How to work the bias switch on a Prologue Classic?

I just purchased a Prologue Classic amp and see it has a bias switch on the right side. There are no instructions on how to use it. I am new to valve amps and would appreciate any help I can get. What does the switch for? How do I use it?
Here is a WAG. It allows you to switch from big power tubes, i.e. 6550's/KT88's etc to smaller power tubes i.e. EL34's/6L6/5881/KT77, etc. The amp probably has two different settings internally, one for each class of tubes, yet within each range employ it's auto bias circuit.

BTW, why don't you just call the USA distributor and ask?
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