Pinout is NOT compatible with 12AX7/ECC83 despite both being Noval twin triodes. Pins 2+3 and 6-8 are swapped relative to 12AX7. An adapter or socket rewiring is required to use in 12AX7 circuits.
Center-tapped heater allows operation at 6.3V (parallel, 360 mA) or 12.6V (series, 180 mA). Heater ballast resistors (35 ohm per lead) recommended in the DC amplifier application to cancel out heater resistance variations.
Special cathode springs reduce microphonic response, verified by tapping at 400-600 G shock levels. Maximum grid current of 10^-9 A ensures drift stability in DC amplifier service.
Developed by Bell Telephone Laboratories for AT&T precision instrumentation. The 420A was Western Electric's designation; Raytheon manufactured the same tube as CK5755 under military contract (JW-5755 for Joint Army-Navy Wet-environment spec).
Both sections must be balanced at the start of operation by adjusting grid bias so plate currents match within 1 µA. The DC amplifier circuit (Fig. 1 in datasheet) uses a 150 kOhm shared cathode bias resistor with 310V supply.