Forbes Tube Theremin
I started this project to build a tube theremin about 10 years ago. I started on the Schematics page of thereminworld.com. There is/was a gif schematic and a text file with some instructions.
I built the theremin but at the time I couldn't get the antenna sensitivity right. The whole pitch range was within a few inches and I hadn't even messed with the volume antenna. Several years later I came across this website that has a lot more details on building the doug forbes theremin than the original article. Turns out the you have to use copper plates as antennas with this design (I had made up some more traditional theremin antennas). Once I made up some copper plate antennas and changed a capacitor value as recommended in the 'build notes' section, the pitch antenna worked great.
I'm still having issues with the volume antenna. The frequency of the fixed oscillator is coupled in with the variable cap in the bandpass filter, so when you go to tune the volume antenna, the fixed oscillator freq varies.
I have my volume antenna working now with a range of about 12-14". That isn't anywhere close to the range that the pitch antenna has, but that seems to be the nature of the design and I'm satisfied. I ended up operating the volume oscillator at about 1.3 MHz using the original cap value from the schematic in the oscillator (I had a larger cap in there in order to achieve 770kHz like the original designer, but his inductor must have been a much higher value than mine). I also reduced the caps going to and from the LC bandpass from 15pF to 3.3pF and added a pot on the grid of the VCA to tweak the threshold. See markup below for changes from the original schematic.