Description

Electrical signals at the plate and cathode of a tube are 180 degrees out of phase with one another and roughly the same amplitude, the cathode being in phase with the grid input signal. The output signal from a Bonham VoPak (voltage pack) is alternatively sampled from the cathode and plate at a rate determined by the varistors\' changes in resistance (i.e. the vibrato speed and amplitude). As the audio signals are (relatively slowly) shifted back and forth in phase angle, an apparent Doppler-like frequency shift occurs, which is the pleasing vibrato effect.