Rose Morris Vox AC30TB "Vintage" Top Boost (1991 - 1993)
Rose Morris Vox AC30TBR "Vintage" Top Boost Reverb (1991 - 1993)
Rose Morris Vox AC-30 "Vintage" Top Boost Head (1991 - 1993)
A Look "Under the Hood"





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Rose Morris Changes Their AC-30 Vendor for the Third Time in Ten Years
Rose Morris was the first owner of the Vox brand that did not have any amplifier manufacturing facilities. After Rose Morris purchased Vox from Dallas Industries in 1978, amp production remained at the Dallas plant in Shoeburyness, Essex UK through 1984. This Dallas manufacturing plant also produced the Vox amps sold by Dallas Industries from 1973 through 1978.

As the former distributor of Marshall amplifiers, Rose Morris had grown accustomed to products built to a high standard of quality and packaged in rugged export cartons. This was not the case for the Vox amps coming out of the Dallas facility in Shoeburyness. Many new Vox amps failed to work out of the box. Flimsy packaging caused many amps to be damaged in shipment. These and many other issues caused the relationship between Rose Morris and Dallas to deteriorate. By 1983 Rose Morris was looking for a new contract manufacturer to build the AC-30.

Vox next turned to a company called Audio Factors to build the AC-30. Audio Factors was also located in the UK, near Leeds. The Audio Factors Vox AC-30 retained the traditional "Contempo" chassis design that traced back to the 1960 origin of the amp. All tube sockets were mounted to the chassis base. The preamp circuitry was incorporated onto a large vertical PC board.

In addition to the AC-30, Audio Factors also built the Venue Series of solid state amps that Vox offered from 1984 through 1988.

Precision Electronics
It wasn't long before the quality of the AC-30 amps produced by Audio Factors started to decline. As a result, Rose Morris shifted AC-30 production in 1988 from Audio Factors to Precision Electronics, a UK electronics manufacturer located in Wellingborough. In addition to the AC-30, Precision Electronics also produced the solid state "Q Series" amps for Vox.

Rose Morris was so pleased with Precision Electronics that they purchased an interest in the company in 1989 and renamed it Vox Amplification, Ltd.

Vox Amplification Ltd produced one thousand AC-30 "Limited Edition" amplifiers in 1990 and 1991. They followed on the success of the AC-30 "Limited Edition" by introducing the AC-30 "Vintage" line up from 1991 through 1993. The AC-30 "Vintage" was available as a 2x12 combo amp with or without reverb or as a separate 30 watt head with a matching 4x12 speaker cabinets. A "Collector" AC-30  combo amp with a solid mahogany cabinet and the "Vintage" chassis was also offered.

It is worth noting that Paul McCartney has toured with Rose Morris era AC-30 "Vintage" heads and matching 4x12 cabinets since the early nineties.

All-In-One Circuit Board
Like the "Limited Edition" AC-30 from 1990 that preceded it, the circuitry for the AC-30 "Vintage" amplifier was designed onto a single printed circuit board.

The green circuit board shown in Figure 1 was from an "Issue 1" version of the AC-30 "Vintage" amp. A quality inspection tag on the PCB dated the amp to 11/27/1991.

The chassis shown at the top of this page had a red printed circuit board. This was the "Issue 2" version of the AC-30 "Vintage" amp. The quality inspection tag on the PCB was dated 5/26/1992.

Figure 1 - Vox "Vintage" AC-30 Circuit Board - Issue 1 (November 1991)

The Vox AC30TB (no reverb) and the AC30TBR (with reverb) "Vintage" series amps shared the same printed circuit board. The components for the reverb circuit were omitted from the AC30TB circuit board.


Figure 3 - AC-30 "Vintage" Circuit Board Issue 1

Figure 4 - AC-30 "Vintage" Circuit Board Issue 2
Figure 5 - Vox AC-30 Circuit Board Issue 3
Circuit Board Issue Numbers
Vox assigned an "Issue" number to the large integrated circuit boards in their AC-30 amps from 1989 through 1993. The issue number was imprinted in the lower left corner of the board.

"Issue 1" circuit boards (see Figure 3) were installed in the early production of Vox AC-30 "Vintage" combo amps and heads in 1991. This printed circuit board utilized the ill-fated PC mount reverb drive transformer that was subject to frequent failure.

"Issue 2" circuit boards (see Figure 4) were installed in the later production of Vox AC-30 "Vintage" combo amps and heads from 1991 through 1993. The location of the main (smoothing) filter caps was changed on the Issue 2 board. Issue 2 circuit boards also utilized the ill-fated PC mount reverb drive transformer that was subject to frequent failure.

"Issue 3" circuit boards (see Figure 5) were installed in the AC-30TBR - Model 1007 and the "Limited Edition" 30th Anniversary AC-30 amplifiers. Unlike amps utilizing the Issue 1 or Issue 2 circuit boards, the Issue 3 board required a chassis mounted reverb transformer that was not subject to failure. They also featured the breakaway power amp mini board.

What Each Tube Does
The tube layout for the AC30TB and AC30TBR "Vintage" amp is shown in the open chassis picture at the top of this page. Here is the function of each tube.
V1
12AX7
Oscillator for the Vib/Trem Circuit
V2
12AX7
½ used for preamp of Vibrato channel
½ used as a gain stage for Vibrato circuit
V3
12AU7
Modulator for the Vib/Trem Circuit
V4
12AX7
½ used for preamp of Normal channel
½ used for preamp of Brilliant channel
V5
12AX7
Top Boost Tone Control Circuit
V6
12AT7
Reverb Drive (AC30TBR only)
V7
12AX7
Reverb Return (AC30TBR only)
V8
12AX7
Phase Inverter
V9 - V12
EL-84
Power Amplifier


Figure 4 - Ventilated Preamp Tube Cage
No More Slider Boards
The Vox AC-30 "Vintage" amplifier eliminated the two piece traditional "Contempo" style chassis and slider board system from the original JMI design. It was replaced with a rectangular anodized steel pan that filled the full inside width of the cabinet.

Barbed nylon standoffs secured the printed circuit board to the inside of the chassis pan. The transformers and choke were bolted to the outside of one end of the chassis pan. The chassis was secured to the cabinet with four machine screws.

A ventilated aluminum cage bolted to the chassis over the preamp tubes (Figure 4). It shielded the tubes from electrical interference. A foam block in the inside lid of the cage applied downward pressure to the preamp tubes, keeping them firmly seated in their sockets. A spring loaded bracket bolted to the chassis surrounded the EL84 power tubes to keep them seated in their sockets.

Problems With the AC30TBR "Vintage" Reverb Circuit
The reverb circuit in the AC30TBR "Vintage" amp had an inherent and fatal design flaw. It is unusual to find a AC30TBR "Vintage" amp that has functioning reverb.

The problem with the reverb circuit can be traced to a board mounted transformer in the reverb "drive" circuit on the "Issue 1" and "Issue 2"  main printed circuit board. Given the part number A262A3E (see Figure 5), this tiny transformer was positioned between the output of V6, the 12AT7 tube that drives the reverb circuit and the input of the reverb pan.

This transformer was designed to operate in circuits producing no more than one half watt. Unfortunately, the normal operating range for the AC30TBR "Vintage" reverb drive circuit is in excess of one watt. This additional current burns the internal windings of transformer, causing the reverb circuit to break down.

A qualified electronic technician can restore proper operation to the reverb circuit by replacing the defective A262A3E with a chassis mounted Hammond 125A transformer. The 125A has the split primary coil required by the AC30TBR "Vintage" reverb drive circuit. The 16 ohm output tap on the 125A works well with the factory installed Accutronics 9BC2C1B reverb pan.

Figure 5 - Drive Transformer for the Vox "Vintage" AC-30





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