Join Date: Jan 2005
Location: San Juan Capistrano
Unfortunately you cannot rely on what it says on the package.
That was my thought when I read B24Spider's request. The tolerance these bulbs are made to, differences between vendors, differences between how the bulbs were made in the 1970's versus today, etc etc might result in very different wattages and luminosities.
If you want to achieve a certain light level, another approach would be to put resistors in series with the filaments. For example if you knew you had a 25.00 watt filament and wanted to reduce its power dissipation to 20.00 watts, my calculations show that putting a 1.44 Ω resistor in series would do the trick (assuming the circuit receives 12.00 volts).
But I don't know whether a modern bulb rated at 25 watts that was "throttled down" to 20 watts would produce the same luminosity as an old-time 20 watt bulb. A better bet might be to use a rheostat to experiment with the level of added resistance to produce the desired luminosity. Then find a fixed resistor with the corresponding value (which might be tough, as resistors in the 1.5 Ω range aren't so common).
San Juan Capistrano, CA
'63 Guilia spider
'65 Guilia Sprint GT
Last edited by Alfajay; 06-12-2019 at 08:07 AM.