I found this pic of how the tubes are installed in the SA-31. Although it's hard to tell conclusively, I would have to think that since the tube has wires instead of pins the wires are most likely soldered to the board. Or possibly they are soldered onto terminals on the part which surrounds their base, and that in turn has pins which plug into some kind of receptacle underneath. But looking at the picture I'm doubtful that it's the latter.
Regards,
-- Al
Regards,
-- Al