Well the best is to start with small and then go to bigger stuff. Since you already have some background in the electronics you could already go with building  with the amp you want. Building amps is fun and educational but also dangerous. So if you are unsure of somethin always ask someone else or on this forums.

i would soo much build an amp by myself.. i soo much like these old tube amp it is driving me crazy .
if i could i would spend all my life for tube amp, and i would like more thant everything to build tubeamp for myself or for re-sell since not much company can make good vintage amp... they usually use pcb, change component for lower cost, and that's bad, as someone said somewhere in the forum, they just made lame attempt to make reissues.

only problem me i have never worked into a tubeamp, but from the instruction on internet on building a jtm45, and from all the info and circuit info and tube info i looked over the internet, i think at end these amp are kind of simple, but i guess that not because they simle it mean simple to build.

i also used to repair and tweak everything that use electricity here since there no tech who i trust around"or who are more calified thant a 5 years old kid for soldering wires".
only difference is there is tube and higher voltage. wich is still a big difference lol."well for me"

there is no school on internet for learn to build and work into tubeamp? lol

Well, if nothing else, you can sell your build for cost or a small loss (it's hard to make money on your first few builds), and fund the next amp with it.   Nothing wrong with having a great guitar amp for your buddies to use. 

Hang on.  There is never anything sad about taking some time for learning.  Amp building does require some basic knowledge but the learning curve isn't too steep to get you building successfully.  Nik provides a layout not a schematic.  There are plenty of pictures of amp builds all over the various builder forums for reference so as long as your choice of build isn't too complex you should do fine.  For a first build I would suggest a head as the chassis will have much more space for inexperienced fingers.  My first build (way back when) was a home brew 18 watt circuit and it came out fine with a very limited amount of know how.  An 18 watt lite is a great first build. 

the metro kits are killer. good parts and good instructions. and a lot of help from the guys at the metroamp forum