Most likely it's a switch between triode and ultra-linear mode. Triode less gain less power more transparent. Ultra-linear nearly doubles the power with larger gain and control.
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Thanks exocet954, I talked to the previous owner and he confirmed its a ground disconnect. Just out of curiousity, have you done any tube swapping? I'm currently using the kt77's and 7199's and what I think is a gz34 as it has no markings. I have a set of el34's but have not tried them. Also, what do you have the bias set at if I may ask? Thanks