This CK-5755 is a double-triode made by Western Electric (USA) for Raytheon. It has same specs as 12AX7A / ECC83 / 7025 but with different pinout for Pin 2+3 and 6-8 (see data sheet). For use as 12AX7 an adaptor or an adjusted socket wiring for pin 2+3 and 6-8 is required. Western Electric marked this tube WE 420A for their own brand. The CK-5755 "features special cathode springs which reduce microphonic response. This characteristic is controlled by a "microphonic stability" test for balance in which the tube is tapped at shock levels of 400 - 600G." read more in the data sheet.
