Yeah the OD, when engaged, produces a pretty big jump in volume as Matt mentioned. When I use it, I typically have it between 10 and 2 o'clock with the channel volume around noon, and use the MV to tame things down. With those settings I get a really great OD tone out of the amp. Not sure how engaging the OD with a footswitch in a live setting would work due to the huge increase in volume. In any case, it sounds really good!
The bias switch layout had me scratching my head for a while. When looking down into the chassis, the "wired" side of the switch is the KT66 side. This allows 2 things...the B+ can bypass the Zener diode since it has a pathy through the switch (i.e. the Zener is shorted), and 2) it engages a parallel resistor (R44) for the bias supply. The no-connection side is 6V6, so when this side is selected the B+ has no where to go except through the Zener, which drops the voltage, and R44 is not added in the circuit. Therefore, from the outside of the chassis, the handle of the switch will be towards the OT when in KT66 mode, and towards the PT in 6V6 mode.
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