Same Octal base and identical pinout, but electrically different: 6SN7 has mu=20, gm=2.6 mA/V, ra=7.7 kOhm vs ECC33 mu=35, gm=3.6 mA/V, ra=9.7 kOhm. Not a drop-in equivalent due to significantly different operating characteristics.
Heater-to-Cathode Voltage (max)Heater-to-Cathode Voltage (max)100V
Max Grid ResistorMax Grid Resistor
Notes
The heaters of the two cathodes are connected in series. When operating from a 6.3V supply, the heaters are in parallel internally; when from 12.6V, they are in series.
Despite sharing the same Octal base and pinout as the 6SN7, the ECC33 has significantly higher mu (35 vs 20) and gm (3.6 vs 2.6 mA/V), making direct substitution inadvisable without circuit recalculation. Many amplifier manufacturers list both as acceptable alternatives where the circuit can tolerate the difference.
Maximum anode voltage is only 300V — considerably lower than the 6SN7's 450V rating. Circuits designed around 6SN7 max ratings may over-stress an ECC33.
Mullard ECC33 tubes are highly sought after by audio enthusiasts, commanding premium prices. Production ran approximately 1949–1960s.
Pin Layout — Octal
1Control Grid (section 2)
2Anode (section 2)
3Cathode (section 2)
4
Socket-Compatible Tubes ⚠ Not electrically compatible