VOX AC-50/4 MKIII, fully restored 1966

A brief history of the VOX AC-50 VOX amplifiers will forever be associated with The Beatles and The Rolling Stones—but not only them. Bands such as The Who and The Yardbirds also played through VOX amplifiers identical to this one. Later, artists including Brian May and Queen, Paul Weller and The Jam, and U2 likewise used VOX amplifiers. The VOX AC-50/4 Mk III was the second of three versions in the AC-50 "Big Box" amplifier head series. Production of the three "Big Box" models at JMI spanned from late 1964 to 1967. The "Big Box" AC-50 models replaced the single-channel AC-50 "Small Box" Mk I units produced between 1963 and 1964. The "Big Box" design introduced a storage compartment at the base for power cables, speaker cabinets, and guitars. The first "Big Box" amplifier head was introduced in late 1964 and named the "AC-50/4 Mk II." A second channel was added to the circuit while retaining the GZ-34 rectifier tube in the power supply—identical to that used in the Mk I "Small Box" models. This dual-channel chassis was also installed in some Mk I "Small Box, Thick Edge" units during the model transition. In 1965, the AC-50/4 Mk II was superseded by the dual-channel "AC-50/4 Mk III," which employed solid-state rectifier diodes. In 1966, the AC-50/4 Mk III was itself replaced by the dual-channel "AC-50 Mk IV." The main difference between the Mk III and Mk IV versions was a preamplifier tube change in the first stage: in the Mk IV, the 12AU7 (ECC82) previously used in the first preamp position was replaced with a 12AX7. The higher gain offered by the 12AX7 allowed the Mk IV to more readily drive the circuit into overdrive compared to earlier versions. This helped shift the AC-50’s reputation from a "clean and chimey" amplifier toward one better suited to post-1960s musical tastes. This AC-50 Mk III, serial number: 6911, chassis number: 3352, has been almost entirely restored, preserving, however, the amplifier’s essential sonic and aesthetic components—the chassis, transformers, front panel, knobs, handle, grille, logo, etc. When we received this amplifier, it was literally a rodent nest. We removed a bagful of tiny, gnawed paper scraps—clearly serving as nesting material for several litters born inside. Nonetheless, it bore indelible marks of lifelong mistreatment, including replacement of components with low-quality or inappropriate parts. Its condition was so poor that we decided on a full professional restoration to restore its original tonal character and specifications—even at the expense of originality for certain components. Resistors, capacitors, and even diodes were all replaced with new ones. The original BY100 diodes—no longer manufactured—were substituted with 1N4007s. All potentiometers were replaced, as the originals were excessively noisy. As for tubes, there was no doubt: none of the preamp tubes matched the original specifications. The input tube must be a 12AU7 (ECC82), while the others should be 12AX7 (ECC83). The amplifier originally contained four 12AT7 (ECC81) tubes. Regarding the power tubes—EL34s—one was cracked, so both were replaced with a new pair. All current tubes are from JJ Electronics/TESLA. We will replace them—at no additional cost—with tubes of another brand, per the buyer’s preference. This amplifier is an authentic relic, impressively 55 years old. After this "surgery," it is certainly ready for another 55 years. Due to its distinctive tone, it is highly prized by bass players—just like its previous owner. A heartfelt thank-you to him, wherever he may be!