Biggest noval power pentode?

[gianis]

EL520 (=EL503) is Magnoval type, not Noval.

EL 520, Tube EL520; Röhre EL 520 ID29781, Beam Power Tube

The EL520 is a rare Noval version of EL503 ,,I had both of them .

 

[Kay Pirinha]

The EL520 is a rare Noval version of EL503 ,,I had both of them .

Nope! The EL520 is the same tube as the EL503, both featuring a Magnoval base.

Rather often tubes designed as EL 503 show a Philips type code, starting with U3 (for EL520) instead of tB (for EL503).

Best regards!