These were used a lot in valve guitar amps for effects. I have made them up over the years using an LDR (Light Dependent Resistor) and a LED and they work a treat.
Yepp, as i said in my post above, used for tremolo in valve amps, like most Fender amps made around and after the mid 60s, though Fender and most manufacturers used a neon lamp for the light source. Some of the amps made later on also used a similar device to do the channel switching, with an LDR and a LED, same as Mike described above.
