Max Guitar is crazy about the older Voxes…and especially the ones with a little extra historical value. This one is from either late 1965 or 1966 (defined by the #09 backplate) and is very collectible and great sounding amp. Serial 4703. similar as used by Paul MCcartney. this however is from a slightly later date.
Amplifiers played an important role in many of the hit records produced during 1964 and 1965, the early years of the “British Invasion.” During this period, the Beatles, the Rolling Stones, the Dave Clark 5, the Hollies, the Animals and U.S. group Paul Revere & the Raiders, among others, took the music world by storm. Their success helped to make Vox the dominant brand of guitar amplifiers in the world at the time.
Vox was so proud of the brilliant frequency response produced by their signature “Top Boost” tone circuitry that they gleefully proclaimed their amplifiers could produce “glass shattering treble” in a 1964 product catalog.
By 1966, however, musical tastes were evolving. The bright, poppish guitar sounds from the early “British Invasion” were being replaced by a darker, overdriven tone. This new “bluesy” tone was developed by guitar amps that incorporated a high gain preamp design. Such preamp circuitry would intentionally overdrive the amp into distortion, a concept previously avoided by Vox.
While Vox did not seem at first to respond to these changing tastes in guitar tone, upstart amp manufacturer Jim Marshall did. Marshall owned a music store in London that was producing high gain 50 and 100 watt amplifiers in the basement of his shop.
Aside from a slightly higher gain preamp, the AC-50 Mk IV was nearly identical to the amp it succeeded, the AC-50 Mk III. The Mk IV circuit was found on Vox schematic OS/163, dated April, 1967. The AC-50 Mk I, Mk II and Mk III preamp circuits all incorporated a 12AU7 tube in their preamp section. The 12AU7 is a low gain tube with an amplification factor of just 20. The low gain design of the 12AU7 helped to form the basis for a very “clean” sounding AC-50 amplifier. The power amp featured two EL-34 output tubes running in fixed bias mode, producing about 45 watts RMS output power. It can be a challenge to install modern EL-34 output tubes in the AC-50. Many modern EL-34 tubes are too tall to fit in the clearance between the chassis and top of the cabinet. To extract the maximum audio power from a pair of EL-34s, Vox pushed the plate voltage to 480 volts. This was still within the “safe operating range” for the Mullard and Brimar tubes Vox originally installed at the factory in these amplifiers.
Dual Amphenol three-pin XLR “round top” jacks were provided in a recessed rear panel for speaker connection. A speaker impedance selector allowed the choice between 8 or 16 ohm total speaker loads.
To connect to a single 16 ohm cabinet, the selector plug should be in the 16 ohm position and the speaker system could be plugged into either jack. If either one 8 ohm or two 16 ohm cabinets were
connected, the selector plug should be moved to the 8 ohm position. The AC-50 does not accept speaker loads less than 8 ohms, so pairs of 8 ohm speakers should never be connected to the amp.