In America, Thomas Organ, the US manufacturer of Vox amps, designed the Royal Guardsman and Super Beatle amplifiers in 1966 as replacements for the UK made AC-50 and AC-100. Thomas also added the Goodmans Midax horn to the speaker cabs for these amps, despite an abundance of treble in their new solid state circuitry.

In 1969, Thomas Organ replaced the Royal Guardsman and Super Beatle amps with the "Series 90" models. The new Series 90 amps no longer utilized the Goodmans Midax horn. A new, shorter horn was sourced from a UK supplier, probably Celestion. Despite the difference in depth, the Series 90 horn had exactly the same screw mounting pattern as the Goodmans Midax horn. A 4uf oil filled capacitor, wired in series with the horn, acted as a 2000 hz 6db/oct crossover.

Unlike the earlier amps, the Series 90 horn was not concealed behind the grill cloth. A exposed plastic multi-cellular grill was mounted in front of the horn (see below).