How Do Converter Tubes Work?

After the variable condenser, the next stage in our superhet circuit is the converter tube. The converter is the most complex tube in our radio. It has three jobs. Help generate the oscillator signal. Mix the oscillator signal with the antenna signal. And amplify the mixed signals.

It’s called a converter tube, because it helps convert the radio station signals into the IF signal. It’s also sometimes called a mixer tube, because it mixes the oscillator and antenna signals.

Let’s take a closer look. As with the rectifier tube, the converter tube has a filament, cathode, and plate. In addition, it has five elements called grids. On the schematic, grid four is not shown. But it exists, and is internally wired to grid two.

The first two grids are connected to the oscillator, which constantly changes voltage in relation to the oscillator frequency. When the signal is fed to the grids, the grid voltage changes in relation to the oscillator signal. When the signal is more negative, the grids repel the electrons coming from the cathode, and little current passes to the plate. When the signal is more positive, the grids attract the electrons, and more current passes to the plate.

Because the grids are changing from negative to positive in relation to the oscillator signal, that same signal is duplicated at the plate. But the signal is much stronger now, because the current flow from cathode to plate, is far greater than the tiny current coming from the oscillator. This is how tubes amplify.

In addition to the oscillator signal on grids one and two, the antenna signal is applied to grid three. So the plate not only has the amplified oscillator signal, it has the amplified radio signal. And as we’ve learned, when two frequencies are mixed, you get four frequencies. The original two. The sum. And the difference, which is the important I.F. signal.

Filtering the unwanted frequencies from the IF is the job of the I.F. Transformers, which I’ll discuss in a later section. But first, I’ll explain grids four and five, and in the next section, how the oscillator works.

Grid four is called the screen grid. It’s positively charged, to shield the electrons from the oscillator and antenna grids, and help them move toward the plate.

As fast moving electrons flow from cathode to plate, some of the plate’s electrons can become dislodged and interfere with the antenna and oscillator control grids. This is called secondary emission, and it can cause instability within the tube.

Grid five is the suppressor grid. It’s negatively charged to repel electrons that dislodge from the plate, and suppress secondary emission

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