Best info on the theory behind biasing is on Randall Aiken's web-site in the Tech Info section.
www.aikenamps.comAlso make sure you check out his safety tips.
Essentially you need to measure the current passing through each valve and calculate the "resting" power dissipation in each valve (power = plate voltage x current). This is normally adjusted to between 50 and 70% of the maximum permissible for each valve (25 W for an EL34) for the usual 50/100W Marshall amps using El34s or 6L6s.
The DSL has test points and bias adjustment pots on the outside of the chassis so you can adjust the bias without opening the amp up. However, depending on how old your amp is, I've found it quite a hard amp to adjust the bias on due to interaction between the adjustment pots.
The JCM800 has a bias adjust pot on the inside so you will need to remove the chassis to adjust the bias, thus potentially exposing yourself to high voltages. You will also need some form of bias probe to measure the current draw.
To measure the plate voltage in either amp you will need a multimeter and have the amp open on the bench and on i.e. with the high voltages exposed.
Alternatively, you can assume that the B+ is around 470-490 for each amp and use this figure for your calculations, which is plenty good enough for rock and roll.
The Weber bias right is a good tool because it allows you to measure both plate voltage and current draw without poking around inside the amp when it is on. Although it might seem expensive, it's probably less then a couple or 3 trips to a tech. It will also let you check that your filter caps have been discharged as you can measure the B+.
Some extra practical tips:
I always leave amps for at least 20-30 minutes (longer is better) when biasing them, checking the bias every few minutes or so. This is especially important in Marshalls which can have a very large time constant in their bias supply, and can take several minutes for the bias supply to reach its final voltage. It also allows the valves to get up to a typical operating temperature. Also generally if anything bad is going to happen to new valves, then it will happen in the first few minutes of operation.
Always have a load attached to the amp (this can be disconnected when removing the amp chassis). I like to plug the amp into a speaker/s so I can hear if there are any problems with the new valves. I also turn down all the controls.
If you are going to poke around inside a live amp (and I can only recommend this if you are
very sure you know what you are doing), make sure that the amp chassis is
very stable and don't use both hands inside the chassis ("one hand in your pocket").
