MOSFET amp PS question

[Andrei_T]

Hi, I have on the bench an ASI Q-Tube70 guitar combo that breaks up really early.

It was a 110V unit and someone tried to "adapt" it to 230V and messed up the PT. It's obvious that he restacked it and now it has 2x 15VAC for the rails. After the rectifier it's +-20V.

It has two IRF243 power mosfets driving the speakers and the +-20V rails seem too low to me. Another thing indicating this is the capacitors that are 3300u/63V. However I have no experience with mosfets and no schematic to consult so I'm asking should't the rails be more like +-50V? for 70W of power?

Thanks in advance,

Andrei

[Steve Conner]

It depends on the speaker impedance. You need an absolute minimum of +/-24V for 70W into 4 ohms, and +/-34V for 70W into 8 ohms. MOSFET amps often need about another 3-6V on top of this. Either way, +/-20V sounds a bit too small