Well, you're not really dividing anything here. You multiply the max dissipation number from the spec of a given tube by 60% to 70% and this resulting number gives you a range to bias your tube to. (Remember when you multiply by a percent, it is a decimal, so 60% becomes .6.) You also multiply your plate voltage by the cathode current and this gives you what the dissipation level is right now in your amp. You want the number to be somewhere within the 60% to 70% numbers from the first equation so that your power tubes are operating within a safe range. There are other ways to set the bias.....this one happens to be the current draw method. I use this method myself because I believe it is the most accurate and the safest way to set the bias. I'll explain other methods below after elaborating a bit more on the current draw method.
The number you are setting for is in watts, and watts are voltage times current. So when you go to figure out the bias on your amp, you measure the plate voltage and it gives you a number. Lets say 550v. To figure out what the dissipation is that your bias is set to right now you have to be able to measure the plate current. Unfortunately, the plate current is difficult to measure directly, but you can measure it indirectly by measuring the cathode current with one of the bias probe meters. The cathode current is the plate current, plus the screen current. The screen current is usually a much smaller number than the plate current and adds to the cathode current in a safe direction, so you can usually ignore that it is even there.
So you use one of the bias probes and it says the cathode current is 40 milliamps. You take 550 times .040 and this gives you a number (in watts) that is the plate dissipation. With this example, that happens to be 22 watts. Next, you look up the tube type in question that you are trying to bias for. You can get this info online or look in a tube manual like the RCA tube manual. As I said before, if it is a KT88, then the max dissipation that this tube type can handle would be 42 watts. The 6550 is less....I think its 35 watts but I forget at the moment and don't have the resources to look up the actual number right now for that type. With a fixed bias amp, you're aiming for 60% to 70% of that total usually. So if you take 60% to 70% of 42, you end up with 25.2 watts to 29.4 watts. With this example the bias is set a bit cold right now. It will function fine, but it may not sound the way you want, so you adjust the bias pot to where the current reading on the probe is a higher number. As you do this, the plate voltage will go down, so you must remeasure the plate voltage, multiply by the current on your meter to see what the dissipation level is, and see if it is within the safe range. Once you have the amp adjusted to where you like the sound, and you do the equation and it turns up that you have it set in a safe range, then you're done. As tubes age, the bias setting can change, so it is worthwhile to check the bias every couple years of use to see if it has drifted a lot and if so you can readjust. To use this bias method, you need a bias probe of some sort to measure the cathode current and a DMM to measure the plate voltage. You could also use this method if you had some 1 ohm resistors in the amp in between the cathode and ground on the power tubes. If so, then you measure across these resistors with your DMM, and it gives you a number just like the bias probes do. Look up "Bias Rite" or "Bias King" and you can see two examples of the bias probes. I prefer the Bias Rite as if you spend a little extra to get the more fully featured one, it will measure the cathode current, and then if you flip the switch it will measure the plate voltage too, so you don't need a seperate DMM.
The reason the plate voltage goes down as you adjust the bias current hotter, or on the flip side, the plate voltage goes up as you adjust the bias current colder is because of ohms law. Ohms law states that the voltage is equal to the current multiplied by the resistance. (E=IR)
If you flip the equation around, it also reads as the current is equal to the voltage divided by the resistance (I=E/R) or as the resistance is equal to the voltage divided by the current (R=E/I) Ohms law also states that the power is equal to the voltage multiplied by the current. E is the voltage, R is the resistance, and I is the current. If there is more current, there is less voltage, and that can be figured out by plugging numbers into the ohms law equation. Keep the resistance the same and plug in a voltage and vary the current and you'll see that as one goes up, the other goes down, and vice versa. For those who don't like to do math then just take my word for it that this is the case. I don't like to do math myself, so when I started learning about this and read books about it, I would assume it was correct and keep reading. Later I was curious and plugged in numbers to see and it is true. Anyway, the part to remember is that if you want to set the bias you have to be aware that as you adjust the cathode current, the plate voltage will change and you have to recalcutate each time you adjust to get an accurate reading.
Alternatively, you can set bias by the grid voltage recommendation in the schematic as Isaac suggested. I don't recommend this method personally, and I'll quote from Randall Aiken's page as to the reason why. "The negative grid voltage method: This method involves measuring the DC voltage on the grid of the output tubes, and setting it to a recommended value. Biasing by negative grid voltage is highly inaccurate because the same grid voltage can produce drastically different plate currents in different tubes of the same type. This method should be avoided."
There is the oscilliscope method too. You would also need a signal generator and a dummy load in addition to a scope to use this method properly, and these are quite expensive. With the amp's output hooked up to a scope, you look at the waveform and set bias pot just right to where the positive and negative halves of the wave line up with the crossover notch in the same spot on both halves. This method should be avoided because it is sometimes very hard to set the amp as the wave is just clipping and you'll get a different result every time you do it. Also, since most people don't have a scope or the time and training to learn how to use one properly, many people nowadays use the current draw method to set the bias as I mentioned above. Randall Aiken has a bunch of info on his site about these method as I've listed below.
There is also a transformer shunt method which requires a DMM or two and some very careful hands. It is quite dangerous so I won't go into detail about it except to say that you should probably use another biasing method unless you really know what you are doing with tube amps and high voltage.
For more reading on biasing amps, Randall Aiken's site has some very good information. Here are some articles on biasing, with the most important one first, and then the master link for this articles at the bottom for more reading on amps.
Greg
http://www.aikenamps.com/Biasing.htmlhttp://www.aikenamps.com/WhatIsBiasing.htmhttp://www.aikenamps.com/Why70percent.htmlhttp://www.aikenamps.com/CrossoverNotchBiasing.htmlhttp://www.aikenamps.com/TechInfo_2.htm
