I had a lot of success with Russian germanium transistors in fuzz circuits, so I thought it wouldn’t hurt to try getting one Russian radio and convert it into a guitar amp. There’s a bunch of makers and models from ex USSR that can be found very cheap. This one cost 2 euros and was in non-working condition, but containing all original parts. It’s got a good looking, although a bit damaged plastic enclosure which I could reuse, which is definitely a plus.
It’s a very compact board and neatly organized. The whole amplifier section of the radio is located around the speaker hole in the PCB. Circuit board design and build quality is noticeably better than the Toshiba 8TL. There are no jumper leads, no underside components and copper side of the PCB is protected with some sort of varnish. Sure, enclosure plastic is of poor quality and most of the screw receptacles broke, but it shouldn’t be too big of a deal to fix with some epoxy.
It’s easy to isolate the amplifier section of the amp. As with most of these radios, the best place to split the circuit is right after the volume pot. You can disconnect the lead coming from the middle lug of the pot and use it as amplifier input lead. As you can see on the schematic below, the amplifier circuit is very simple and slightly different than other radios I took apart. The first two stages are directly coupled. There’s interesting feedback look between the interstage transformer and the driver stage and another one from the output transformer back to the driver stage. There’s no temperature compensation or Zobel network (capacitor or capacitor and resistor in parallel with the output transformer primary) on the output transformer primary that is typical for similar circuits. I assume that the curious feedback loops provide plenty of stability, so there’s no need for Zobel network. There’s very few components between the guitar and the speaker, and sometimes that’s good.
On the photo below you can see the pot wiring. I’m not too keen on using the decades old pot to control the volume, but we can use it to determine where the amplifier circuit actually starts on the PCB by tracing where the middle lug lead goes. In this case, it’s all the way to the furthest possible spot on the PCB – not so good. To save on current consumption I cut the trace that powers the radio section (the exact spot is marked on the schematic).
One thing that raised my eyebrows, though, is the fact that there’s no heat sink of any kind on the power transistors. Moreover, all 4 audio transistors have some sort of plastic “skirt” on which seems to make heat dissipation even less efficient. But they don’t seem to get hot at all, so I guess it’s fine.
Amplifier section of the radio has only 3 electrolytic capacitors, all are 30uF. Two are for power filtering and one for negative feedback loop between the output and driver stage. I replaced all three with nice 47uF Nichicon FW capacitors. More filtering should be better and I doubt there will be noticeable difference with slightly higher NFB capacitor value. Amplifier input capacitor is 1uF and I wasn’t sure what kind it was. It’s marked as a polarized capacitor on the schematic, but doesn’t look like other electrolytics – it’s not enclosed in a metal can and shows no polarity markers. The amplifier was working with it, but I didn’t want to take chances and replaced it with a nice Wima box-style 1uF poly film capacitor.
The whole thing is pretty compact and I needed free space for input and output jack and for DC jack. I ditched the puny little speaker which left enough room for the output jack. For DC jack I just widened the hole that used to be headphone output and for input jack I had some room where the battery used to be.
To minimize noise, I shielded plastic back side of the enclosure using conductive adhesive copper tape. It is grounded at the DC jack (negative center pin, positive shell comes in contact with the copper tape). Most of the front side of the enclosure is aluminum speaker grill which is grounded through output jack. Without grounding, metal would not act as a Faraday cage which keeps the noise out.
I didn’t believe that these tiny transformers can produce useful tones, but it actually turned out pretty cool. It’s slightly less loud that most other radios I’ve converted but the tone is very good. Clean and warm and gets into SRV territory when pushed hard.