via HACKADAY: Spy Transceiver Makes Two Tubes Do the Work of Five

Here at Hackaday, we love following along with projects as they progress. That’s especially true when a project makes a considerable leap in terms of functionality from one version to another, or when the original design gets more elegant. And when you get both improved function and decreased complexity at the same time? That’s the good stuff.

Take the recent improvements to a vacuum tube “spy radio” as an example. Previously, [Helge (LA6NCA)] built both a two-tube transmitter and a three-tube receiver, either of which would fit in the palm of your hand. A little higher math seems to indicate that combining these two circuits into a transceiver would require five tubes, but that’s not how hams like [Helge] roll. His 80-m CW-only transceiver design uses only two tubes and a lot of tricks, which we admit we’re still wrapping our heads around. On the receive side, one tube serves as a mixer/oscillator, combining the received signal with a slightly offset crystal-controlled signal to provide the needed beat frequency. The second tube serves as the amplifier, both for the RF signal when transmitting, and for audio when receiving.