Today I used my Tektronix 130 L/C meter as a theremin. It works very well. I connected one side of an 8-ohm home stereo speaker to the case ground of the 130, and the other side was connected by a 1K resistor and a 1000uF capacitor in series, to the top of R71, which is the cathode of the multivibrator that squares up the output of the mixer. You can see this subcircuit in this partial schematic of a Type 130. BAMA has the full manual. I chose to tap the signal at that point because I figured it would be a relatively low-impedance point and it only has about 40 volts of DC. Of course one would like to have a dedicated buffer amplifier, blah, blah, blah. At first I tried to use a straightened paper clip as the antenna. It was not very responsive. So I used a 12"x12" sheet of FR-4 PC board material. It forms a nice capacitor with my hand and the theremin is quite responsive.