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Discussion Starter #1
....to match the others on my 1955 Lancia.

I am looking for older two filament 12V brake/turn signal bulbs with 20/3 watts rating and "standard" BAY15d base.

Philips 12500 and Osram 7241 are the designations of the original bulbs, but later production uprated them to 21/5 watts. They don't play well with the 20/3 bulbs, and they're also noticeably brighter.

Maybe someone has a box of these gathering dust on a shelf. Or even a couple sitting around?

My thanks,
Steve
 

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Unfortunately you cannot rely on what it says on the package. I recently tested Osram (Sylvania in USA) 7528 Long Life listed as 21/5 and 7528 (regular) listed as 25/5. When tested at the prescribed (Osram literature) 13.5 volts they both came out to be 26 watts.
 

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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).
 

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I assume his interest is "heat". That is why I investigated bulb wattage. I don't want to melt unreplaceable lens.
 

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>:)A person could do a Rube Goldberg and put a rheostat in the circuit.
 

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Discussion Starter #7
(osso- Thank you for the move.)

Thanks all, panic temporarily averted.

Actually, my primary consideration is originality; as the Spider is a preservation car, even the headlights are the original 45/40 watts. Like I said, dim bulbs. Plus, it's a nice, soft look, sort of like the car.

LED lights would be way too bright/harsh, and though I have some spare NOS tail-light lenses, they're way too full of pigment, they'd look out of place; it wears the originals that are moderately bleached, but not deformed or crazed. The swirls are great. And carl750 is right about heat; I'd be afraid LEDs are way too hot for old plastic.

Another consideration is electrical; the Marelli DN22 single field generator, also fit to the early Alfa 1900, puts out a whopping 200 watts. When you add up all the components that could be in use at the same time, such as a variety of lights, Mousebreath™ heater fan, wipers, etc., well, it gets real close to it's limits. If I changed all four corner lights to 21/5 I'd be adding 12 watts to the load, 6% of capacity. Doesn't sound like a lot, but when you're close to the edge...

I suppose if I get truly desperate I could try adding resistors, but I'd really like to avoid the mickey mouse. Digging through my shelves yesterday I was lucky enough to find a pair of NOS Aurelia B10 sedan front turn signals, of which one still had it's "3/20"W bulb! I may have used the other one already, years ago (I really don't remember, I thought they were empty). I should test it to see what it's really rated at (carl750, can you point me to/supply me with Osram's test info? Thank you.), and if I can, find some way to test it's luminosity before I put it in. At least then I'd know what I might someday be trying to match.

So, I'm good for the moment, but it's just a matter of time before another burns out, so I'm still looking. Just not quite as desperate...
 

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I'd be afraid LEDs are way too hot for old plastic.
Not that I'm trying to sell you on LED's, but... the fact is that LED's consume far less power than incandescent bulbs and as such, run much cooler. So if your primary goals were to put less strain on that antique generator and to ensure that rare lenses don't get melted, LED's would be a great solution. But if originality is your primary concern, then no, they won't look right.

Another consideration is electrical; the Marelli DN22 single field generator, also fit to the early Alfa 1900, puts out a whopping 200 watts.
Holy smoke! That's only 16.7 amps @ 12V. I guess you only use the air conditioning when it's really hot.
 
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