Further on above - I made some tests recording the sound of the amp with a range of bias settings. This is the same few licks, stepping the bias in the order 0.9V, 0.75V, 0.6V 0.45V and 0.3V.
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i was playing as consistently as I could, on the OD1 channel - and you can hear the sound diminish at the lower settings.
The amp was miced from a seperate cab for the recordings, since it's own speaker would not have sounded good with all the amp guts hanging out to tweak the bias!
John