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Vintage Radio

Last month, we looked at the development of AC mains power supplies for domestic radios and the all-important power transformer. In Pt.2 this month, we look at some of the other aspects of power supplies that a restorer needs to understand.

By Rodney Champness, VK3UG

Understanding the power supplies in valve receivers is an important aspect of restoration. It’s important that you know what you are doing when restoring such equipment, as incorrect servicing can cause a fire or even result in electrocution.

The power supply circuits used in valve radio receivers are usually relatively simple. They typically consist of a transformer, a rectifier like a 5Y3GT, two 16µF electrolytic capacitors and a 12 Henry choke between them. This was almost an industry standard for many years. Later sets used rectifiers such as the 6X4 and a resistor in place of the choke – see Fig.1.

Replacing like for like components with the same ratings is usually quite reasonable when restoring such circuits. However, because the circuits are relatively simple, many people are often lulled into thinking that any odd-value component can be used to replace a faulty part.

A typical power supply

Fig.1 shows the circuit of a typical power supply as used in many late-model valve mantel receivers. Let’s take a look at the design requirements necessary to ensure reliability for this type of supply, starting with the power transformer.

Generally, the power transformers used in valve receivers are conservatively rated and it is rare to hear of them burning out. Most (but not all) power transformers have a tapped primary winding that (depending on the connections) can accommodate mains input voltages ranging from around 200V AC up to about 250V AC at 50Hz. However, some transformers were designed to work safely on 40Hz mains, as used in Perth many years ago.

Because they are conservatively rated, most transformers in valve receivers will withstand somewhat higher currents than originally intended. This may be brought about, for example, by substituting a valve that draws greater heater and HT currents than the valve originally specified. However, although they may withstand moderate overloads in the short term without too much fuss (but run warmer in the process), this is not recommended long term.

In any case, the transformer should not run hot in normal use.

Voltage regulation

The voltage drop across a valve rectifier varies significantly between low load and its maximum specified load. The figures provided as examples in the following paragraphs depend on the resistance of the transformer’s primary and secondary windings, the type of rectifier used and the value of the first filter capacitor. These values are typically set down in valve data books.

For example, a 6V4 rectifier connected to a 600V centre-tapped transformer secondary winding will have a DC voltage of 424V on its cathode with no current being drawn. This slumps to approximately 300V at its maximum rated current output of 90mA.

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