Brans Vademecum lists UU6 and V41 as equivalents. UU6 uses MO base — not pin-compatible.
Same heater voltage as UU5 (4V) — the UU4 is distinguished by its lower maximum anode voltage rating (350V vs 500V), not by heater differences.
Indirectly heated cathode — one side of the heater is internally connected to the cathode.
The UU5 can be used as a replacement for the UU4 (higher ratings are always safe downward), but the UU4 must not be used in UU5 circuits that exceed 350V RMS.
Vdrop ≈ 35V estimated from Mazda UU4 datasheet regulation curves (Vtr=2×350V, Io=120mA → Vo=400V; ideal cap-input ideal ~495V; regulation includes ~60V of cap-charging loss, leaving ~35V pure tube drop). Cross-checked against Telefunken AZ4 (4V heater, 125mA/plate rating) which has Vdrop=35V. Source: Mazda UU4/UU5 datasheet.
Pin Layout — B4
1Anode 1
2Anode 2
3Heater and Cathode
4Heater
Socket-Compatible Tubes ⚠ Not electrically compatible