[Amps] A Naive Question...

[Kim Elmore]

When I look at transmit tube ratings, I see that they're always 
specified in watts, which makes perfect sense because the grid structure 
can dissipate only so much heat before all manner of Bad Things will 
happen. Every amp I've seen measures grid current, and the spec sheets 
specify currents for "typical" operating conditions for the various 
tubes. But... How do I convert grid current to grid dissipation? Do I 
need some kind of grid impedance to do that?

Kim N5OP