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Vintage Radio

The development of AC mains power supplies was an important step in the evolution of domestic radio receivers. Understanding how they work is important for vintage radio restorers, especially if the power supply has to be modified in some way.

By Rodney Champness, VK3UG

Perhaps the most common modification to a vintage radio’s power supply is the substitution of a different rectifier valve. This may be necessary if the original type is no longer available or is difficult to obtain. Before substituting a rectifier valve though, it’s important to first determine if the replacement is indeed suitable.

Considerable care is also necessary if a valve rectifier is to removed and converted to a solid-state circuit using diodes.

Different voltages

Valve radio receivers invariably require a number of different voltage rails to supply various parts of the circuit. What’s more, the current requirements for these voltage rails can vary widely, depending on the circuitry that’s being powered.

Click for larger image
This photo shows two common mains trans-former styles from the valve radio era. The one on the left is an above-chassis mounting type while the other is a through-chassis type.

Originally, the necessary voltages in radio receivers were supplied by primary and secondary batteries. The capacity of the batteries depended on the current drain at the particular voltage required. For example, many old radios typically needed just 10mA at 90V for the high tension (HT) voltage supply, whereas a current of 2-3A may have been required to heat the filaments (usually at voltages of 1-5V).

As a result, the HT battery consisted of many small cells of limited capacity in series, while the filament or low-tension (LT) battery commonly used two or three large wet cells with perhaps 100 amp-hours (Ah) capacity.

In short, batteries were used to power the earliest valve radios and also to power the various valve portable radios that were later developed.

Unfortunately, the high power consumption of battery valve receivers meant that the cost of powering such receivers was quite high (this also applied to the later portable sets with their specially-designed “battery valves”). As a result, set manufacturers and experimenters looked at ways of supplying the necessary power to a radio from the mains. In the end, a fairly standard circuit quickly evolved and this was used in a wide range of receivers during the valve radio era.

Of course, running a set from the mains supply restricts where the set can be used. In most cases though, that didn’t matter because the set was installed in a fixed location and the aim was to eliminate the use of batteries which were expensive.

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