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.
Will Vincent st-70 upgrade
I just purchased a used beautiful Will Vincent upgraded st-70 power amp. This is my first tube power amp so I am somewhat of a novice when it comes to these units. On the top of the amp between the power supply transformer and output transformer there is a toggle switch. What does this toggle switch do? I don't want to mess with it not knowing its function.