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.
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.
Thanks for the info Marakanetz, curious since it is not labled is it ok to switch between modes while the amp is on?
It's OK to switch even when the amp is playing so you'll find yourself better setting. If you idle the amp keep the toggle switch on triode mode once you hear the difference.
If you only have one set of outputs that switch could, alternatively to Marakenetz's comments, change the output impedance from 8 to 4 ohms ala some Cary amps. Just a WAG though. Maybe another owner will actually know.

The toggle switch is for hum control. Try in one or the other mode for least hum...I own a Vincent ST70 amp as well.
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
Hum control. I never would have guessed, but why do you need hum control and how does it work?
Hah, it's simply disconnects ground if there's a ground loop similar to cheater plug.

I have Gold Lion KT66 tubes. The soundstage just blew the doors open with this amp vrs the EL34's. Have yet to try KT77's Will Vincent recommends bias at 1.40 for EL34/KT66
tubes. Will is a great guy and his amps are awesome. He can be reached at 208-746-9650.
Thanks exocet954. I currently have the bias set at 1.35 on the KT77's. I believe I will try a set of Gold Lion 66's. So far I am enjoying this tube amp immensly, I have a Mac C220 tube amp running the input side and a pair of B&W CDM9NT's as my monitors.
I've only heard great reports on the Gold Lion KT77's. I switched to Gold lion KT66's several months ago and have been very impressed with the results on the Will Vincent
Stereo 70, Quicksilver Mid Monos and Quicksilver Mono 60's...All the best!