EF50 One Valve Radio Power Supply Using AC Transformer

The EF50 heater filament requires 6.3 V at 0.3 A, and the 7806 voltage regulator is capable of providing 6 V and up to 1 A. Some of the newer LDO voltage regulators are more efficient and capable of providing up to 1.5 A of current and do not dissipate heat so much. Therefore, keeping that in mind, ideally, you should use the best component you have available. In addition, a heatsink for the voltage regulators is vital as they do heat up.

The success of this project very much depends upon the quality of the transformer used, because not all transformers perform the same. One requirement is that it must be capable of providing up to two to three amperes of current and approximately 12 V to 15 V AC or thereabouts. You should avoid any lesser specifications that might struggle to provide the necessary current. An ideal transformer should be an isolated type where the mains wires go directly into it and it should have thermal cutout safety features. In addition, one with an RF shielding is desirable, so it does not radiate mains hum or electromagnetic field that could interfere with the operation of the ferrite rod. The power supply should also have safety features such as a fuse to cut the power in case of shorts.

Most hobbyists like me have a box with salvaged transformers, and when I first built this radio, I connected a transformer that was supplying only 8 V DC. The voltage and current were both insufficient and all I heard was mains hum indicating an excessive loading on the transformer. I therefore removed that and connected another capable of providing up to 2 A and 15 V. With this transformer, there was no mains hum and the sound was loud and clear.

For the bridge rectifier module I was using a recycled component that came out of a power amplifier. With these types of components, the current and voltage rating are critical parameters to consider and this particular one was rated at 8 A and 200 V. For the power smoothing capacitor, you could use 1000 µF/25 V, or larger to provide as much smoothing as possible.

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