RCA Radiola Superheterodyne
By W.L. Carlson, R.S. Holmes, and N.E Wunderlich
The designers of this receiver
The following material has been reproduced from the November 1930 issue of Radio News. In 1930 RCA had released its patents on the Superheterodyne to its licensees, opening the market to greatly improved radio receivers. This RCA model was "state of the art" in 1930. It is also interesting to read the understanding of "high fidelity" described here.
>In order to give the excellent reception to which the present day broadcast
listener is entitled, a receiver must give much better performance than would
have been considered satisfactory a few years ago. It must be able to bring in
the distant stations without interference from powerful local stations, and at
the same time give high quality reproduction of programs coming from nearby
The ease of obtaining high amplification and selectivity
in an intermediate frequency amplifier is chiefly due to the relatively low
frequency used and to the fact that the characteristics of such an amplifier are
independent of the broadcast frequency to which the set is tuned. In the 1930
RCA Radiola superheterodyne an intermediate frequency of 175 kilocycles has been
chosen as the best compromise between amplification, stability, selectivity and
PRE-SELECTION and the RADIO-FREQUENCY AMPLIFIER
In order to eliminate extra responses in a superheterodyne, pre-selection at the
incoming broadcast frequency is required. In any type of receiver it is desirable to have some
selectivity before the first tube in order to eliminate inter-ference, such as
secondary modulation. It is also desirable in a super-heterodyne to have a
relatively high signal level at the grid of the first detector (or frequency
Following the radio-frequency tube is a capacity coupled radio-frequency transformer, which, with the tube, gives a uniform amplification of about thirty over the broadcast band.
OSCILLATOR and FIRST DETECTOR
The oscillator circuit is a conventional one
for use with a three-element tube. It consists of a tuned grid circuit with a
plate feed-back coil coupled to it. The grid of the tube is connected to the
mid-tap of the tuned grid circuit to minimize change in oscillator frequency
with tubes. The oscillator is self biased by means of a grid leak and blocking
HiFi? Note that at 5 kHz the response is down about 50%
The function of the intermediate-frequency amplifier is to furnish the major portion of the amplification and selectivity of the receiver. It consists of three transformers and two amplifier tubes.
The first intermediate-frequency transformer is connected to the "local-distant" switch so that in the "local" position the selectivity is impaired slightly in order to prevent side-band attenuation. In the "distant" position the transformer is extremely sharp and offers greater attenuation to frequencies outside the desired band.
The transformer consists of a tuned primary connected in the plate circuit of the first detector and a tuned secondary connected in the grid circuit of the first intermediate amplifier tube. The secondary is partially shielded from the primary to loosen the coupling and improve the selectivity. The transformer is mounted in a copper can to keep the losses at a minimum, while at the same time shielding the transformer windings from other parts of the circuit. When the "local-distant" switch is thrown to the "local" position a resistor is thrown across the primary and another in series with the secondary to broaden the resonance of the transformer and reduce its amplification.
The second and third inter-mediate-frequency transformers are both alike, and consist of tuned primary and tuned secondary coupled tightly enough to give a broad top resonance characteristic with high attenuation to frequencies outside the desired band. These transformers are mounted in iron cans to shield them from other parts of the circuit and to add sufficient loss to prevent double peaks in the resonance characteristic. All three transformers have adjustable capacitors across both primary and secondary for accurate tuning.
Plate circuit rectification and a single stage of audio amplification are used in this receiver. This system eliminates the grid leak detector and two audio stages commonly used in the past. Plate circuit rectification eliminates the loss of high acoustic frequencies due to the time constant of the grid leak and condenser. The low audio gain causes a correspondingly low a.c. hum and decreases the tendency for microphonic howl.
The "Power Plant" described in the text. Tubes used are one 80 and two 45s.
The blasting and breaking up of the sound output, when tuning through a local station, is ordinarily due to overloading of the output tubes. This disturbance is reduced to a minimum in this receiver by designing the audio system so the detector overloads at about the same time as the audio tubes.
The two -45 tubes in push-pull are used in this receiver, thus providing a large output without distortion.
LOUD SPEAKER AND CABINET ACOUSTICS
The loud speaker used in this receiver is an improved electro-dynamic instrument especially designed to give excellent quality and high output without distortion.
Very great care has been taken in designing the cabinet for this receiver in order that the reproduction may be brilliant and faithful. Holes have been bored in the cabinet under the loud speaker to prevent cabinet resonance, and the dimensions of the cabinet are correct for the loud speaker used.
Overall response curve. Typical of the era.
"FUNCTION OF THE LOCAL-DISTANT" SWITCH
Switch The adjacent channel selectivity and the fidelity of broadcast receivers are so related that it is impossible to emphasize either of these characteristics without a corresponding sacrifice in the other. In general, the characteristic most desired in a receiver, when receiving distant stations, is good selectivity, so that the station may be satisfactorily received without interference from adjacent channels. In receiving local or powerful distant stations, where interference is usually less, the most desirable characteristic is good quality. The "local-distant" switch in this receiver is arranged to perform this function.
At the "local" position the receiver is still very sensitive, but its selectivity is impaired slightly in order to get better tone quality. The receiver should normally be operated with this switch in the "local" position. In the "distant" position better sensitivity and selectivity are obtained, and throwing the switch to this position will give better volume on extremely weak signals and on all signals the frequency of which is but slightly different from powerful local stations. The difference in tone quality of the two positions of the switch, while noticeably better at the "local" position is still very good at either position. At times when the noise level is high, throwing the switch to the "distant position," even when receiving a local station, may give better results because of the greater selectivity of the receiver in this position.
Volume control is accomplished b varying the control grid bias on the radio-frequency and first intermed-iate-frequency amplifier. This gives a balanced reduction in amplification at both radio and intermediate frequency when the volume control is reduced.
The amount of shielding required in an ordinary receiver is determined, to a large extent, by its amplification or sensitivity. The purpose of the shielding is to eliminate feed-back which would cause instability or oscillation. In general, the higher the frequency to which an amplifier is resonant the more likely it is to be unstable. In a superheterodyne most of the amplification takes place at the low intermediate frequency, so that much less shielding is necessary than would be required at broadcast frequencies for the same sensitivity.
In the power supply a -80 rectifier tube is used. The filter system is especially
designed to eliminate a.c. hum from the loud speaker. The loud speaker field is
used as one of the inductive elements of the filter, so the total rectified
current flows through its winding, providing a powerful magnetic field.
The dial indicator is of the projection type and is calibrated directly in kilocycles so that the receiver can be turned directly to a station once its frequency is known.
TESTING THE RECEIVER
In the construction of any receiver, the methods of testing the
various components determine in a large measure the ultimate results to be obtained
from that receiver. The Radiola superheterodyne is indicative of the truth of this