RCA Radiola Super-Heterodyne AR-812
Edwin H. Armstrong and Harry Houck were the main engineers to bring forth the first RCA superhet in March 1924. It was an ingenious model of the superhet that operated on the second harmonic principle.
The large number of tubes used in the standard superhet was a significant factor in limiting its use for home broadcast reception. The tubes in a standard superhet were only performing one function. Armstrong reasoned that the price of a superhet could be significantly reduced if some of the tubes could be used for double duty, reducing the total number of tubes required. It was called reflexing, and it wasn’t anything new to the superhet; Armstrong and his associates tried to make the oscillator tube do double duty in their first experiments in France in 1918. It wasn’t very successful because the input tuning circuitry and the oscillator were adjusted to nearly the same frequency (to produce a low IF), and this resulted in interference between the two tuning controls. Houck solved the problem by using the second harmonic of the oscillator to mix with the received signal.
The front center panel of the AR-812 folds down to reveal the six-tube catacomb. Most of the electronics for the radio, including the IF transformers, were sealed in wax inside the catacomb. The catacomb sealed out moisture and protected the various components, but it also kept prying eyes out! This was state-of-the-art technology in 1924!
At right is a photo of the open left battery compartment. The right side had a similar door to hide batteries. With an internal loop antenna hidden behind the catacomb and a handle on top of the radio, this was considered a portable radio. The UV-199 tubes helped reduce battery consumption.
Close up view of the right-side oscillator tuning dial. Close up view of the Radiola AR-812 nameplate.
Simplified wiring diagram of the RCA second harmonic superheterodyne from the 1926 Radio News Superheterodyne Book.
The basic operation of the RCA reflexed second harmonic superhet is as follows (refer to the simplified diagram): The signal is tuned in with the loop and its associated tuning capacitor (this cap constitutes the main tuning control on the left front panel). Capacitor C is a small value and it readily passes the high frequency signal to the grid of the first tube, a radio frequency amplifier. The first tube also does double duty as the first IF amplifier, as will be seen. The radio frequency transformer (RFT) in the plate of the first tube is designed to pass all signals of the broadcast band to the second tube.
The second tube is the heterodyne oscillator, and it also does double duty as the first detector. The second harmonic of the oscillator and the received signal mix inside the tube and they produce the IF signal (40 KC). The oscillator tuning capacitor constitutes the main tuning control on the right front panel. The plate current of the oscillator tube contains the IF frequency, and this frequency is impressed on the primary of the first IF transformer, IFT1. The secondary of IFT1 feeds the grid of the first tube and the tube amplifies the IF frequency. As mentioned before, the first tube performs double duty as the first RF amplifier and the first IF amplifier. The two signals are impressed on the grid in shunt fashion, or parallel. Capacitor C is small, which readily passes the high radio frequency, but at the same time it blocks the low intermediate frequency. The secondary of IFT1 acts as a high impedance for the RF signal. Therefore, the two very different frequencies have minimal effect on each other.
The plate current of the first tube has a combination of RF signal and IF signal. The RF signal is developed across the primary of the untuned radio frequency transformer (RFT) and is passed on to the second tube. The primary of the radio frequency transformer has a low impedance for the IF, so the IF signal is passed on to the primary of the second intermediate frequency transformer, RFT2. The primary of the second intermediate frequency transformer acts like a high impedance to the RF sigmal, so again there is minimal interaction between the two signals. Like mentioned in the section on the Cockaday, reflexing works because high and low pass filters route the different signals where they need to go. For another example on reflexing see the Cockaday circuit.
It was a complex matter to design a reflexed second harmonic superheterodyne. It essentially eliminated two tubes from the circuit, and saved a little bit on battery consumption. The set may have cost a little less for the consumer as well. However, at $269 bucks a pop (minus batteries), the real winners were Armstrong, Houck, and RCA!
Photograph of Edwin H. Armstrong with a prototype of the RCA second harmonic superheterodyne. This photo was in the May 1924 Radio magazine.