#1
Hey. I bought a JCM900 4101 second hand off ebay a few weeks ago and have decided to replace the valves. How would I do this? Would I need to bias them or are they 'cathode biased'? (whatever that means)

They are 4x EL34 (thinking of getting a matched quad of JJs) and 3x 12ax7 (again thinking of JJs)

Also do the preamp tubes need to be matched???

Thankssssss Ug
What's the difference between a chickpea and a lentil?
I've never paid to have a lentil on my face!
#2
Any way I could talk you out of JJ's for the power section? You can do better. And no, the pre's don't have to be matched, some places sell 'em that way though for a few extra bucks. Honestly, since you just bought a used amp, and IF you have the money, leave it with a tech you trust to give it a once over and replace the tubes, usually a very good idea.
#3
Hmm whats wrong with JJ's?
What's the difference between a chickpea and a lentil?
I've never paid to have a lentil on my face!
#5
IIRC, the JCM900's have adjustable bias pots inside the chassis. That being the case, find an amp tech to replace the power tubes for you. The preamp tubes are easier: you can just plug and play all you want, and not have to worry about bias or about matching anything.

Regarding specific tube types and brands: it's all preference. It would be nice and easy if there was some universally accepted hierarchy of tubes that was indisputable, but there will never be such a thing. Brand A isn't really any better than Brand B, it's just that some people prefer Brand A and some people prefer Brand B. I personally like current production SED's for my power tubes but my roommate won't have anything but old stock Mullards, regardless of what it costs him. Once again: it's all preference.
ESP LTD EC-256 and a Fender Deluxe VM