This is the first vintage radio I tried to convert into a guitar amp. It’s Toshiba 8TL-463S Japanese radio that features 2SB54 preamp transistors and 2SB56 poweramp transistors.
Looking at the gut shots, it seemed to have beefier transformers than most of other radios, which helped make the decision to get it.
Comparing the Toshiba schematic with Mullard’s, it looks very similar to Mullard amp and even has few improvements, like temperature controlled bias (note the thermistor marked TH M8601) for output transistors and more elaborate negative feedback network.
It took about an hour to carefully disassemble the radio and salvage the circuit board without damaging it. These things were not build to be serviced easily. Initially I cringed when I saw board mounted transformers, but luckily, there’s a hidden “L” shaped steel profile that serves as a heat-sink for power transistors and takes the weight of the two transformers.
With radios and some other devices, you’ll need to figure out which part of the circuit is the amplifier and isolate it from the rest. Luckily, in most cases, it’s easy to do. In most radios, amplifier circuit starts right after the volume pot. In this case, there’s an phono input jack that bypasses radio input, so that’s a good place to cut the circuit.
Tracing the board back to the schematic, I figured out which leads were important for the amplifier section and I removed all others. It left me with everything I needed – input lead coming from “Phono” jack, ground lead, power lead and speaker output.
I hooked it to a 9V battery and my 1×10″ 8ohm cab and surprisingly it worked! However, volume was a bit lower than expected. A quick check indicated that electrolytic capacitors have dried out (no wonder, they are over half a century old!), so I had them replaced with nice Panasonic and Sprague capacitors. Most of western world transistors I’ve come across become leaky over time, so I had low expectations from these, but measuring their bias right around the half of the supply voltage, it seems like they aren’t too leaky. Replacing the capacitors helped bring back the volume to the level I expected – noticeably louder than normal TV level when connected to a proper 8ohm guitar speaker.
The old wood/vinyl enclosure was too dirty and smelly, so I decided to get rid of it and rehouse the PCB into a different enclosure. An empty Oreo cookie tin box turned out to be perfect size and it kinda looks cool.
In terms of gain, with a Strat the sound is pretty clean, but higher powered pickups from the Ibanez give enough voltage swing to drive it into a mild, creamy overdrive. However, amplifier input impedance is rather low for passive pickups, so volume drops very quickly as you turn the guitar volume control down and impedance mismatch makes the sound a bit dull. Like with other radio-conversion amps, it really needs a pedal in front to shape the tone, get lower impedance signal and potentially push the amplifier a bit harder if we want more crunch.