I tried to bias up a pair of Mazda 6v6's in my amp last night using a Bias Probe. The reading kept fluctuating up and down and when it would settle down, I would try to tweak the bias into the correct range. When I did this would set off another series of fluctuations getting me no closer to the target range. I did this for about a 1/2 hour and never did get them set-up right. What causes this? Are the tubes shot? I just got them from an ostensibly reliable supplier so I can probably send them back. Oddly enough they sounded pretty damn good, but I wonder for how long?