Same Loctal base and same pinout but lower ratings (70 mA DC out, 22 V drop, 0.5 A heater) — 7Z4 can substitute for 7Y4 but not vice versa at full load
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 (peak)450V
Voltage Drop (typical)Tube Voltage Drop (at 100 mA per plate)40V
Notes
JEDEC base 5AB — Loctal 8-pin lock-in base, same pinout as 7Y4.
Nominal heater 7.0 V / 0.96 A; datasheet-rated at 6.3 V / 0.9 A for the 6.3 V bus. Both are supported.
Unipotential cathode with ±450 V peak heater-to-cathode rating — higher than many 6 V rectifiers and permits stacked-heater designs.
Max AC plate-to-plate supply voltage: 650 V RMS (cap input) or 900 V RMS (choke input) — i.e. 325 V / 450 V per plate. Verify filter topology before applying the higher rating.
DC output voltage regulation from half load to full load is approximately 35 V (cap input) or 15 V (choke input) per the RCA 1947 datasheet.
Tung-Sol 1948 datasheet explicitly states tube voltage drop of 40 V measured with the tube conducting 100 mA each plate — this is the definitive Vdrop figure.
Filter-input capacitor larger than 40 µF may require additional plate-supply impedance above the 75 Ω min per plate to avoid exceeding the 300 mA peak plate current rating.
US military designation VT-181 per MIL-HDBK-213A; British CV number CV1790.
Pin Layout — Loctal
1Heater
2No Connection
3Plate 2 (Anode 2)
4
Socket-Compatible Tubes ⚠ Not electrically compatible