what you need to ask yourself is why you want to do it.
do you want to save money?
if so, dont bother, you're unlikely to save any money and its a lot of work, especially the hours upon hours of study just trying to figure out how tube amps work.
if you are an electronics nerd/masochist/"the-guy-who-spent-his-entire-childhood-playing-with-Lego-blocks-and-building-crazy-stuff" (like me
) and are genuinely interested in acually BUILDING (not just "owning" a preamp), then go right ahead, its great fun!
taking the pre-amp section out of an existing amp wouldn't really be achievable, especially if its PCB based. building from scratch is do-able, but might not end up being practical, the box would be quite big i'd imagine as you'd have to accomadate the power-transformer and tubes in a DIY fashion, which wouldn't be as tight and compact as how the factory dudes do it
best to google for "guitar pre-amp schematics" or something.
i've built a Marshall 18watt and am half-way through a custom amp build at the moment,
its a lot of fun, expensive, painful and hard work, but definatly fun
hope that helps somewhat,
... actually, i'm curious about this, i'm gonna search the interwebs for a good schematic, i'll post here if i find anything use-able.
Tube amps have around 500 volts going on in them.
