I suspect that the bias was not tested after the tubes had sufficient warm-up time when installed. I usually set the bias low initially and then set it again after an hour of operation.
If the power supply isn't regulated (and in this case I mean the power supply to the output section, which includes the filament circuit) if the AC line voltage is higher, the bias will read higher too.
So you have two explanations of the higher bias. At any rate I would not be alarmed.
As tubes age the bias level tends to drop rather than increase.
If the power supply isn't regulated (and in this case I mean the power supply to the output section, which includes the filament circuit) if the AC line voltage is higher, the bias will read higher too.
So you have two explanations of the higher bias. At any rate I would not be alarmed.
As tubes age the bias level tends to drop rather than increase.