INteresting changes to the 5Y3 in this thread. I am with Mik and Oakland A....if I plug a Gibson humbucker into a healthy stock 5Y3 turned up, I can get those NEil Young tones. THe distortion will get that 'bubbling' loose thing going on and feedback just the right amount to sing those overdriven tones as long as you want. I have never heard another amp do quite what the 5y3 does, although there are some little Gibson's that get close to that Young sound.
Voltages? IF I were building one, I would opt for MErcury Mag's power transformer that yields vintage-type B+ levels. These tweeds like to see 100VAC....they lose soem 'brown' sound with higher voltages.