Plate voltage doesn’t change. Bias current changes. If you know what the bias cureent is then current x 417= plate dissipation. 6l6 has a max plate dissipation of 30 watts. I like a little margin so I use 25. 25/417= 59mA which is what I would use for my maximum bias current. this is why I really don’t like auto bias. you can’t just set your bias at 55 and know it is right.
Jerry
edit to add that KT88, 6550 and KT120 all have a higher rated plate dissipation power (watts) so high may be too much bias current for the 6L6, resulting is reduced life and possible red plating.