I noticed that some 153 block signals have resistors underneath them and some do not. What is the difference between the two different types?? Were they made in different years?? I have 2 of my own, but they do not have resistors. If I was to get some with resistors, whats the difference? They wire up the same??? Thanks.
Before 1950 they used 6-volt lamps. The resistor was put in series with the common to drop the transformer voltage to a suitable level. Those signals also had sockets for miniature screw-base lamps. The later signals used 14-volt bayonet-base lamps.
Only the later signals can be operated directly from a control rail without being rewired. The earlier signals need a 153C contactor (or a relay).