tubes converter    det / pre - amp          audio amp   rectifier
0             4         x                  x                  x           x
1             4         x                  x                  x  (selenium)
2  5 (standard)         x                  x                  x           x
3             6         x                  x                  x           x
4             6         x                  x  x x (push - pull)           x
5             8         x  x x (push - pull)  x x (push - pull)           x
6             8         x                  x  x x (push - pull)           x

Table as JSON:
{"tubes":{"0":"4","1":"4","2":"5 (standard)","3":"6","4":"6","5":"8","6":"8"},"converter":{"0":"x","1":"x","2":"x","3":"x","4":"x","5":"x","6":"x"},"det \/ pre - amp":{"0":"x","1":"x","2":"x","3":"x","4":"x","5":"x x (push - pull)","6":"x"},"audio amp":{"0":"x","1":"x","2":"x","3":"x","4":"x x (push - pull)","5":"x x (push - pull)","6":"x x (push - pull)"},"rectifier":{"0":"x","1":"(selenium)","2":"x","3":"x","4":"x","5":"x","6":"x"}}