Mark McKeown built these custom vacuum tubes by request to replace the 120 tubes in the RCA Theremin. The tubes are a solid statement replacement and eliminate the volume lag of the original tubes. He has some left from the project and is selling them for $150 each.
Here's a description from Mark's 2006 article on his restoration of an RCA Theremin:
The RCA Theremin uses the 120 tube to vary the audio gain by varying the tube filament voltage (i.e. filament temperature) and therefore the amount of current flowing through the tube to the plate of the audio amplifier tube. Many early 1920’s radios used a rheostat to control the filament voltage and indirectly the same method to control the volume. As the resonant frequency of the large coil is varied by hand capacitance approaches the resonant frequency of the volume oscillator, more current flows in the large coil and induces more current in the secondary of the large coil which powers the 120 filaments.
To purchase one, you can contact Mark directly at email@example.com.