You could only use DC, not AC, for the lamp, because the blocking diode would pass half of any AC on the busy lamp lead, causing the A relay in the line circuit to chatter.
That's why the original BSP called for switching the lamps from 10 volt ones to 24 volt ones, as the most prevalent source of DC was -24 from a typical key system supply.
But, of course, there were resourceful I/R people who quickly figured out how to derive -10 Vdc and then the field hassle of carrying 24 Volt lamps down in the bottom of the tool box was solved.
The 10Vdc was manufactured by putting a full-wave rectifier on the output of a Trimline transformer, (output 6-8 Vac nominal) and grounding the positive side through a forward-biased BMF diode to drop the voltage another 0.7 Volt. On average, it gave slightly more than 10Vdc, but the lamps survived due mainly to the voltage drop across the many feet of 24G IWC through which the lamps were powered.
