The AC-50 "Big Box" speaker cabinet and trolley evolved from the AC-30 sized cabinet that Vox originally sold with the AC-50 single channel amplifier heads.
Several signs point to the initial "Small Box" cabinet design being the result of a hurried effort to supply a speaker system to the Beatles for their new 50 watt AC-50 heads.
These original "Small Box" speaker cabs had two 15 watt Celestion Alnico blue speakers, yielding a total power handling capacity of only 30 watts. This was not well matched to the far greater 50 watt output of the AC-50. Additionally, the cabinets were not sufficiently deep to enclose the back of the Midax horn, leaving it exposed and protruding from a hole in the back of the cabinet.
The new AC-50 "Big Box" cabinet corrected both of these problems. Two 25 watt, 12" Fane speakers allowed a total cabinet power rating of 50 watts to match the output of the head. The overall depth of the cabinet was increased to properly enclose the Midax horn.
A Goodmans Midax horn with a 2 uf oil filled crossover capacitor was included in the AC-50 "Big Box" speaker cabinet. This horn was included in the speaker cabinet to compensate for the reduced amount of treble available from the AC-50 amp head. This lack of treble was especially evident when comparing an AC-50 to a "Top Boosted" AC-30.
A tubular steel, chrome plated swivel trolley on swivel casters was also developed for the AC-50 "Big Box" cabinet.