Thomas Vox 35 Watt RMS Power Supply/Power Amp Module
Viscount and Buckingham Amplifiers
A Look "Under the Hood"

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Thomas Organ designed this 35 watt RMS modular power
amp in 1966 for use in their new Vox Viscount and Buckingham solid state guitar amplifiers. The Viscount and Buckingham amps would replace the British made Vox AC-30 and AC-30 Super Twin tube amplifiers in the North American market.

Power Amp Module
The chassis of the 35 power amp module used in the Viscount and Buckingham resembled those in the 60 watt Royal Guardsman and 120 watt Super Beatle amplifier. They even shared the same basic chassis. While similar in appearance, the Viscount and Buckingham power amp modules utilized germanium output transistors while the Royal Guardsman and Beatle power amps used silicon output transistors.

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 through the mid 1960s, 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 some of the lower powered U.S. Vox amp models would save developmental time and engineering costs for Thomas.

To help protect the germanium output transistors in the Viscount and Buckingham from overheating, Thomas Organ designed a heat sink for each output transistor that was twice as wide as those used in their amps powered with silicon output devices.

The availability of reasonably priced silicon power transistors forced germanium transistors into obsolescence by the end of the sixties. As a result, further production of germanium transistors essentially ended by 1970. The only germanium transistors available today are the remnants of sixties production and this supply of these transistors continues to dwindle. As a result of limited supply, replacing germanium output transistors in a Vox Viscount or Buckingham amp has become quite expensive. While a matched pair of original factory spec Delco DTG-110 germanium output transistors will be nearly impossible to locate, NTE Electronics Inc. offers a suitable substitute with their part number NTE104. A matched pair of these NTE104 output transistors for a Viscount or Buckingham will set you back nearly $70.

The Viscount and Buckingham power amplifier also incorporated an interstage transformer (see photo above). The interstage transformer isolated the preamp circuitry from the power amp circuitry and was the brain child of Thomas Organ/Vox engineer Sava Jacobson. The interstage transformer served two purposes. It served as a "phase splitter," dividing the audio waveform into positive and negative components to drive the "push-pull" power amp output circuit. Secondarily, it isolated the preamp circuit from the output amplifier with a transformer, making the power amp section virtually immune to failure from voltage spikes coming from the preamp.

A shielded cable with an RCA plug interconnected the power amp module to the audio output of the preamp section. (see photo above)

Power Supply
The DC power supply for the amplifier was also housed on the power amp chassis. The DC power supply utilized a power transformer with a 120 VAC primary and a secondary of approximately 40 VAC. A bridge of four discrete diodes provided rectification while a pair of 2500 uf 40 volt smoothing capacitors further cleaned up the AC ripple. The basic operating voltages coming from the filter caps were +31 VDC and -31 VDC. A ladder of voltage dropping resistors and additional filter capacitors provided appropriate DC voltages for the preamp section, supplied through a nine conductor modular plug connecting the power amplifier and and preamp modules (see photo above).





North Coast Music manufactures many replacement and restoration
parts
for the Vox Viscount and Buckingham under license from Vox. These parts
are available exclusively at North Coast Music. Some are shown below.




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