Hoffman Amps
Mike's Tube Amp Pages decent list of links too.
Schematic Heaven (Supposed to be a schematic for a Ampeg SVT there somewhere)
There's plenty more out there too, do a bit of imaginitave googling, it's not hard to find.
All that said...
If you're not already familiar with electronics, it's usually advised to start with a much simpler project such as an effects pedal of some sort. A couple of reasons...
a) Not as intricate, amps are much more complicated so effects are a good idea to start with, some are less to assemble than just the preamp section of a full amp.
b) Voltage. Effects generally run on 9VDC, not easy to electrocute yourself on a 9V battery while the 100-450VAC and occasionally more in an amp is dangerous. Tube amps can kill, solid state can get ugly. IF you really want to get into it with an amp, try for something fairly simple, building a 10 watt practice amp is a lot simpler than a 100 watt or more stage amp. since you're talking bass, it'll probably be over 100 watts, possibly up to 300 or more.
Get a feel for the basics with a simple project, visit some of the electronics related forums and dig through the back posts, lots of guys out there sharing info on forums, quite often you don't even have to join or post to find out a lot of good info, just read through lots of dead threads.
And do pay close attention to the voltage warnings concerning tube amps, if you want to tinker with tubes...if the filter capacitors are not properly drained tube amps can be deadly...literally. One very good piece of advice to remember is keep one hand in your back pocket when even thinking about touching anything inside a live tube amp. A live amp of any kind for that matter...
