Substituting for the 7591A

While the 7591 output tube is becoming more scarce and expensive, it is actually one of a family of three tubes with essentially identical characteristics. The three tubes are the 7591, 7868, and 6GM5. Ratings are identical, and the only electrical differences are very, very minor differences in interelectrode capacitances. The 7591 has an octal base, the 7868 has a 9 pin Novar base (9 pin compactron), and the 6GM5 has a 9 pin miniature base (technically a Neonoval base, since it has a 9 pin miniature configuration on a larger glass bulb). Basing diagrams and outline drawings are provided below. Note that I copied the outlines from available sources, and they are not to scale, so read the numbers, do not look at the pictures. Substitution (of a 6GM5 for a 7591, for example) is a simple matter of either changing sockets or making adaptors by following the basing diagrams. It is interesting to note that RCA lists the basing diagram for the 7868 as "9RW" and Sylvania lists it as "9NZ". Both are identical.