Proper biasing can make a sonic difference. As little as a 5ma difference lower than optimum can change my sound from neutral/warm to neutral/cool. 5ma over optimum bias can shorten tube life quite a bit. Bais requirements change as the tubes age and, depending on use and tube types, should probably be checked at least every 6 months (I check mine every 2 months). Checking bias will also give you advance warning of an aging tube - you can change it before it dies or shorts out. The latter is well worth avoiding.
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