Univox BT-505 Bass Amp

This humble looking solid state amp may have been Univox's first all solid state amplifier. This particular unit appeared to be of about mid-'60s vintage, making it a very early solid state bass amp. The power transistors and driver transistor were all Sony brand. The preamp transistors were very odd looking small packages. The amp appears to have been made in Japan.

It's a pretty cool chassis. You can service it pretty easily as you can access both sides of the circuit board and the solder side is well labelled for the power amp, and partially labelled for the preamp.

The amp proudly says "All Silicon Transistor" on the front panel. This amp made a lot of awful noise when I first turned it on. There was a bad connection on the circuit board to the filter caps for the driver and preamp stages. While I was in there, I thoroughly checked out the power amp, and sketched out the circuit. It is a totem pole type output, and it is very important to check that the resistors haven't drifted much. These had not. I pulled the transistors, checked them out of circuit, including checking that their Vbe matched. I reinstalled them with fresh mica insulators, fresh heat sink grease, and fresh insulated washers too as the old ones cracked when I tried to put them back.

Note the unusual mounting of the power transformer, and how it connects via a tube socket.