Vox V1031 Cambridge Reverb Amplifier
Vox V1032 Cambridge Reverb Amplifier
Vox V1081 Berkeley II Amp Head
"A Look Under the Hood"


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The Vox V1031 Cambridge Reverb amplifiers and the V1081 Berkeley II amp head shared the same chassis and circuitry. These amps were produced in 1966 and 1967. The V1032 Cambridge
Reverb replaced the V1031 model in late 1967, adding an "E-Tuner" reference pitch generator to the V1031 circuitry. All three amplifiers were produced by the Thomas Organ Company of Sepulveda CA under license to Jennings Musical Industries of Dartford Kent UK.

Chassis
The steel chassis for the V1031/V1032 Cambridge Reverb and V1081 Berkeley II housed the all of the preamp, power amp and power supply circuitry. The chassis was comprised of three pieces: the chassis pan, the control panel and the power amp heat sink.

The stamped and formed steel chassis pan was the backbone of the chassis. It housed the 25-5274-2 printed circuit board assembly. It also supported the T1 power transformer, 5000 uf main filter cap and the T3 reverb transformer. This formed steel chassis pan was also shared with the Vox V1021 Pacemaker and V1011 Pathfinder solid state amplifiers.

The second part of the chassis was the control panel, including the controls, jacks and switches. It was fastened to the top of the chassis pan with two machine screws.

The third part of the chassis was the power amp heat sink. The germanium output transistors and the T2 power amp driver transformer were mounted to the power amp heat sink. The power amp heat sink was suspended beneath the main chassis to facilitate air circulation and fastened to the pan with three machine screws.

The chassis design of the V1031/V1032 and V1081 made servicing a bit of a challenge. The printed circuit board was wired to both the power amp heat sink module (located beneath the circuit board) and the control panel (located above the circuit board). Wires connected to the circuit board from both above and below made it difficult to access the foil (or back) side of the board to install and solder replacement components.

Standardization and Modular Construction
In late 1965, faced with the task of quickly developing an entire range of solid state U.S. Vox amplifiers, Thomas engineers aimed for manufacturing efficiency through standardization. They designed a universal preamp printed circuit board (PCB) given the Thomas part number 25-5222-2. This board would form the basis for a two or three channel modular preamp and control section for their higher powered amps. Thomas also developed 30, 60 and 120 watt power amplifier modules to combine with their new universal preamp section.

When the universal preamp section was combined with the 30 watt RMS power amp module, the Viscount and Buckingham amps were created. When the preamp section was combined with the 60 watt RMS power amp module, the Royal Guardsman and Westminster arrived. Adding the 120 watt RMS power amp module to the universal three chanel preamp section formed the Super Beatle.

Thomas also designed a single printed circuit board for their lower powered solid state Vox amps: the Pathfinder, Pacemaker, Cambridge Reverb and Berkeley II. Given the Thomas part number 25-5274-2, it consolidated the preamp, tremolo, reverb and power amp circuits onto one circuit board. A color coded diagram that highlights the layout of the 25-5274-2 circuit board is shown above.

When the 25-5274-2 board was populated with the electronic components required for the V1031 Cambridge Reverb and V1081 Berkeley II amplifiers, the completed circuit board assembly was given the Thomas Organ part number 84-18382-1 (see diagram above).

The populated version of the 25-5274-2 circuit board used in the V1011 Pathfinder amp was nearly identical to the preamp board used in the Pacemaker. However, as the Pathfinder did not incorporate MRB, the components related to the MRB circuit were not installed. The Pathfinder circuit board was given the Thomas part number 84-18382-0.

It is also worth noting that Thomas Organ designed some unused circuit traces onto one end of the 25-5274-2 PC board. I suspect that this area of the board might have been under consideration for the future addition of a "Distortion Booster." The unused area of the 25-5274-2 PCB resembles the individual Distortion Booster circuit board (Thomas part number 25-5277-2) installed in the V1141, V1131, V1151 and V1121 preamps.

Several of these unused traces were adapted for use in the E-Tuner circuit that was added to the V1032 Cambridge Reverb in 1967.

Power Supply
The V1031/V1032 Cambridge Reverb and V1081 Berkeley power supply utilized a full wave rectifier using two diodes and a power transformer with a center tapped secondary. Primary DC smoothing was accomplished by a large, vertical 5000 uf filter capacitor. Two additional 500 uf capacitors provided additional smoothing. The main supply voltage was 33.5 VDC but two series resistors in the power supply provided additional power taps at 27 and 17 VDC.

Power Amplifier
The 18 watt power amplifier circuitry for the V1031/V1032 Cambridge Reverb and V1081 Berkeley II had two stages: driver and output.

Power Amp Driver Circuit - The driver circuit included two transistors and a driver transformer. The power amp driver transistor (Q7) was designed to increase the gain in the preamplifier audio signal. The factory replacement part number for this transistor was 86-5075-2, but Thomas often refered to this transistor simply as "G."  It was housed in a TO-3 style "top hat" case. Thomas added cooling fins to ward off heat related failures (see photo at top of page). The industry standard equivalent part number for this silicon transistor is 2N2219.

The output from the driver transistor was sent to the driver transformer (T2). The driver transformer served two roles. It acted as the phase splitter, sending one half of the audio wave to each output transistor. The driver transformer also isolated the output transistors from spikes coming from the preamp circuitry.

Power Amp Output Circuit - The power amp output stage of the V1031/32 Cambridge Reverb and V1081 Berkeley II included a pair of germanium output transistors in an OTL (output transformerless) circuit. The output transistors and the T2 driver transformer were mounted to an "L" shaped steel heat sink that was suspended beneath the chassis pan to facilitate cooling.

The commercial use of germanium transistors dates back to the mid 1950's. While germanium transistors were dominant in the design of solid state devices for the next ten years, dependability was always an issue. They were especially prone to self destruct if they became hot. Early Motorola auto radios equipped with germanium transistors were known to suffer heat related failures in cars when exposed to nothing more than the summer sun with the windows closed. Given this issue and considering that the solid state Thomas Vox amps were designed in late 1965, it seemed a curious choice to include germanium output transistors in some of their circuits. By this time, stable and dependable silicon transistors were commonly available.

I suspect that the designs for the lower power germanium based power amp stages used in these solid state Vox amps were

adaptations of the power amp circuitry used in Thomas organ consoles. The power amp circuit designs used in these organs could easily be several years old, developed prior to the arrival of silicon transistors. Incorporating these preexisting power amp circuits into the smaller U.S. Vox amp models, such as the Cambridge Reverb and Berkeley II, would save developmental time and engineering costs for Thomas.

The service manuals for the V1031/V1032 and V1081 offered 86-5089-2 as the factory replacement part number for the germanium output transistors in these amps. These internal Thomas parts numbers became meaningless after Whirpool closed the Thomas Organ parts warehouse in 1979. However, matched pairs of replacement germanium output transistors for the Cambridge Reverb and Berkeley II are still available from NTE Semiconductors under their part number NTE-121MP. Search the web to find an NTE dealer near you.

Reverb Pan
The full length, medium delay reverb pan for the V1081 Berkeley II amp head was mounted vertically to the inside front panel of the head cabinet (see photo at left). The same pan was mounted to the bottom of the V1031/32 Cambridge Reverb amp.

The Thomas replacement part number for the original two spring reverb pan was 23-5002-2. For those seeking to purchase a replacement pan for a Cambridge Reverb or Berkelely II amp, the modern Accutronics replacement part number is 4FB2A1C.


MRB
In addition to reverb and tremolo, The V1031/32 Cambridge Reverb and V1081 Berkeley II included an effect called "MRB, or "Mid Resonance Boost." MRB accented mid frequencies by passing the preamp signal through a tuned "LC" (coil/capacitor) circuit.

The MRB circuit provided in the Cambridge and Berkeley was a simplified version of the MRB circuit originally designed for the three channel Viscount, Buckingham, Royal Guardsman and Super Beatle amplifiers. MRB was included in the second, or "Brilliant" channel of these amps. A three position rotary switch on the rear control panel allowed the selection of one of three capacitor values for the tuned LC circuit. By varying the value of the capacitor in the LC circuit, the mid range boost frequency would be centered at either 450, 600 or 750 hz. The MRB effect was actuated either by a control panel rocker switch or by the remote foot pedal.

The simplified version of the MRB circuit for V1031 Cambridge

and V1081 Berkeley II had a fixed, single pole LC circuit. The MRB boost frequency was preset at ~600 hz as there was only one capacitor to "tune" the mid frequency point. The effect was actuated exclusively from the three button foot pedal included with the amplifier.


E-Tuner
Thomas added an "E-Tuner" feature to the V1032 Cambridge Reverb. Acting as an electronic "pitch pipe" of sorts, it provided a tuning reference pitch matched to the frequency of the E string of a guitar. The E-Tuner circuit was externally mounted to the bottom of the chassis pan and included an adjustable inductor, one transistor, two high stability capacitors, and several resistors. The circuit was powered by wires coming from the circuit board.

What Year Was My Cambridge Reverb or Berkeley II Built?
After looking at hundreds of Thomas amplifier serial numbers, I have come to the conclusion that the first digits appear to provide a clue to the year the amplifier was produced. Serials starting with "9" were from the first full year of production, 1966. Those starting with "I0" or "10" were produced in 1967 while those starting with "I1" or "11" were built in 1968. Following this same pattern, a serial starting "I2" or "12" indicates 1969.





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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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