Same rectifier function and electrical ratings — the 5993 datasheet explicitly identifies it as 'Generic Type 6X4' — but different base: 6X4 is B7G (7-pin miniature), 5993 is Noval (9-pin miniature), so not pin-compatible.
6Z31
Listed as substitute in Sams — electrically equivalent to 6X4 family but on a different (B7G) base.
EZ90
Listed as a substitute in Sams and Philips tables — same electricals as 6X4/5993 but EZ90 is B7G, not Noval, so not pin-compatible.
Electrical Specifications
Absolute Maximum
Anode Voltage (max)AC Plate Voltage per Plate (max RMS, choke-input)450V
Heater-to-Cathode Voltage (max)Heater-to-Cathode Voltage (max DC)500V
Voltage Drop (typical)Tube Voltage Drop (derived from plate I/V curve at 35 mA per plate)22
Notes
Bendix TE-10. The 5993 is described by Bendix as 'Generic Type 6X4' and is designed to replace the 6X4 'in applications where severe environmental conditions are encountered, especially in airborne equipment'. It is NOT a pin-compatible drop-in: the 6X4 uses a 7-pin miniature (B7G) base, whereas the 5993 uses a 9-pin miniature (Noval / T-6½) base with a different pin arrangement — 5993 pin 1 = plate, pin 9 = plate, pins 2 & 7 = heater, pin 5 = cathode, pins 3, 4, 6, 8 = open.
Sams and Philips lists show 6Z31/EZ90 as substitutes. These are 7-pin miniature (B7G) tubes — they are electrically equivalent but NOT pin-for-pin drop-ins; they require a different socket.
Designed for 10 000 h conservative life and 5 000 h typical life expectancy. Each tube undergoes a 45-hour run-in under overload, vibration and shock to eliminate early failures.
Bendix Red Bank construction: four-pillar mount, triple bulb-snubbing micas, heavy plates with strengthening ribs, solid extruded ceramic heater insulator, large cathode with extra vibration-damping tab.
Maximum ratings per the Bendix datasheet (design-center): peak inverse voltage 1500 V, peak plate current 230 mA per plate, peak surge 1.6 A per plate, DC heater-cathode potential 500 V, cathode heating time 45 s.
Typical operation: 325 V rms per plate (cap input) or 450 V rms per plate (choke input) giving 70 mA DC output. These match 6X4 ratings — the tube is electrically identical; only construction, base and environmental ratings differ.
Tube voltage drop (≈22 V at 35 mA per plate, i.e. 70 mA DC output) is derived from the 'Average Plate Characteristics' curve in the Bendix datasheet (one plate only): at 35 mA plate current the plate voltage reads approximately 22 V. This matches the published 6X4 Vdrop and the rectifier_reference.json value for the generic 6X4 family.
Note: the curated reference table lists max_AC_V = 260 V for the 5993 — this appears low compared to the Bendix datasheet, which states 325 V rms per plate (conservative typical) and 400 V rms per plate (maximum) from Rating Chart I. The Bendix datasheet takes precedence; 260 V may be an error or a conservative derating in the source table.
Pin Layout — Noval
1Plate 1 (Anode 1)
2Heater
3No Connection
Socket-Compatible Tubes ⚠ Not electrically compatible