Dim Dial Light Bulbs



Some early AC/DC radios, the Bush DAC90A for example, use 3.5 Volt 0.15 Amp dial light bulbs. The problem is that these bulbs are no longer manufactured. Even if they were still available, they were so dim that they did nothing to light up the dial. The Bush DAC90 is even worse with only one pilot bulb to light the dial. It is possible to use a series resistor or voltage divider network to drop the mains voltage to suit readily available 6.3 Volt bulbs, but resistors get very hot. It is also possible to change the value of the original shunt resistor across the bulbs in the DAC90A, but that can cause more problems. I love the warm glow of the lamps from the radio dial so, I looked for a solution.


I’ve found a source of 6 Volt transformers which will power two 6.3 Volt dial light bulbs nicely. The encapsulated transformers create virtually no heat and are physically small, fitting inside most radios without making any permanent modifications to the chassis or wiring. The bulbs will give a constant brightness and light up the dial beautifully.


The bulbs will be at full brightness from switch-on with no dimming or slowly reaching a dull glow as the radio warms up. Apart from the obvious benefit of having a decently lit dial, the other advantages are numerous. Firstly, 6.3 Volt bulbs are inexpensive and readily obtainable. With the original design, the shunt resistor will become very hot if the dial lamps fail, and some radios will not work at all if one or both lamps fail. With the transformer fitted, there will be less heat generated inside the radio and, should the bulbs fail, the radio will still work with no detrimental effects such as overheating. If one bulb fails, the other will still work.


There is no permanent modification required to your radio. The last thing you want is to devalue your vintage radio by permanently modifying it. With my modification, the radio can be put back to its original design at any time simply by removing the transformer, swapping a couple of wires around and fitting the original bulbs.


The cost of supplying and fitting the new transformer and bulbs to your radio will be in the region £25, depending on the type of radio you have. I have the transformers and bulbs in stock, so email me or give me a ring and arrange to bring your radio over to the workshop for this truly effective and worthwhile modification.


Apparently, some engineers are fitting their own version of my bright dial light system. This is fine, but the modification must be carried out correctly. The total voltage and dropper resistance required for the valve heater chain has been carefully calculated to include a shunt resistor and two 3.5V 150mA dial light bulbs. Unless the total resistance of the shunt resistor and original bulbs is taken into account when modifying the circuit, the valve heaters may be damaged by excessive voltage or operated at a voltage which is too low. An excessive heater voltage will drastically shorten the life of the valves. A heater voltage which is too low will have a detrimental effect on the reception and volume of radio stations.


I’ve heard that some engineers are increasing the brightness of the original dial lamps by incorrectly adjusting the mains voltage setting on Bush DAC90, DAC90A and DAC10 radios. The mains voltage in the UK is permitted to vary between 216 and 253 Volts – the norm being 230 Volts. If your radio has been set to run on 220 Volts and the mains reaches 250 Volts, it will be over-run by 30 Volts. The bulbs will certainly be a lot brighter, but this will cause overheating and drastically shorten the life of the valves and other components. Be warned, only allow a qualified engineer to work on your treasured Bush radio.

I don’t mean to bore you with the above warnings but safety and correct operation of your radio is of paramount importance. I care about vintage radios, and how they are treated, repaired and modified. The modifications I carry out are easily and completely reversible. If you care about your vintage radio and you’d like it repaired or restored, then talk to me about it. Check out the before and after photos below.