What is bias a tube amp ? Is it difficult?

Find out ahead of time how to bias the particular amp you're looking at buying - they are all different.
Some auto bias, so it's a total non issue for the owner.
I have a Rogue Zeus, which has a LOT of tubes. Rogue puts a meter right on top, built in. You just flick a switch for the tube you want to check, and adjust. It's that easy. Rogue might do this on all of their amps, I'm not a dealer - I don't know.
In my mind there are some advantages of being able to check bias - you should let the tubes warm up for a while before checking - auto bias checks them at the power up cycle. Also, you can check tubes periodically, and see if one changes faster than the others, and is on the way out.
I use my amp daily, and need to adjust them every 4 -6 weeks - but not by much. You feel like you are giving the thing a tune up - which I guess you are!

Biasing is NOT difficult. Own a JoLida 502B.

I don't understand auto biasing vs manual biasing? Why wouldn't all tubes be auto/self biasing? What is it about the nature of an amp, or the manufacturer, or whatever, that one would produce an amp without this feature? thanks in advance, warren :)

Warren,

My amp designer states that auto-biasing impacts the sound of an amp, and not positively. Nick makes the best sounding electronics I've been privileged to hear, regardless of price, so I believe him. In top level gear manual biasing is probably preferable.

Aside from any degradation caused by unneeded complexities in the circuit, our own ears tend to confirm this from another direction. We readily hear any small changes we make in bias levels, so if the amp were constantly monitoring and adjusting...

Biasing and balancing four power tubes once every 6-8 weeks takes all of 10 minutes. It's no big deal, as others have said. YMMV of course.

Well made amps don't require biasing very often, and it's a good idea not to go crazy with this simply because frequent biasing will prematurely wear out the pots.

The CJ and Rogue amps I have are basically set it and forget it. I check the bias once every six months or so on the Cronus but with the CJ I haven't had to touch the bias for almost year now. And I am running Winged C Svetlana EL34s in each.