I was lucky enough to be given this great little piece of equipment to be transformed into a guitar amp. These Bell & Howell Fimosound 385 units are famous for being great platforms to be converted for guitar use. It has really nice transfomers and all the appropriate sockets to accomodate a guitar friendly design.
Although exciting, the sight of the point to point original circuit is intimidating. Lots of out of spec components crammed into a tiny space, with waxy capacitors looking more like candles than caps anymore. The amp has had a little bit of work done to it before but was in no safe, usable condition. With so many untrustworthy components a rebuild seemed like a better choice and the owner agreed.
The owner wanted a fendery type of sound with plenty of clean headroom. The amp was pretty much gutted clean and I began trying the first versions of the circuit. With two 12ax7s and a 5879 pentode(a total of 5 gain stages!) at first the amp sounded like a fuzz pedal…pretty cool but not what we were going for.
As a solution for that, I added a switch that lets the user choose the 12ax7 triode (both stages in parallel) or the brighter, higher gain 5879 pentode as the first preamp stage. This gives two different types of tone and breakup qualities. The tone control uses the switched pot as a tone bypass switch and I added a second toggle in the back of the amp with three negative feedback variations. The 6V6s were cathode biased at around 10 watts per tube so the amp sounds pretty loud and clear.
The amp turned out pretty sweet in the end. Take a listen to it in this short demo:
p.s: The power light isn’t flashing in reality, it’s how my camera sees it : )