As I said earlier, the bias voltage is the most important voltage in your amp. Measuring the bias voltage doesn't really tell you anything though....you need to measure the plate current to get anything meaningful. It is hard to do this directly though, so the best method is to measure the cathode current, which is made up of the plate and screen current. I'll elaborate on this more below.
Thanks for posting the link to Randall Aiken's biasing page Ken. I agree completely with Randall about biasing and I would urge everyone to read his article as it has everything you need to know in it.
The basics are that there are normally three ways to bias a tube, of which two are often used for power tubes. These are fixed bias, and cathode bias. They are almost never used together in an amp at the same time, but there are always exceptions, and the early Vox AC50 used cathode bias and fixed bias at the same time on the power tubes. For preamp tubes, they are almost always cathode biased, or grid-leak biased. They CAN be fixed biased, but it is more expensive to do so and usually doesn't sound as good, so no one ever does it that way. 95% of preamp circuits are cathode biased, and grid-leak biasing was used in some older amps, but isn't used often today in audio because it is noisy and has problems with large inputs. It is used quite a bit in RF stuff, but we won't go there. Note also that the bias method used is not the same as the Class the amp is biased into. Many people misunderstand that part of it. Now on to the methods.
Cathode bias is achieved by using an appropriately sized resistor connected between the cathode of the tube and ground so that the cathode will always remain positive as compared to the grid. A Vox AC30 is cathode biased, and all four power tubes share a common resistor, which is why you can't pull two tubes to reduce power like you can in some other amps. The resistor value is chosen so that when all four tubes are working, it is an appropriate value. If you pull two tubes, the resistor is now half of what it should have been and the two remaining tubes will idle too hot and burn themselves up. A Fender Champ is cathode biased too, but it only uses one power tube and IS Class A, whereas the AC30 is NOT Class A.
Cathode bias "usually" means that the tube will set it's own bias level and you don't have to change anything when you put a new tube in. However, since tube tolerences aren't always in spec, especially with modern tubes, it can be a good idea to see what the tubes bias to and make sure it is within a good range for the tubes. The easiest way to do this is to buy a Weber Bias Rite from webervst (
www.tedweber.com). It allows you to plug a seperate socket into the tube socket, and then plug the tube into the bias rite socket. Then you fire the amp up and it will measure the cathode current. Look to the Aiken article for more on how this works. It is a good idea to use matched tubes for a Vox AC30 because it will reduce hum levels and will allow the tubes to work better in relation to each other, and you'll develop max power this way also. If you use unmatched tubes, if the tubes are far enough unmatched, it will cause one or more tubes in the quad to run too hot or too cold and can cause tube failure, and they will also hum more when unmatched. While I said it can be useful to check the actual bias current in a cathode biased amp, I've never done so in my AC30 and I've had it 14 years and have replaced the power tubes several times and have not had a problem yet.
Fixed bias is achieved by grounding the cathode of the tube, and injecting a preset negative voltage into the grid of the power tube. This negative voltage, assuming it is set correctly, will make sure the grid is negative in relation to the cathode in all phases of operation, which is what allows the tube to function. The negative voltage at the grid is usually achieved by a tap on the power transformer. Many Fenders had a 70v tap dedicated for the bias. By hooking up the caps and diode appropriately that positive 70v gets changed to a negative 70v, and gets sent into the grids of the power tubes. You can make this voltage set at a particular value by choosing a resistor value and setting up the circuit so that it is non-adjustable. This is what Mesa Boogie often does, but in order for this to work every time, you need to use graded and selected power tubes so that the operating level of the tube is always the same within a margin of error. It can also be a problem because Mesa chooses the resistor value very conservatively, because they don't want their amps blowing up. This results in your tubes often running very cold, and the sound is closer to crossover distortion than most would like. The tubes do live longer this way though. You can use a resistor and a pot together with the pot working as a balance control, and this balances the bias level between the two tubes for least hum, but the overall bias voltage is mostly unchanged, and the overall bias level can still be set too cold or too hot, depending on the specs of the tubes you use.
The other way to set it up is to use a pot in place of the hard set resistor. This will allow you to adjust the bias voltage up or down within a range to accomodate different tubes. This is how Fender and Marshall traditionally did it. This method also benefits from the use of matched tubes. You can have one pot for each tube, and this will allow you to use unmatched tubes and set the bias appropriately for each. You can also use a 1 ohm resistor in between the cathode of the power tubes and ground....one for each tube. This allows you to measure the cathode current with a DMM and is basically the same as what the Bias Rite and others do. This is how I've set up one of my project amps that I'm working on.
When changing power tubes, you should always change the whole set at once. In most amps, the tubes work against each other....one pushing while the other is pulling. If you've used a pair of power tubes for 6 months, and one fails, and you put a brand new one in it's place, the balance between the pair is off unless you spend the time to match up the current draw of the new one to the old one and have the equipment to do so. If you replace a pair of tubes, the almost never will bias to the same level, so you should always set the bias with a fixed bias amp. With a cathode biased amp, you would want to put the new tubes in, and then fire the amp up and see if the plates of the tube get red while you play. You need to do this in a dark room to be able to see well enough, and you also have to have the amp rigged up so that you can see the plates of the tubes. This is sometimes hard to do while still having the speaker cable plugged in, but the speaker cable has to stay plugged in or you will destroy your amp. Preamp tubes last much longer than power tubes and they don't have to be changed unless they fail or become noisy or microphonic.
On a fixed bias amp, the 70% rule applies usually. What this means is that you should aim to bias your power tubes no hotter than 70% of their maximum dissipation at idle. The tube manual has a spec for the tubes that shows the max dissipation. For a 6L6GC, this max is 30 watts. 70% of 30 is 21, so you should bias no hotter than 21 watts. So lets say your plate voltage in your amp is 450v. You put some new tubes into your bias rite, and plug them into the amp and you measure the cathode current as 20ma. You do some math and you see that this is a little cold. (20ma is .020, multiply this times 450, which equals the plate dissipation, which is in this case 9 watts). So you decide to adjust for more cathode current and change it to 40ma. When doing this, the plate voltage changes too so it must be remeasured. You can do this with a DMM, but it is safer to get a Bias Rite that measures both. Anyway, lets say the plate voltage dropped to 400v. So 440v times .040 is 17.6 watts, which is closer to the 21 that is the max. You could try it here and see how it sounds and if it sounds good, it is good. You could adjust hotter up to the 70% max and if you like the sound there, then leave it there. The reason you don't want to go above 70% usually is that for a fixed bias amp, the plate current varies considerably in operation, and you have to allow some room for it to swing above the idle condition without maxing out. 70% is usually a safe value. Some tubes need to be lower than this, and others can be higher. JJ tubes often can be biased hotter, but keep in mind that the hotter you bias, the shorter your tube life.
In general terms, biasing cold (low current) makes the amp run closer to cutoff, and makes more crossover distortion get into the signal. Crossover distortion sounds unpleasant and should be avoided. The tubes will last longer but the sound is often uninspiring and farty. Biasing hot (high current) if you go too far will cause the tubes to burn up. Biasing on the hot side but still in the safe range will allow the tubes to distort sooner than otherwise and will make them sound warmer and more full. I usually try to set up my amps as cold as possible, but still where they sound good. This way the tubes last longer. I find on most amps that this tends to be in the 65% to 70% range.
Well thats all I can think of at the moment. Read the Aiken article as it has everything you need to know in it. Hope this was helpful.