Rose Morris Vox AC30 Top Boost and AC30 Top Boost Reverb (1979 - 1985)
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.
Shortly after the purchase, Rose Morris established a corporation in the UK called "Vox Limited" to produce new Vox products.

The AC30 Top Boost Reverb amp chassis shown on this page is an example of the first generation AC-30 produced by Rose Morris. RM offered this version of the AC-30 from 1979 through 1984. The component date codes of the chassis shown on this page suggest it was produced in early 1982.

This amp was manufactured for Rose Morris by Dallas Industries in Shoeburyness, Southend-on-Sea, Essex, England. This same facility produced the Vox amps sold by Dallas Arbiter from 1973 through 1978.

Chassis
This first generation Rose Morris AC-30 chassis featured an anodized 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

this same chassis for their 1973-1978 production of hand wired AC-30 amplifiers.

The RM, Dallas and the original JMI AC-30 chassis were all nearly identical except the Dallas and RM chassis versions were slightly taller.

The RM, Dallas and JMI chassis were each bolted to a slider board that allowed the chassis to be removed like a drawer for service. The RM slider board included a foil ground shield and a vent screen under the power tubes to improve flow-through cooling.


Power Supply
The original design for the JMI AC-30 power supply included a GZ-34 rectifier tube. The GZ-34 is a full wave rectifier tube that converts AC voltage from the secondary of the power transformer to pulsed DC. A natural attribute of the GZ-34 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. All AC-30 models produced prior to the Rose Morris ownership of Vox included a "capacitive pi filter" DC power supply.

In an attempt to reduce manufacturing costs, neither the GZ-34 rectifier tube or the choke were included in the power supply redesign of the first Rose Morris AC-30.

The GZ-34 tube was replaced with four IN4007 silicon diodes. Installing a GZ-34 rectifier tube and chassis mounted tube socket might add $10 USD to the cost of building an AC-30. Replacing the tube and socket with four IN4007 diodes would cost just pennies.

However, the installation of diodes in the AC-30 power supply was not without benefits. Diodes generate no heat and have a much

longer service life than a GZ-34 rectifier tube. Additionally, the electrical efficiency gained from the diodes helped the Rose Morris AC-30 amp to produce upwards of 40 watts of RMS output power.

The choke was replaced with a 22 ohm 10 watt resistor in the first generation Rose Morris AC-30. Deleting the choke from the AC-30 power supply saved Rose Morris more than $10 USD per amplifier. The 22 ohm supply resistor actually simulated some of the voltage sag lost by the removal of the GZ-34 rectifier tube.

A 120V/240V voltage selector switch was mounted on the upper rear edge of the chassis. It was visible when the upper back panel of the cabinet was removed.

Circuit Board Construction
Early Vox AC-30 amplifiers featured hand wired, point-to-point circuit construction. The leads of all the electronic components were manually wrapped around hook shaped mounting lugs. All interconnections were made "wire to wire" and hand soldered. Building an amplifier in this fashion was labor intensive and costly to produce.

Stolec Industries produced Vox amplifiers from 1970 to 1972. The redesigned Stolec era AC-30 was built on two large printed circuit boards. One board contained the preamp circuitry, the second contained the power amp. All of the tube sockets were mounted directly to the circuit boards. The heat of ten tubes caused these circuit boards to warp, causing premature failures of the amplifier.

Dallas Industries bought Vox from Stolec in 1972. Dallas recognized that the reputation of the AC-30 had been damaged during the Stolec period. Dallas decided to restore confidence in the AC-30 by taking it back to the original hand wired, point-to-point construction used in the sixties. The hand wired AC-30 was produced by Dallas Arbiter from 1973 through 1978.

When Rose Morris purchased Vox in 1978, every avenue to cut production costs was reviewed, including a return to printed circuit board construction. Dallas approached Rose Morris with an innovative redesign of the AC-30 that replaced the hand wired tag strips used on earlier AC-30 amps with similar printed circuit strips. Components could be easily installed on the printed circuit strips prior to assembly into the amp. The completed boards would then be hand wired to tube sockets that were mounted to the chassis. This offered Rose Morris the best compromise between a hand wired and a printed circuit amp.


Reverb
The AC-30 Top Boost Reverb model added a full length Accutronics delay line to the standard AC30 Top Boost model. The reverb circuit was powered by two ECC83 tubes and a hand wired circuit mounted to the inside bottom of the chassis. The standard AC-30 Top Boost model would not include the additional tubes, circuitry or reverb pan.

No additional holes needed to be provided in the top panel to accomodate a reverb control. To create a position for the "Reverb" control, Rose Morris combined the "Vib/Trem" and "Speed" controls on to one six position rotary switch. The allowed the "Reverb" control to be mounted in the top panel position formerly used for the "Speed" control.

Control Panel
The control panel used on this first generation series of Rose Morris AC30 amps was finished in gray paint. The control nomenclature was silk screened on the panel in gold paint.

The AC-30 control panels installed on JMI, Stolec and Dallas Arbiter amps featured an anodized finish that offered greater resistance to scratching.




Links to Additional Rose Morris AC-30 Amp Topics in the Vox Showroom
  • The 1979 - 1984 Rose Morris AC30 Top Boost Reverb
  • 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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