Kevin O'Connor at
http://www.londonpower.com sells tube-based Guitar and Bass Pre-amp kits, you have to buy a power supply and the eyelet board construction modules too. The kit comes with no chassis included so you have a lot of flexibility on how you want to build it. I've had my Concert Bass chassis out of the shell and I'm willing to bet that there's room in there especially if you rip out the old dead pre-amp board. I'd rate the kit as an intermediate project since the instructions leave a few gaps for you to fill in (a Heathkit it aint)
I recently built the bass preamp kit and put it in a separate "mini-head" style chassis that sits on top of my Concert head. I plug the tube pre-amp out into the power-amp-in jack on the back of my Concert Bass head and I'm pretty satisfied with the results. Much warmer sound out of the tube pre-amp, you can crank the distortion on the 12AX7 tube as high as you can stand and then pass that whole signal down to the Concert Solid State power amp section where it gets plenty loud.
The components that O'Connor sells in his kits aren't the most heavy duty in the world, so if your bothered by quarter watt resistors in your pre-amp cause some macho tube-head says they all have be one-or two-watters, well its your call. I've been running my preamp on 10 an hour a day for 2 months with no problem. Don't know what happens in a three hour gig though. I'm just an at home player. Hope this idea helps.
