Power Supply for Battery Valve Radios

  
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Many later radios use four 7-pin valves and require a 90V HT supply at typically 12mA and a 1. 5V LT supply at 125mA or 250mA depending on the valves used. The original batteries are sadly no longer made. One possibility is to make up a 90V battery using ten PP3 batteries in series for the HT, and to use two D-cells in parallel for the LT. This has the advantage that the set
Power Supply for Battery Valve Radios - schematic

remains portable. However, the cost of the batteries is not insignificant - around £10 even when paying trade prices and probably over £25 if buying retail. The most popular solution is to make up some sort of mains power supply, and a suitable design is presented here. It should be possible to make the unit for around £30, and the running costs are negligible. Depending on the components used and the size of the battery compartment in your radio, you may be able to make the unit small enough to fit in place of the batteries. The photographs on this page were kindly supplied by Richard Newman, and show his completed unit. Comments, suggestions and modifications made by Richard and a couple of other constructors are given at the end of the page. To keep the size and cost to a minimum, a single transformer us used to provide both the HT and LT supplies. Since transformers with the correct voltage secondaries are not now available, I have chosen a standard transformer and designed appropriate circuitry to achieve the voltages required. A 12VA transformer having two separate 15V windings is used. This allows the two supplies to be completely separate. This is necessary because the supplies are generally connected via biasing resistors within the radio, so any connection between them within the power supply would affect the operation of the radio. For this reason, a transformer with two separate 15V windings MUST be used - do...



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