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UK VOX AMPS History of Vox in the UK JMI Era Vox Amps: 1959-65 JMI Era UL Series Hybrid Amps: 1966-67 JMI-VSL Era Solid State Amps: 1967-73 VSEL/Corinthian Era Vox Tube Amps: 1968-70 Birch Stolec Era Vox Amps: 1970-73 Dallas Era Vox Amps: 1973-78 Rose Morris Era Vox Amps: 1978-92 Korg Era Tube Amps 1993-25 Korg Era Solid State and Hybrid Vox Amps:1999-25 Korg Era Vox Valvetronix Amps - 2001-16 US VOX AMPS VOX ORGANS VOX GUITARS Vox Solid Body Elec Guitars: 1960-88 Vox Semi-Acoustic Elecric Gtrs: 1964-68 Vox Acoustic Guitars: 1964-70 Vox Bass Guitars: 1962-87 Voxton Guitars & Basses: 1969-70 Vox Gtrs & Basses, Korg Era: 1998-25 VOX FX & MISC View All Products Vox Tape Echo & Reverb Units:1963-68 Vox Microphones and Mixers Vox Boosters Vox Wah Wah Pedals: 1967-2025 Vox Effects Pedals: 1965-2025 Vox Tonelab & Stomplab FX Processors Vox Amplugs Vox Ampliphonic Accessories: 1968 Vox Accessories Vox Promotional Items VOX CATALOGS VOX MANUALS MY VOX BAND SEARCH WHAT'S NEW

The Vox Solid State Escort 50 Lead Amplifier - A Look "Under the Hood"

Cabinet In contrast to the premium finger-jointed, 13-ply Baltic birch cabinets manufactured by Vox in the 1960s, the Escort 50 Lead enclosure used simpler butt-jointed ¾" particle board. Large internal wooden cleats reinforced the cabinet joints. The rear panel was ⅛" masonite with a cut-out for storing the power cable. A 12" Vox branded Fane speaker was mounted to the left side of the baffle as viewed from the rear of the amplifier. The off-center placement of the speaker probably served to offset the weight of the mains transformer on the opposite side. Chassis The Escort 50 Lead featured an inverted "L" shaped, 16-gauge steel chassis that anchored the amp's circuitry. In addition to housing the electronic components, the chassis also functioned as a heat sink for the output transistors. Circuit Board A single phenolic printed circuit board housed the preamp circuit, tone controls, power amp drivers, power supply and the mains transformer. This board was secured to the chassis by the five PC mount potentiometers. Screws on each end of the chassis fastened it to the amplifier cabinet. Single Rail Power Supply The Escort 50 Lead amplifier shown on this page featured a mains transformer factory-wired for 220–240 VAC operation with alternative 120 VAC leads for operation in North America tucked beneath the end bell. Its single, untapped secondary winding fed an IN5404 diode bridge rectifier in conjunction with a 1Ω anti-surge resistor and a 1000 µF smoothing capacitor to generate a +70 VDC rail to supply the power amplifier circuit. This +70 VDC rail voltage was then stepped down to +20 VDC to power the preamp stage via a 2.7kΩ resistor and a 20V Zener diode. The Escort 50 Lead incorporated two internal 2A fuses: one protecting the primary winding of the power transformer and another for the secondary. 50 Watt Quasi-Complementary Power Amplifier Circuit The Escort 50 Lead employed a quasi-complementary power amplifier circuit using two 2N3055 NPN output transistors to emulate a standard NPN-PNP push-pull pair. It accomplished this by using a traditional Darlington pair (BD537 NPN driver transistor + 2N3055 NPN output transistor) for the upper half of the wave and a Sziklai pair (BD538 PNP driver transistor + 2N3055 NPN output transistor) for the lower half of the wave. This Sziklai pair configuration effectively simulated a high-power PNP output transistor, bypassing the high costs and reliability issues associated with early PNP power transistors. The audio signal from the output transistors then passed through a 1000uf electrolytic capacitor before reaching the speakers. This capacitor was critical to isolate the amplifier's DC biasing voltages from the speaker output. Without this capacitor acting as a barrier, the DC biasing voltage from the 2N3055 output transistors would instantly burn out the speaker's voice coil. Preamp Circuit The entire preamp circuit, including the tone controls, was powered by a single RC4136 quad opamp. The RC4136 included four independent amplification circuits in a compact 14-pin DIP (dual in-line) package. The RC4136 significantly streamlined the layout of the circuit board and completely eliminated the need for discrete transistors in the preamp circuit. Schematic Download the factory schematic for the Escort 50 Lead by clicking here.