Guys !!!!!!!!!!!!!!!
No need for that. All you need to do is to change transformer wiring.
Send me a pic of the transformer you have and I'll let you know which leads you have to solder together....
As a rule of thumb here:
1. leave the thicker wires from the transformer to the board alone
2. de-solder the wires from the transformer to the switch and IEC
3. check and see if you have these 4 color codes: black, brown, white, and red(orange). Most likely you have them
4. Solder the brown and white cables together....insulate with electrical tape
5. solder the red / orange wire to the IEC
6. solder the black wire to the switch
Done, 220 - 250 V.
7. (maybe) run the amp for a day and put your hands on the chassis. If it's just warm, you're ok. However, if the chassis is cold or very warm (your judgement call) then it will need rebiasing.