did a simple job for a friend. he has some extra 50’s Jensen speakers and wanted to use them with this GT40 amp.
he told me GT40 has mono and stereo output mode. so, we decided to leave one of the 6.5″ speakers as it was. then put a long thread 1/4 jack on the other output.
the only place I found for the internal/external switch was the inside of the bass reflex vent on the back. the back panel itself is too thick to attach the switch.