I have a Marshall JTM30, and i want to put in some KT66s, the amp need to be biased or not? Right now I have EL34s in it, and it originally came with 6L6s, so I'm not playing the amp until I know for sure if it needs to be biased or not. Replies greatly appreciated.
I am not sure of your amp, but this is what I read on a Marshall Forum....

"The JTM30 is a fixed bias amp, so the only way to "adjust" the output section is using different tubes or modifying the amp. What makes you want to change what tubes you are using? I would stick with what the amp was designed for unless you have a reason to change it. The Sovtek 5881 isn't really anywhere close to the original 5881 released by Tung Sol, it is a lot more like a 6L6GC according to the datasheets and its performance in actual circuits. Marshall knew this when they designed the amp around the Sovtek 5881s, which had the best availability at the time of manufacture. And, the 6L6 is much taller than the 5881, so you'll need to take off the metal grate on the amp to fit them in."

Fixed bias means that you have to manually change the bias for different tube AFAIK. Cathode bias means that the circuit adjusts to handle the different tubes.

Just though I would say that because I am pretty sure that's how it is.