Rose Morris Vox AC30 Top Boost with Green Printed Circuit Boards - 1979
A Look "Under the Hood"

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The UK musical distributor Rose Morris (RM) purchased the Vox brand from Dallas Arbiter in 1978. The AC-30 shown on this page was one of the first new Vox amps offered by Rose Morris.

Dallas Arbiter Sells Vox to Rose-Morris
Dallas Arbiter had high hopes for Vox when they purchased the iconic marque from Birch Stolec in 1973. Under Dallas, the AC-30 "Top Boost" amp returned to the original point-to-point, hand wired tag strip construction of the JMI days. Dallas hired Reg Clark, the original JMI sales manager to be the director of sales. JMI founder Tom Jennings was also brought on as a technical consultant. A manufacturing facility for Vox was established in Shoeburyness, Essex. Despite their best efforts and intentions, Dallas decided to divest itself of Vox just five years later in 1978. The sale was announced to the trade in the advertisement at right.

Several factors led Dallas to the decision to sell Vox. A subsidiary of Dallas Arbiter, CBS Arbiter, was the UK distributor for Fender. Fender viewed Dallas Arbiter's involvement with Vox as a conflict of interest and asked Dallas to drop the Vox brand. It was not difficult for Dallas to comply with this request as both the sales and profitability at Vox had been steadily declining. To further complicate matters, Thomas Organ retained the distribution rights for Vox in North America. This prevented Dallas from exporting Vox amps to the US, the largest music market in the world.

Rose-Morris had been the distributor for Marshall Amplification throughout the 1970's. The relationship between manufacturer and distributor had been strained for years. Rose-Morris lived in the constant fear of losing Marshall. When Vox went up for sale in 1978, Rose-Morris seized the opportunity and purchased the brand. If Marshall appointted a new distributor, Rose Morris would still have Vox. Rose-Morris shortened the company name from "Vox Sound Limited" to "Vox Limited."

As Rose-Morris had had no manufacturing facilities, it welcomed an offer from Dallas to continue production of Vox amps in their facility at Shoeburyness, Essex. This facility had also produced the Vox amps sold by Dallas Arbiter from 1973 through 1978. The AC-30 Top Boost amp chassis shown on this page is an example of the first generation Rose Morris AC-30, produced in 1979.

Circuit Board Construction
Due to it's popularity in the Vox line, the AC-30 would undergo close scrutiny by Rose-Morris. They hoped to use their experience with Marshall to reduce manufacturing costs.

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). However, the tough lessons learned from the Stolec era (1970-1972) AC-30 proved that this was not a wise choice. Stolec mounted the tube sockets directly onto the PCB, causing many heat related board failures. This design flaw gave the Stolec AC-30 the reputation as the least dependable version of the AC-30 ever produced.

The Dallas era AC-30 (1973-1978) replaced the Stolec model. The circuitry was constructed on hand wired tag strips utilizing the same point-to-point assembly technique employed by JMI Vox in the sixties (see Fig. 1). The tube sockets were mounted to a JMI style chassis, improving heat dissipation. Hand wiring improved dependability and serviceability but was costly to produce.

When Rose-Morris purchased Vox in 1978, every opportunity to cut production costs was considered. Dallas offered an innovative redesign of the AC-30 that replaced the hand wired tag strips with similar PCB strips (see Fig. 2). Made from green 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 controls and tube sockets. This design offered Rose Morris the best compromise between a hand wired and a PCB amp.

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)

Chassis, Control Panel and Slider Board
The original 1960's era JMI Vox AC-30 chassis design had an 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. Dallas Arbiter used a similar chassis for production of AC-30 amplifiers for Rose-Morris. This Rose-Morris chassis was about 1/8" (~3mm) taller than the JMI Vox chassis, making it incompatible with a JMI style AC-30 cabinet.

The Dallas/Rose-Morris AC-30 included two additional holes in the chassis near the power transformer. These holes were used to mount an additional pair of 12AX7 tubes for the optional reverb circuit. If the amp did not include reverb, the holes were left empty, as shown in the chassis photos at the top of this page.

The gray finish of the earliest Rose-Morris AC-30 control panels appears to be silk screened. The gray finish was susceptible to scratching and nicking, unlike the anodized or etched and filled gray control panels of the JMI Vox era.

Five bolts secured the Rose-Morris AC-30 chassis to an original style slider board for ease of service. A ground shield, made of aluminum foil affixed to the top of the slider board, 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.

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 first edition Rose-Morris 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 Rose-Morris 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) Rose-Morris could simplify the mains transformer by eliminating three primary voltage taps.

Links to Additional Rose Morris AC-30 Amp Topics in the Vox Showroom
  • The 1979 - 1984 Rose Morris AC30 Top Boost Reverb - A Look "Under the Hood"
  • 1979 Rose Morris Product Catalog
  • 1981 Rose Morris Product Catalog
  • 1985 Rose Morris AC-30 Top Boost Reverb

  • North Coast Music offers many factory licensed cosmetic replacement
    and repair for the Rose Morris Vox AC30. Some are shown below.

    Chrome plated rigid AC-30 stands

    Vox Logo (NCM-025)

    Two pin replacement corners for the AC30, manufactured by North Coast from the original mold

    Model "Flags"

    Vox replacement handles for the AC-30

    Replacement Vents for the AC30

    Vox cast aluminum "Egg" foot switch

    Brown Vox Replacement Grill Cloth

    Black Vox Replacement Grill Cloth
    Gold Fascia Strip for the AC-30

    AC-30 Amp Covers

    Foam lined plywood road cases for the AC-30, carpeted exterior

    Celestion Alnico Blue Replacement Speakers

    Celestion Green Back Replacement Speakers

    Replacement Vinyl for the AC30/6

    White and gold cabinet piping for the AC-30

    Replacement Feet for the AC30

    Schematics for Rose Morris Era AC-30 Amps



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