Rebuilding a Piece of the First Digital Voice Scrambler

In SIGSALY, the analog input voltage is fed into a five-tap logarithmic resistor divider ladder, whose taps drive the thyratron grids. Photo: NSA

Top Secret Installation: The complete SIGSALY voice scrambler was a huge machine that relied on many analog-to-digital converters. After I added power resistors to solve some problems with filament voltages and fixed a few wiring errors, the quantizer began to work a bit, but only two of the five thyratrons triggered, the microphone preamp had full-scale oscillations once a second (“motorboating”), and the anode power was heavily overloaded as each thyratron fired.

Source: spectrum.ieee.org