Vox V1083 Berkeley III Amp Head
"A Look Under the Hood"

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The Vox V1083 Berkeley III head and the Vox Ampliphonic Orbiter Music Stand shared the same basic circuit design. The V1083 Berkeley III and Vox Orbiter debuted in Vox catalogs
and price lists published in early 1968. Both were produced by the Thomas Organ Company of Sepulveda CA under license to Jennings Musical Industries of Dartford Kent UK.

Modular Circuit Board Construction and Standardization
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 modular circuit board construction and standardization.

Thomas Organ designed a universal preamp printed circuit board (PCB) that would form the basis for a two or three channel modular preamp and control section for their more powerful amps. Given the Thomas part number 25-5222-2, it was incorporated into the Viscount, Buckingham, Royal Guardsman, Sovereign, Wesrminster and Super Beatle amplifiers.

Thomas also designed a modular printed circuit board that would be incorporated into all of the single channel solid state Thomas/Vox amps: the Pathfinder, Ampliphonic Nova, Pacemaker, Cambridge Reverb and Berkeley II. Given the Thomas part number 25-5274-2, this circuit board consolidated the preamp, tremolo, reverb and power amp driver circuits onto one circuit board.

When the decision was made in 1967 to add a second channel to the popular Berkeley II amplifier, yet another modular circuit board was developed. Given the Thomas part number 25-5319-2A (see circuit diagram above), it was incorporated into the V1083 Berkeley III, V116 Scorpion and the Vox Ampliphonic Orbiter music stand.

The steel chassis for the V1083 Berkeley III 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 (see photos above).

The steel chassis pan was the backbone of the chassis. It housed the 25-5319-2A printed circuit board assembly. It also supported the T1 power transformer, two 2500 uf main filter caps, the external portion of the E-Tuner circuit and the T3 reverb transformer.

The Berkeley III steel chassis pan was a standardized part that was also incorporated into the design of the V-1 and V1011 Vox Pathfinder, V-2, V1021 and V1022 Vox Pacemaker, V-3, V1031 and V0132 Vox Cambridge Reverb, the V-8 Berkeley Super Reverb and the V1081 Berkeley II 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 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 V1083 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.

Two Channel Preamp
The addition of a second channel to the V1083 Berkeley III made it a more formidable competitor to the Fender Tremolux and Bandmaster amplifiers popular at the time.

The "Normal" and "Brilliant" channels of the Berkeley III were voiced for separate and unique tonalities.

The full-range response of the Berkeley III Normal channel made it suitable for guitar or microphone. Reverb and tremolo were included in the Normal channel but not in the Brilliant channel.

The Brilliant channel was designed to accentuate treble tresponse. This brighter channel voicing was accomplished by C40, a .003 uf capacitor located in the circuitry of the Brilliant channel. This capacitor carved some of the low and mid frequencies out of the signal without affecting the treble. The attenuated bass and mid response allowed the MRB (Mid Resonance Boost, explained in detail below) effect to really stand out when engaged. MRB was limited to the Brilliant channel.

Power Supply
The V1083 Berkeley III power supply utilized a four diode bridge. Primary DC smoothing was accomplished by a two large, vertical 2500 uf filter capacitors (C49 and C50). Three additional capacitors provided additional smoothing (C51, C52, C53). The main supply voltage was 24 VDC but three series resistors in the power supply provided additional power taps at 22, 20 and 17 VDC.

Power Amplifier
The V1081 Berkeley II amp head was rated at only 18 watts RMS. Thomas Organ pumped up the output power on the V1083 Berkeley III to 32 watts RMS. The power amplifier circuitry for the Berkeley III had two stages: driver and output.

Power Amp Driver Circuit - The power amplifier driver circuit was primarily located on the 25-5319-2A circuit board. The power amp driver circuit also included the T2 driver transformer, mounted to the power amplifier heat sink.

The predriver transistor (Q7) was Thomas part number 86-5044-2, also known by the manufacturer as a "Green-Yellow" transistor. It was an NPN signal transistor that increased the gain coming from the preamp. Exact replacements are available from North Coast Music.

The driver transistor (Q8) was designed to further increase the gain in the preamplifier audio signal. The factory part number for the transistor was 86-5102-2, but the factory part may be replaced with an NTE-152 transistor from NTE Electronics. The GE industry standard part number for the Q8 silicon driver transistor is D28D3. A brass clamp bolted the top of transistor Q8 to the chassis and allowed the chassis to serve as a heat sink.

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 - Prior to designing the Berkeley III, Thomas Organ incorporated germanium transistors into the output stages of their Pacemaker, Pathfinder, Cambridge Reverb and Berkeley II solid state amplifiers. These germanium output transistors were susceptible to premature heat related failures. The power amp design of the Berkeley III eliminated germanium in favor of more dependable silicon output transistors. The power amp remained 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 service manuals for the V1083 offered 86-5101-2 as the factory replacement part number for the silicon output transistors. These internal Thomas parts numbers became meaningless after Whirpool closed the Thomas Organ parts warehouse in 1979. However, output transistors for the Berkeley III are still available from NTE Electronics under their part number NTE-390.

Reverb Pan
The full length, medium delay reverb pan for the V1083 Berkeley III amp head was mounted vertically to the inside front panel of the head cabinet (see photo at left). The exact same pan was used in the Berkeley II and V1031/32 Cambridge Reverb amps.

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 Berkeley III amp, the modern Accutronics replacement part number is 4FB2A1C.

In addition to reverb and tremolo, The V1083 Berkeley III 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.

MRB was 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 V1083 Berkeley III featured a simplified version of MRB. Unlike the three way MRB circuit from the larger Vox

amps, the Berkeley III used a single fixed capacitor in the LC circuit to select a non adjustable 600 hz mid frequency boost point. As in the three channel Thomas Vox amps, MRB was only included in the "Brilliant" channel. MRB was actuated exclusively from the three button foot pedal included with the amplifier.

Thomas included their "E-Tuner" feature in the V1083 Berkeley III. 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 (see the circuit layout diagram near the top of the page).

What Year Was My Berkeley III 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.

Errors in the V1083 Berkeley III Schematic
I have discovered four misnumbered components in the factory circuit board layout and chassis illustrations for the V1083 Berkeley III.

1) Resistor R29 (100 ohms) is incorrectly labeled R20 in the reverb return circuit of the factory parts layout. This error was corrected in the circuit board illustration near the top of this web page.

2) Resistor R37 (22ohms) is incorrectly labeled R27 in the power amp driver circuit of the factory parts layout. This error was corrected in the circuit board illustration near the top of this web page.

3) Resistor R53 (680 ohms) is incorrectly labeled R63 in the E-Tuner supply circuit of the factory parts layout. This error was corrected in the circuit board illustration near the top of this web page.

4) Capacitor C50 (2500 uf) is incorrectly labeled C48 in the "Chassis (Top View)" illustration.



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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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