Same Loctal base and same pinout but higher rating (100 mA, 40 V drop, 0.9 A heater) — different thermal profile, not an electrically identical drop-in
Voltage Drop (typical)Tube Voltage Drop (at 70 mA per plate)22V
Notes
JEDEC base 5AB — Loctal 8-pin lock-in base. The '7x' prefix in American tube numbering is the convention for Loctal-based tubes on a 6.3 V heater (nominal 7.0 V, operated at 6.3 V).
Unipotential (indirectly heated) cathode with Vhk_max = 450 V, so the heater can share a common 6.3 V winding with signal tubes.
Maximum DC output current is only 70 mA — comparable to a 6X5/84 and significantly lower than 5 V rectifiers like 5Y3/5U4. Verify total B+ draw before substituting.
Maximum AC plate voltage depends on filter topology: 325 V rms per plate with capacitor-input filter, 450 V rms per plate with choke-input filter.
Tung-Sol 1948 datasheet explicitly states tube voltage drop of 22 V measured with tube conducting 70 mA each plate — this is the definitive Vdrop figure.
Capacitor-input operation: if the filter capacitor exceeds 40 µF it may be necessary to increase the plate-supply impedance above the 150 Ω minimum per plate to keep peak plate current within the 210 mA rating.
Sylvania describes 7Y4 as 'GT equivalent 6X5GT' — similar electrical characteristics, smaller envelope, more rugged Lock-In construction.
Pin Layout — Loctal
1Heater
2No Connection
3Plate 2 (Anode 2)
4
Socket-Compatible Tubes ⚠ Not electrically compatible