‘an amplifier‘ is not enough to go by. Say you had an Ampeg SVT, with a Plate voltage of about +600VDC, and a US Marshall, with +450VDC on the Plate. The SVT would be fine around 35mA per tube, and the Marshall around 45mA per tube. Big difference. 45mA per tube in the SVT won’t last long, and 35mA in the Marshall would sound bad. The tubes would last forever, but sound bad doing it.
You should be familiar with what is going on inside of your amplifier before you poke your fingers in there, and know some ‘numbers’ before you can know what the bias ‘should’ be.