Life extension for incandescent pilot lights running on lower voltage

[wa2ise]

In the July 11th 2012 TVTechnology trade journal page 27, they have an article on bulb life vs applied voltage. Reducing applied voltage on a light bulb greatly extends life. Efficiency gets bad, but for pilot lights in radios, that's not much of a concern.

life/rated life = (rated voltage/applied voltage)^13.1

An example: In my Tasma Baby, I replaced the 6.3V pilot lights with bulbs rated for 7.2V, which will make them last 5 times longer.

Conversely, a light bulb run at 1.1 times its rated voltage will only last 20% of its rated life.

So if your mains run a little high, and the pilot lights seem to burn out often, try using the 7.2V bulbs (intended for flashlights). In radios with power transformers, not hot chassis radios. Radio shack has them in stores.

Or use a resistor to drop a volt from the 6.3V heater winding to run the bulb.

For hot chassis AA5 radios, a resistor in parallel with the #47 bulb will reduce the voltage across the bulb. Hard to calculate this resistor, as the current thru the bulb and heater tap isn't a nice sine wave or DC, but a combination of sine wave and large current spikes from the rectifier. Try something like 22 ohms and see if the bulb gets too dim. Only real way to estimate the effective voltage across the bulb is to light up another #47 off a 5.3V power supply, when the brightness of the bulb in the set looks the same you have the resistor roughly right.