If anybody wants to build an amp and you have no electrical experience, I would suggest you get assistance from somebody who does. There are voltages in excess of 350V dc in some of these amps, some as high as 600v I believe. (Doc, correct me if that's an exaggeration.)

I'm not sure why anybody would bother with a 12 volt / 240 volt initial input transformer, unless you want to use it for busking.

The danger of these amps isn't the 240v AC primary winding voltage, ('tho that IS lethal...) it's the high DC voltage in the circuit wiring that catches people out. Even if you had a 12 volt transformer feeding the amp, the circuit voltages would still be in the 300-400 volt range.