The Vox AC-30 Top Boost Amplifier (1970 - 1973) - Birch-Stolec Era
A Look "Under the Hood"

The Birch-Stolec era AC-30 was the earliest effort to build an AC-30 using printed circuit construction techniques. The chassis, slider board and control panel from the JMI and VSEL era AC-30 were adapted by Birch-Stolec to accept their new twin printed circuit board design. Rectangular holes in the chassis provided access to the printed circuit board mounted tube sockets.

The tube complement remained unchanged from the original JMI design. It included five ECC-83 and one ECC82 in the preamp area (these tubes were removed from the amp in the photo above), four EL-84 for the output stage and one GZ-34 rectifier tube.

Various books have reported that the Birch-Stolec AC-30 was the first AC-30 redesign to eliminate the GZ-34 tube recitifier. This appears to be only partially true. The pictures shown here offer evidence that the GZ-34 rectifier tube was included in some Birch-Stolec era AC-30 Top Boost amplifiers.

Please note the "Lemark" label on the choke seen above. Lemark Industries was another company owned by Birch-Stolec Ltd.

Above is a view of the outside of the Birch-Stolec AC-30 preamp printed circuit board. Servicing this amp can best be described as a nightmare. All of the electronic components were mounted on the inside of the board, requiring the board to be taken out of the chassis for service. Accessing the inside of the preamp circuit board required the removal of all the control nuts plus a series of screws that fastened the circuit board to the chassis.

This is the component side of the Birch-Stolec AC-30 preamp board. The tube sockets were mounted directly to the circuit board. This design conducted the heat from the tube sockets directly to the circuit board. The design also placed mechanical stress on the board whem removing and installing tubes.

A second printed circuit board was used for the power amp section of the Birch-Stolec AC-30. Notice the brownish discoloration of the circuit board under the sockets for the output tubes. The Birch-Stolec design required the printed circuit board to dissipate the heat conducted from the output tube sockets. The discoloration on the circuit board is a result of this heat.

By comparison, the tube sockets on JMI and VSEL AC-30 amplifiers were mounted to the chassis. Heat from the output tube sockets was dissipated by the chassis.

Photo Credits
The use of these chassis photos by the Vox Showroom was generously allowed by Champ Electronics, Nottingham England. These images were used with permission and remain the intellectual property of the copyright holder.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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