JMI Vox AC-50 Single Channel Four Input Amplifier Chassis
"A Look Under the Hood"




The AC-50 utilized the classic Vox amp chassis design. The base of the chassis was made of formed steel, supporting the transformers and power tubes. A vertical piece of aluminum, bent into the shape of an inverted "L," was bolted to the steel base and to the tops of the transformers to support the preamp and control panel sections of the amplifier.

This unique Vox chassis design yielded many benefits. The steel base section of the chassis provided the strength and rigidity to support the heavy power and output transformers. The output and rectifier tubes were mounted vertically to this steel chassis base, facilitating proper ventilation and providing an excellent heat sink for the tube sockets. Locating the hand wired preamp circuitry on the inside of the grounded vertical aluminum structure electronically isolated the sensitive, high gain preamp circuitry from stray electronic fields eminating from the transformers and power anplifier circutry. The chassis was structurally rigid and not easily susceptible to damage.

The quad input, single channel AC-50 chassis used one half of a 12AU7 (ECC82) as the first stage of the preamp. The signal was then fed to one half of a 12AX7 (ECC83) which was used as an additional gain stage. The second half of this 12AX7 powered the tone circuit. A second 12AX7 was used as a phase splitter. Two EL34 power tubes provided about 45 watts of output power. A GZ34 tube served as the power supply rectifier.

A removable "plug" type voltage selector allowed the amp head to be used in any country in the world.




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