I built this small linear LED regulator a while back. Initially I built it to upgrade my ОИ-19 microscope illuminator to LED, but I decided to use the remaining light bulbs until they break before I start upgrading it to LED.
I used a couple of soviet potentiometers to go with the theme (СП4-1), it’s a simple standard linear current regulator based off two transistors. The gate current can be adjusted with the internal potentiometer, which allows programming the maximum LED current (up to 800mA). The main transistor (TIP41C) had it’s legs break off, when I desoldered it from somewhere (was probably bent for a heat sink) so instead of throwing it away I just soldered some wires onto it.
The input goes through a bridge rectifier (with heat sink) and is smoothed by a capacitor, this allows operating the lamp also from an AC supply (6-9VAC). The total dissipation should not exceed 3W, otherwise the heat sinks get too hot. As such the input voltage needs to be quite low for higher current LEDs.