Another route might be a commercially-available hi fi tube preamp in kit form, so you'll have all the parts/instructions already, no need to fabricate a chassis (hassle!), no worry about earthing (notoriously difficult to get right and keep noise floor low) presuming it's a quiet design already.
A stereo line preamp with its two channels in series would typically give you 40db or so of gain - not a great deal but maybe usable, and could be increased with the addition of an input transformer (which would give you a better impedance match between mic and input), not to mention an iron mojo factor.
A phono preamp would have more gain, especially with the RIAA network removed (which you'll have to do anyway).
Phantom power would have to be supplied externally, but units for this purpose aren't expensive. Never built a phantom power supply myself, but I know high tolerance resistance matching is essential so a good meter would be required, plus the hassle of sqeezing it under the hood and possibly inducing noise if not implemented properly.
Sorry, long-winded post, but building from scratch with a schematic is a bit of a crap shoot at best, but it can be done. Nothing like the satisfaction of having built your own gear - 100% tube, hard-wired, just like the old school stuff.
