I have found myself in a confusing situation regarding old (pre-1997) 35-bulb mini light sets. I recently took home a 35-bulb set of a friend’s (probably from the 1980s or early 1990s) to replace some burnt out and faded bulbs on it. This morning I started to replace some bulbs on it. I tried using modern 3.5V 0.42W bulbs to replace the originals. I quickly found out that this is not the right type of bulb, as they run extremely bright in the set. I tried 2.5V 0.425W bulbs instead (I know some older sets advertised being compatible with “2.5-3.5V” bulbs) and they seemed to work better, but run a bit dim. I ran into a similar situation with a set of my own as well (from 1996). In this instance, the 3.5V 0.42W bulbs blow out immediately when inserted, and the 2.5V 0.425W ones seem to work fine. 3.5V is technically the right voltage for a 35-light set, and all of my modern thick-wire (post-1996) sets work fine with the 3.5V 0.42W bulbs. I am almost wondering if the 3.5V bulbs used in the old sets are of a higher or lower wattage than the modern ones. There is an energy-saving 2.5V bulb (0.25W) that is commonly used on sets longer than 150 lights, but I have never heard of anything similar for 3.5V. Maybe it’s just so old that it’s lost to time? Or is it possible that the 2.5V bulbs are really the right type for these older 35-bulb sets? Even if they aren’t, is it safe to use them in these sets or will they cause the other bulbs to run at a higher voltage?

It is not about the voltage rating, but how fast the lamps heat up

The 3.5V lamps you used have lower thermal mass (possibly due to thinner filament), so they heat faster than the other lamps in the string

As the lamps heat up their resistance rises, and so does the share of the voltage they take from the series circuit. They heat up more, the resistance rises more, and so on. Suddenly they get a voltage which is not 3.5V but several times higher, before the other lamps warmed up at all

It will likely be ok if you use identical new lamps for all of the lamps in the string at once, as long as there is no variation between the individual lamps that will cause the same effect or non-uniform filament inside the lamps themselves

There atleast 3 main different types (wattages) of 3.5v mini bulb

The original ones (which were a lower current/wattage)
The 'superbright' type (which were indeed brighter, and a higher current/wattage)
The newer ones (which were a lower current/wattage than superbright (and somewhat lower than the original))
A 'energy saver' version does exist too, but is rare.

With 2.5v bulbs there are 6+ types!
All these different bulbs makes it confusing & hard for people.

Then you have different voltages ontop of that

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The key with mini bulbs is matching current on bulbs...
Say (as pure example) the set had 100ma bulbs, and you put in a 150ma bulb, that bulb would be dimmer.
Now say you did the opposite, put a 100ma bulb in a set with 150ma bulbs, that bulb would be brighter.
...and here's where things get interesting:
If you were to put a 12v (or any other voltage) 100ma bulb in a set with 2.5v 100ma bulbs, all would light at their correct brightness!
So as long as total bulb voltage added up to 120(or more) you can mix different voltages of bulbs in a set with zero issues.
But the thing is, light companies decided to go with different current ratings for different voltages of bulbs. (and ontop of that different currents of the same voltage)