/3, 45 and 78 RPM. It was originally priced at $240.00. The combined AM/FM design lacks the necessary circuits for good FM performance. This lack was not necessarily obvious near the big cities in 1960, where there were few, but powerful, FM stations. Out in the countryside, its shortcomings were painfully obvious. The audio section of the set was near state-of-the-art, and normally delivered good fidelity with low distortion, significantly better than the average broadcast receiver of the day and an advantage when playing records. Tube linup: ECC85 dual triode RF amplifier, ECH81 pentagrid converter and triode in one tube envelope, EBF89 duodiode (second detector) and pentode (IF amplifier), ECC83 dual audio triode for tone controls, ECC83 dual audio triode for first audio stereo amplifier, two EL95 beam power tetrodes for speaker amplifiers, EM84 "tuning eye" indicator.

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