No i an not affiliated with them, the bottom part was quoted from their site. From your pic you have the Dual Bias Tool which their page states has "two Mv Probes" which i would take to mean that it measures the millivolts across the cathode by inserting a one ohm resistor in series with the cathode. The problem with this is that unless you know what the anode voltage is the Mv reading is useless. The Mv reading will need to be converted to Ma then multiplied by the anode voltage to give you the plate dissipation. Some people add 5Ma to account for the screen grid current that also flows through the tube.
These probes were designed to make biasing easy and safer but without knowing the plate voltage you are in the dark.
The formula for figuring out dissipation is V x C = P. For example, if your plate voltage is 460 and your current reading is 35Ma ( .035) you get
460x.035 = 16.1 watts. Now say the plate voltage is 480. This is what you get, 480 x .035 =16.8 watts. Not really a huge difference but now let assume we add in the screen grid current of 5 Ma, now we get 460 x .040 = 18.4w watts. now plate voltage at 480, 480 x .040 = 19.4 watts.
As you can see the difference in the plate voltage and the current makes a big difference in the final reading.
Now you need to remember that the power tube has a point at which it gets too hot current wise. The standard value to stay below is 70% of MAX dissipation. So if you are using a 6l6GC tube which is rated at 30 watts, your 70% number would be no more than 23W. For any tube going beyond the 70% zone will runs then very hot and shortens the life of the tube quite a bit. If you go past say 85% you start running the risk of red plating and the tubes life is measured in minuets.
This is from my site dealing with Marshall 6100's but this can be applied to many amps as long as you can solder and get to the tube sockets and can safely work around high voltages. It is a little more involved taking the chassis out to bias but on most amps you have to take out the chassis anyway because the bias adjustment pot is on the power amp board inside! With this method you just leave the resistors in there. Go to the page biasing a 6100.http://home.myfairpoint.net/vze1u14s/pe ... rtservice/
Hope this helps.