Once power tubes are fully burned in, the biasing checks are only done maybe once every two or three months, and they rarely fluctuate by + or - 3mv in most cases. The biasing is pretty simple in my opinion.
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I just got a Spirit II 2 weeks ago. Biased it ater it warmed up the first day (tubes were already broken in). Made a few minor adjustments a few days later. I checked it today and it is holding just fine. I see no need to check it again for couple of months. I agree with Blackwilliard, biasing is pretty simple.