The amount of power dissipated in the plate element of a vacuum tube. At idle, or quiescent conditions, it is equal to the DC plate current multiplied by the DC voltage difference between the plate and cathode elements. When the tube is amplifying a signal, the average plate dissipation depends on several things, including the quiescent bias point, the amount of signal voltage between the plate and cathode, and the class of operation. Average plate dissipation can either increase, decrease, or remain the same at full power, depending on these things. In a class AB or class B amplifier, the power dissipation increases, because the signal swing above and below the quiescent point is not the same (the tube is in cutoff for a portion of the cycle) and in a true class A amplifier the plate dissipation decreases at full power, because the plate current and plate voltage are 180 degrees out of phase, so the product of the two is zero when one is at max and the other at zero, and is maximum at idle.