Magnatone offered vibrato in several amps beginning in 1957 Magnatone's approach was more advanced and a better sounding effect than anything its competitors ever attempted.
Don L. Bonham came over from Pacific-Mercury with his vibrato ideas and immediately applied them to the new line Custom "200" Series amplifiers, as well as a new line of organ tone cabinets. Bonham's vibrato circuits were the subject of several patents he filed in his own name (unlike Pacific-Mercury, Magna allowed Bonham to patent the circuits on his own and then licensed the patents from Bonham).
In sales literature, Magna called the vibrato "F.M. Vibrato" for frequency modulation vibrato). Whereas most other companies incorrectly labeled volume modulation as vibrato, Magnatone's vibrato was true pitch shifting vibrato. This kind of advanced technology was more complex and expensive to produce, and no doubt lead to the premium prices of the Magnatone amplifiers.
The key component to the Magnatone design was the varistor. At minimum, the design required three triode sections and for the more advanced, six triode sections were required.
Below is an explanation of Bonham's design by Tony Price, Chief Engineer of Estey Electronics from 1963 to 1965, and Product Engineer of the 1963-1967 Custom Series amps:
Bonham's principle was quite simple in practice, and well-crafted in practice. 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.
The uniqueness of Bonham's invention was that vibrato no longer had to be applied at the audio source (a finger-modulated string or a lever-adjusted bridge, e.g.), but could instead be super-imposed onto a flat tone such as an accordion's output. A rotating Leslie speaker produces a similar Doppler effect mechanically, but is expensive and seems forever associated with the Hammond organ.
Don Bonham faced a lot of trial and error identifying varistors of the right resistance range, a reliable voltage source to vary that resistance, a set of components with predictable tolerances, and a low enough production cost.
The pinnacle of vibrato design was the F.M. Stereo Vibrato used in the M14 and M15 stereo amps. This design used two sets of the F.M Vibrato designs for each channel. Dedicated to this task was a 12DW7, and two 12AU7s and eight varistors! There was an additional "stereo vibrato" switch that put in phase or out of phase the oscillator voltage for the two pairs of varistors in each vibrato unit. This created a deepened sense of vibrato. This was a subject of a Bonham patent in which he described the circuit as a way to "improve vibrato by reducing the effect of wandering pitch on reiterated tones."