Superb and VERY RARE genuine RES664d output power final Pentode valve made in Germany for Italian Air Force supply (Reggia Aeronautica). First series of 664d with an absolutely fascinating "balloon shape" style; similar in structure and appearance to the famous RE604 Triode. These valves were essentially used in German military HAM-radios but not only. DIY HI-END AUDIO designers judge this tube as highly performing, with great energy and detail. Not to be missed!
EQUIVALENT: Dn1004 = TP4100 = E443N = L491D = PP4100 = G100 = PM24C