Ken Shirriff powered up a TR-3 trigger module from the 1948 IBM 604 Electronic Calculator and got it to flip. The TR-3 is a vacuum tube flip-flop built around a 2033 dual triode, which packs both inverters of the flip-flop circuit into a single tube alongside the supporting resistors and capacitors. Shirriff reverse-engineered the schematic, wired it up, and confirmed it triggers, though it demands a clean, stable input pulse to do so reliably.
The TR-3 is not the thyratron module Shirriff documented in June 2026. Thyratrons handle high-current switching at the system periphery. The TR-3 is core logic, the kind of circuit that made the 604 compute. That distinction matters because it shows IBM was building programmable electronic arithmetic in 1948 using analog vacuum tube bistable circuits, not clean Boolean gates.
By 1954 IBM scrapped this flip-flop design in the IBM 650, replacing it with diode logic. The reason is exactly what you would expect: analog flip-flop timing interactions are fussy, diode logic is not. The full writeup on righto.com traces the schematic in detail and the YouTube demonstration shows the module triggering live. Read it to understand why the path from 1948 tube logic to diode-transistor logic in the first transistor computers was not a leap but a correction.
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