Actually, Bob used 6 frequencies. Each of the 3 channels, had a Gated Pulse waveform. And, if you've ever listened to your electricity, you'll hear both tones being reproduced and can tune to combinations that produce musical chords.

Then, of course, the shared toroid and LC tank were subject to hetero-dyning as well. Exponential harmonics are created from that effect. I built a 4 channel version, similar to Bedini/Rife 4wave mixer. Ironically, it's the same exact process of modulation.
What's the difference between Gating and Amplitude Modulation? 1 picoVolt
Stan's use of gradient voltage bursts, effecting Resonant Action in the spherical cavity, were also a form of tonal modulation. Leo Fender did it in 1970 with the infamous Bias Wiggle
Solid State Version:
Vacuum Tube Version:
Listen to your voltage