Vox Sound Ltd (Dallas) Vox AC30 Top Boost with Green Printed Circuit Boards
A Look "Under the Hood" - 1977-78

© 1996 - 2023 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.
Dallas Arbiter had big plans when they purchased Vox Sound Ltd. from Birch Stolec in 1973. As a first matter of business, Dallas restored the AC-30 "Top Boost" amp to the original point-to-point, hand wired tag strip construction (see Fig. 1) used during the Jennings Vox period.

However, the high labor costs of hand wiring the AC-30 started to have an impact on profitability at Dallas. A new, more efficient way to construct the AC-30 was needed or the future of the amp might be in jeopardy. While it was obvious that replacing the hand wiring in the AC-30 with a printed circuit board would improve manufacturing efficiency, this also came with a downside. Tube amp circuits built on printed circuit boards often suffer from reduced dependability and are tougher to service.

In theory, the easiest and most efficient way to build an amplifier such as the AC-30 would be to construct it on a single printed circuit board (PCB). This would eliminate the interconnecting wires or "flying leads" between the circuit board, tube sockets and controls. However, lessons learned from the Stolec era AC-30 (1970-1972) proved that this style of PCB construction could create problems. Stolec mounted the tube sockets directly onto the PCB, causing many heat related board failures.

Dallas developed an innovative redesign of the AC-30 that replaced the hand wired tag strips with similar PCB strips (see Fig. 2). Made from green translucent phenolic material, these PCB strips retained the original layout of the hand wired tag strips but required less time to assemble. The PCB tag strips were populated with electronic components prior to installation in the chassis and then hand wired to the chassis mounted controls and tube sockets. This design offered Dallas the best compromise between a point-to-point, hand wired amp and one using a PCB.

Chassis, Control Panel and Slider Board
The original 1960's era JMI Vox AC-30 chassis design had a passivated steel base to support the weight of the transformers and a vertical aluminum stamping in the shape of an inverted "L" to mount the preamp circuits and control panel. The lower chassis included four cooling slots near the EL84 tube sockets. A grounded brass shield enclosed the input jacks, minimizing against electrical interference. The Dallas chassis was about 1/8" (~3mm) taller than the JMI Vox chassis, making it incompatible with a JMI style AC-30 cabinet.

This Dallas AC-30 chassis included two additional holes in the chassis for tube sockets. These sockets were used to mount an additional pair of 12AX7 tubes for the optional reverb circuit. If the amp did not include reverb, one of the holes was used to allow wires from the output transformer to pass inside the lower chassis.

Unlike the "etched and filled" gray control panels of the JMI Vox era, these later Dallas AC-30 control panels were simply painted gray with silk screened nomenclature. This made them easily susceptible to scratches and damage.

Fig. 1 - Point to point hand wired tag strip - Dallas AC-30 (1974)

Fig. 2 - Green PC board "tag strip"- Rose-Morris AC-30 by Dallas (1979)
Five bolts secured the Dallas AC-30 chassis to an original style slider board (Fig. 3) for ease of service. A ground shield, made of aluminum foil affixed to the top of the slider board, further minimized the effects of external electrical interference.

The slider board also included a rectangular vent hole that was covered with an expanded aluminum grill. This vent hole in the slider board worked in conjunction with four slots in the chassis adjacent to the output tubes to provide flow-through cooling.

Fig. 3 - Slider Board
Power Supply Comparison
The major components in the original JMI Vox AC-30 power supply included a power (mains) transformer, a GZ34 rectifier tube, a 10H choke, two large filter or "smoothing" capacitors and a five position voltage selector switch.

The primary side of the original JMI Vox AC-30 power (mains) transformer had five windings that accommodated the various mains voltages throughout the world. The secondary side of the original transformer had a B+ (high voltage) winding, a 6.3 VAC filament heater winding for the preamp and output tubes and a 5 volt filament winding for the GZ34 rectifier tube.

The GZ34 was a full wave rectifier tube that converted AC voltage from the B+ winding of the power transformer to pulsed DC. A natural attribute of the GZ34 rectifier tube was the tendency to exhibit a "sag" in output voltage when the amp was pushed toward the limit. These momentary drops in voltage caused a bit of audio compression to occur in the output of the AC-30. Many feel that the audio compression created by an overdriven AC-30 is an essential component of Vox tone.

The original JMI power supply also included a choke (sort of a single winding transformer) wired between two large smoothing capacitors. Installing a choke between two filter capacitors creates what is called a "capacitive pi" filter power supply circuit. This design does a superior job of cleaning up the pulsed DC coming from the rectifier tube.

The power supply in the Dallas AC-30 featured on this web page retained the choke, smoothing caps and voltage selector switch from the original AC-30 power supply but eliminated the GZ34 rectfier tube. The tube was replaced with four solid-state 1N4007 silicon diodes. Significant cost savings for material and labor could be realized by eliminating the GZ34 rectifier tube, tube socket and the 5 volt filament tap on the power transformer. The four 1N4007 diodes replacing the GZ34 tube would cost less than a dollar.

While losing the natural compression of the GZ34 rectifier, swapping it for diodes had benefits. Unlike a GZ34 tube, diodes rarely fail. Further, the electronic efficiency of the diodes increased the output of the Dallas AC-30 to nearly 40 watts RMS.

The control panel mounted five position voltage selector switch from the original AC-30 power supply was replaced with a two position slide switch mounted on the upper rear edge of the chassis. It was visible when the upper back panel of the cabinet was removed. By reducing the selectable mains voltages from five (115, 160, 205, 220 and 245 VAC) to just two (220 and 120VAC) Dallas could simplify the mains transformer by eliminating three primary voltage taps.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

Any and all material presented herein is protected by Copyright.
© 1998 - 2023 The Vox Showroom and North Coast Music, all rights reserved

The images and editorial content in this web site may not be copied or reproduced
in online auction sites such as eBay, Reverb and Craig's List. Sellers may provide a link
to the Vox Showroom web site if they wish to refer to this copyrighted material.

URL: http://www.voxshowroom.com/uk/amp/ac30_dallas_pcb_hood.html