Vox V119 Churchill PA Head - A Look "Under the Hood"

Here's a look "under the hood" at the Vox V119 Churchill PA
head. The Churchill was the first self contained, powered mixed offered by a US guitar amplifier manufacturer.

The Churchill PA head was comprised of three separate modules; the mixer, the speaker connection panel and the power amp.

Churchill PA Mixer Module
The six channel mixer module was supported by the steel front control panel of the Churchill PA. The mixer module included the controls, VU meters and reverb circuit. The reverb pan was mounted to the floor of the cabinet, directly behind the control panel. The impedance matching transformer was also mounted to the mixer module.

Two multi pin modular plugs and an RCA audio out jack connected the mixer module to the power amp and speaker connection modules. Two additional cables connected the reverb pan to the mixer module.

A "C" shaped aluminum structure wrapped around the high gain preamp circuitry, shielding it from electronic intererence (see photo at right). This aluminum structure also provided a secure place to mount the massive impedance matching transformer. A removable cardboard panel provided access to the microphone gain circuitry.

The Churchill PA Speaker Connection Panel
The rear of the Churchill featured the speaker connection panel (see photo below). Terminals were provided for 2, 4, 8 and 16 ohm connections. A jumper wire joined the output from these screw terminals to four XLR "speaker out" jacks, wired together in parallel pairs. The two pairs of speaker jacks were then wired to "SPKRS" switch located below the terminal block. The SPKRS switch could then be set to connect the parallel pairs of jacks in either series or parallel. 25 V. and 70 V. terminals were also offered for speaker connection. 25 and 70 volt or "constant voltage" speaker systems are often found in permanent commercial sound installations such as churches and schools.

The Churchill PA Power Amplifier and Power Supply
The modular power amp used in the Churchill PA also powered the Super Beatle / Beatle series of amps and the V1182 Westminster Bass. In addition to the power amp circuit, the DC power supply for the amp was also a part of this module. See an image of the Churchill / Beatle / Westminster modular power amp module below.

The power supply circuit starts at the 120 VAC front panel rotary power switch. From the power switch, the line voltage flows through cables and a nine pin connector to the power transformer, located on the power amp module. A grounded aluminum panel shielded the mixer module from hum producing fields eminating from the transformer.

The single tap power transformer dropped the 120 VAC line voltage to about 40 VAC. A full wave diode bridge and twin 5000 uf smoothing filter capacitors provided +31 VDC and -31 VDC voltages to supply the power amp circuit. 27 VDC, 18 VDC and 14 VDC power taps were created through the use of additional filter capacitors and dropping resistors in the power supply. The 18 and 14 VDC supply voltages powered the mixer module.

The output section of the Churchill was originally designed to be an OTL (output transformerless) solid state power amp circuit rated at 120 watts RMS, 240 watts peak. The circuit was powered by four 2N3055 power transistors configured in push-pull pairs. A 2N3054 transistor was used as a driver to the four output transistors. The 2N3054 and it's bias adjustment pot were mounted to a heat sink near the audio isolation transformer (see photo below).

Unlike the Beatle and Westminster amps, which connected the output from the OTL power amp circuit directly to the speakers,

the Churchill used an impedance matching transformer between the amp and loudspeakers. This transformer was mounted to the back of the mixer chassis (see photo at top of page).

The audio isolation transformer was fitted into the output amplifier circuitry between the 2N3054 driver transistor and the 2N3055 output transistors. This transformer served two functions. It was the "phase splitter" for the power amplifier and it protected final stages of the power amplifier from DC surges caused by electronic failures in the mixer module or driver stages of the amplifier.

The Churchill power amplifier circuit was comprised of four 2N3055 output transistors arranged into two push-pull pairs. One pair of output transistors amplified the "positive" side of the audio output signal, the other pair amplified the "negative" side. The phase splitting function of the audio isolation transformer divided the audio signal from the mixer into positive and negative components before directing the signal to the push - pull pairs of output transistors.

As a transformer only allows AC currents to pass from its primary to secondary coils, the audio isolation transformer prevented any DC voltage caused by a circuit malfunction to be passed from the mixer module or 2N3054 driver circuit to the output stage oft he amplifier.

North Coast Music manufactures many replacement and restoration parts

for the V819 Vox Churchill PA Amplifier under license from Vox. These parts are
available exclusively at North Coast Music. Some are shown below.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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