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.