Thomas Vox 120 Watt RMS Power Supply/Power Amp Module
Super Beatle, Beatle, Churchill, V1182 Westminster Amplifiers
A Look "Under the Hood"

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Thomas Organ designed this 120 watt RMS modular power
amp in 1966 for use in a number of new US Vox solid state amps they were planning to introduce. The module was incorporated into the Super Beatle, Beatle, V1182 Westminster and Churchill amplifiers. These solid state models would serve as replacements for the all tube JMI Vox AC-100 in the US market.

Power Amp Module
A shielded cable with an RCA plug connected the power amp module to the preamp module, supplying the preamp audio signal to the power amp. The signal then passed through a 50 uf decoupling capacitor, a 2N2924 NPN predriver transistor and a 2N3054 NPN driver transistor before entering an interstage transformer.

The interstage transformer was the brain child of Thomas Organ/Vox engineer Sava Jacobson and served two purposes. It served primarily as a "phase splitter," dividing the audio waveform into positive and negative conponents to drive the power amp output transistors. Secondarily, driving these transistors with a transformer made them virtually immune to failure from voltage spikes coming from the preamp.

The output from the interstage transformer was then directed to four silicon 2N3055 NPN power transistors in a push-pull OTL (output transformerless) circuit. The output transistors were mounted to large stamped aluminum heat sinks to protect them from thermal overrun. Fortunately for those making repairs, replacement 2N2924, 2N3054 and 2N3055 transistors are relatively inexpensive and are still commonly available from electronic supply houses.

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 pair of aluminum and steel plates, bolted to the side of the amp chassis, stood between the transformer and preamp section of the amplifier. These grounded plates served to electronically shield the preamplifier from hum enducing fields emanating from the transformer.

A bridge of four discrete diodes provided rectification while a pair of 5000 uf 40 volt smoothing capacitors further cleaned up the AC ripple. The basic operating voltages coming from these filter caps were +31 VDC and -31 VDC. A nine conductor modular plug connected the power amp and preamp modules. This plug supplied DC voltage from the power supply to a ladder of voltage dropping resistors to provide appropriate DC voltages for the preamp section.



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