Recently, I was helping a friend put together a collection of radio related matchbook covers for display at the VRPS convention, and came across one which touted the new "GE Colorama Tuning", which was supposed to be some advance in accurate tuning devised by General Electric. The thing was, I had never heard of it, and had no idea what it might be.
After the convention was over, I was helping with some general clean up, and our delightful president Jim Sargent offered me a sheaf of papers, and asked if I'd like to have a Rider's Index. I'm a sucker for this stuff, so I accepted it. A couple of days later I had time to look at what was there, and it indeed contained a Rider's index, sans covers, but also some original GE service literature. A few of the models had the Colorama Tuning feature!
With some excitement, I immediately read the literature on one of them, and while it described what the circuit did, no explanation was given for how it worked. So, I looked at the schematic, found the circuit, and in a couple of minutes figured out how it worked. The circuit is so simple, and yet so clever, I thought it would make a nice writeup. So, here it is.
The tuning dial markings of a radio with the Colorama Tuning feature are illuminated by lamps. When tuned off station, the tuning dial markings are illuminated red. As one tunes to a station, the color of the markings gradually turns from red through white to green when the station is perfectly tuned. The way this works is there are four red lamps and three green lamps behind the dial, with each green lamp between two of the red lamps. In between stations, the red lamps are fully illuminated and the green lamps are off. As one tunes to a station, the red lamps are gradually dimmed and the green lamps are gradually brightened until the red lamps are off and the green lamps are fully on.
The way the Colorama Tuning Indicator circuit works is both simple, and clever. It relies upon the fact that in any series circuit, the total current is the same in all portions, and that in parallel circuits, the current divides amongst the paths available to it. The circuit uses a special inductor as one part of a parallel circuit, in order to apportion the current available amongst the lamps. See Figure 1.
The GRs in circles represent the green lamps, while the RDs in circles represent the red lamps. The tube and the transformer looking thing are the control circuitry for the lamps. Let's consider the lamp circuitry first, then the control circuitry.
As you can see, the three green lamps are in series, with the series string in parallel with a coil. The red lamps are in series by twos, but the two series strings are in parallel. The unusual connection of the lamps is key to the operation of the circuit.
In order to understand this clever circuit, it's best to examine the unusual connection of the lamps first. So, for the moment, disregard the tube and the thing which looks like a transformer, and concentrate on the lamp connections. Imagine for the moment, that we simply cut the connection to the coil connected to the lamp circuitry. This is the effective circuit when the radio is tuned to a station, and the coil is made not to conduct current. See Figure 2.
Each lamp requires a specific voltage (6.3 V) at a specific current (150 mA) to be fully illuminated. The three green lamps in series therefore require three lamp voltages at one lamp current to be fully illuminated.
Two red lamps in series require two lamp voltages at one lamp current to be fully illuminated, so the parallel connection of the red lamps requires two lamp voltages at two lamp currents, one for each branch, to be fully illuminated.
If we apply four lamp voltages to the circuit as shown, since the voltage is adequate to push the requisite current, the green lamps will fully illuminate. However, they will only conduct one lamp current at that voltage, and that is not enough to illuminate the red lamps. R29 and the red lamps do, however, eat up one lamp voltage.
When illuminated, the lamps have a resistance of 6.3 V / 150 mA, or 42 Ω. When not illuminated, however, they have a resistance of less than 1 Ω. Since the red lamps are more or less "cold", they have low resistance, and nearly all of the voltage appears across the green lamps. The red lamps will glow only dimly, if at all. The dial will glow green.
Consider Figure 3. Suppose we were to make the coil to conduct current such that its only effect would be its resistance. This is the effective circuit when the radio is tuned between stations, as explained below. Its 30 Ω resistance would be in series with a 15 Ω resistor, for a total of 45 Ω, which is suspiciously near the illuminated resistance for a single lamp. This is not a coincidence, as we shall see.
As explained above, the four red lamps require two lamp voltages to illuminate. The coil resistance and 15 Ω resistor together make up another lamp equivalent. So, the red lamps in series with the resistor require four lamp voltages for the lamps to be illuminated, just as the three green lamps do. Let's ignore the green lamps for a moment, and consider the implications.
If the green lamps were not in the circuit, the red lamps would see just the voltage they need. Two lamp currents would flow, one each through the two parallel branches of red lamps. This current would induce two lamp voltages across the total resistance of the coil and resistor. This is the absolute maximum voltage which could appear across the three green lamps. However, they require three lamp voltages to fully illuminate. So, they get only 2/3 the voltage they need, at most. However, since they aren't fully illuminated, they have a low resistance which is in parallel with the coil's resistance, and the voltage is even less. The green lamps will glow only dimly, if at all. The dial will glow red.
If we can modify the behavior of the coil connected to the lamps in such a manner that it either does or does not conduct alternating current, we can control which set of lamps is illuminated. It is time to consider the circuitry which controls that coil.
The control portion of the Colorama Tuning circuit is shown in Figure 4. A moment's look at the sizes of the bypass capacitors will convince the observer that there are no alternating voltages or currents present in this circuit. It is a pure DC circuit. The only external connections are to pure DC voltage sources. Appearances notwithstanding, L27 is not part of a transformer.
The circuit uses the tube to control the direct current flowing through L27. The control voltage comes from the Automatic Volume Control (AVC) voltage developed by the detector circuitry. When the radio is tuned between stations, the AVC voltage is low, near zero volts. When the radio is tuned to a strong local station, the AVC voltage changes to perhaps -12 V.
From the tube data sheet, we see that when the radio is tuned between stations, the the tube passes the maximum current it can, given the values of B+ and the total resistance of L27 and R28 (about 7,000 Ω). This comes to perhaps 15 mA. When the radio is tuned to a strong local station, the plate current drops to about 2 mA. Let's consider L27 and L28 more closely.
As shown in Figure 5, coils L27 and L28 share a common core. However, since only direct current flows in L27, they do not function as a transformer. How do they function? They function as a saturable reactor.
A saturable reactor is an inductor whose effective inductance can be controlled. To understand what that means, one needs to know how inductors function in AC circuits. Inductors have a sort of AC resistance in addition to their DC resistance, which limits the current that can flow through them, though without generating heat or consuming any power. This happens because of the changing magnetic fields which surround the inductor.
It is commonly known that electric currents generate magnetic fields. We call magnets using this effect electromagnets, and the most common one encountered in vintage radio is the field coil of electrodynamic speakers.
What is commonly known ain't necessarily so.
The truth is that electric currents create a stress in the volume of space surrounding them. This stress tends to magnetize anything in that volume. The amount of magnetism which results depends upon how magnetizable the objects in that volume are. Space itself is very slightly magnetizable, and so an electric current induces a magnetic field in empty space. Iron is much more magnetizable than space, so a current flowing in a coil of wire surrounding an iron core generates a much stronger magnetic field than one without such a core.
The way this works is that iron atoms themselves are tiny magnets. Normally, these atoms align themselves so that their magnetic fields are parallel to, but oppositely oriented (anti parallel) to the nearby atoms, so that from a distance there is no net magnetic field, and we say that the iron is not magnetized, though there are many many magnetic fields actually present. If there is a current flowing nearby, the magnetizing force it induces causes some of the atoms to align with it, thereby generating a stronger magnetic field than would be induced in empty space. The stronger the magnetizing force, the more atoms become aligned, and the stronger the induced magnetic field. Clearly, this cannot continue forever.
At some point, all the atoms are aligned, and we say that the core has become "saturated", by analogy with a sponge which contains as much water as it can hold. In this state, increases in the current cause increases in the magnetic field only by increasing the magnetic state of space itself. The core no longer augments the field any further, and it's as if it weren't present. So long as the core remains saturated, changes in the magnetizing force result in small changes in the magnetic field, as if the core were not present.
Consider a coil of wire with a direct voltage suddenly applied to it. Initially, a current attempts to flow, limited by the resistance of the coil. However, this current induces a magnetizing force, resulting in a magnetic field. A changing magnetic field creates an electric force in the volume of space surrounding it. The electric field pushes on the electrons constituting the current. This is the so-called motor effect, which is used in all electric motors.
As it increases, the induced magnetic field pushes on the electrons in such a way as to oppose any change in the current. Since we just applied the voltage, the building magnetic field "pushes back" on the electrons. So, the current in the wire does not instantly jump to the value predicted by DC considerations, that is only the voltage and resistance. It increases gradually, until it eventually (asymptotically) reaches its maximum value predicted by DC theory. The stronger the induced magnetic field, the more forcefully it pushes on the electrons, and the more slowly the current increases. So, with a core present, the current increases only very slowly, but without a core it increases very rapidly. This property of resisting changes in current is called inductance, and it is increased by the presence of a magnetic core.
If we apply an AC voltage to the coil of wire, we see that the current will never manage to achieve the value predicted by DC theory, since the voltage applied reverses polarity regularly. The more quickly the voltage reverses (the higher the frequency) the less time the current has to build up, and the less AC current flows through the inductor. Likewise, with a core present, the current increases more slowly, and consequently less AC current can flow. The presence of a magnetic core increases the effective inductance, and reduces the amount of AC current which can flow.
L27 and L28 are two inductors sharing a common core. Although they do not act as a transformer, they do still interact. In particular, the DC current flowing in L27 induces a magnetic field in their common core. This current is variable from practically zero, up to an amount which can saturate the core. This varies the effective inductance of L28. An inductor whose effective inductance can be controlled in this manner is called a saturable reactor.
When the radio is tuned to a strong station, the AVC voltage is at maximum negative value, and the plate current is essentially zero. Since no current flows in L27, it has no effect on the core, and the inductance of L28 is maximum. Very little AC current can flow in L28, and it is effectively not in the circuit. We saw the results of this above: the green lamps are lit, and the red lamps are dark.
When the radio is tuned between stations, the AVC voltage is essentially zero, and the current in L27 is a maximum. This saturates the core, and the inductance of L28 is likewise a minimum. It offers very little impedance to the AC current flowing through it, and effectively shorts out the green lamps, which are dark. The red lamps, however, are fully lit, as shown above.
When the radio is being tuned to a station, the AVC voltage takes on intermediate values, and the effective inductance of L28 changes from a very low value to a high one. During this time, the relative brightness of the red lamps gradually decreases, and that of the green lamps gradually increases. When both are equally lit, the impression is that the dial is illuminated with white light.
This simple but clever circuit uses the AVC voltage developed when the radio is tuned to a station to vary the current through a saturable reactor. Because of the clever arrangement of the lamps, the available current gets apportioned either to the green lamps, or to the reactor and red lamps. The result is that the color of the tuning dial changes from red, when the radio is tuned between stations, through white to green as the radio is tuned to a strong station. In this manner, the color of the tuning dial illumination acts as a tuning indicator, hence the name Colorama Tuning.