[dave walsh]

You may need more knowledge and experience to make either of them at all safe. Ampro. It's not being kept secret but "lets be positive" isn't really the right approach. Being positive [voltage wise] is actually what you need to avoid, especially on guitar strings.. maybe in damp conditions? The projector doesn't seem suitable anyway and the amp you have is basic to say the least. Mains Transformers were [and still are] expensive so it wouldn't be realistic or safe to build a proper PSU in now anyway. This is a Forum that gives advice but not blindly.

50 years ago, in the UK, AC/DC circuits were used in thousands of Radios, Record Players and TV's to enable the use of different mains supplies and save on costs. [You can read them up here or elsewhere]. In fact this was very safe as no metal parts were exposed and people were more cautious. Even if the power leads were reversed and the chassis was live it was ok but certainly would never be allowed these days.

Amps and Tape recorders though, or anything you could plug into externally-like a microphone, guitar or person, always had a full conventional PSU with a MAINS [not isolating or auto] transformer circuit used for safety and with an earth lead.

Dave W