Tube radios have a certain charm. Waiting for them to warm up, that glow of the filaments in a dark room. Tubes ruled radio for many decades. [Uniservo] posted a video about the history and technology behind the 1920’s era Clapp-Eastham C-3 radio. This is a three-tube regenerative receiver and was advanced for its day.
If you are worried he won’t open it up, don’t despair. Around the ten minute mark, your patience will be rewarded. Inside are three big tubes full of getter and bus bars instead of wires. Add to that the furniture-quality case, and this is a grand old radio.
One interesting thing about this receiver is that it uses a special kind of transformer known as a variocoupler where a coil rotates inside another to adjust the regeneration. It turned out that the tubes were newer than the radio, so [Uniservo] replaced them with more age-appropriate tubes.
Unfortunately, the radio is silent for now because of open audio transformers. We hope he’ll get it working and make another video of it actually operating.
Regenerative receivers have pretty good amplification performance with a low parts count. That’s because the amplifier operates near oscillation where the gain at the selected frequency is very high. It is pretty easy to build your own using technology a little newer than these tubes. If you want to dive into the theory, we’ve done that, too.