A Vox MC100/6 from mid 1965

The Vox MC100 in the JMI catalogue of 1965

The standard JMI publicity shot of the Vox MC100/6 public address amplifier.

A general page on early Vox public address amplifiers .

Thanks to Steve, some pics of a first generation Vox MC100/6 assembled for JMI by Triumph Electronics in mid 1965 - the lone survivor from a set of four ordered by a TV Studio in the UK for its live shows. The dimensions of the amp are exactly the same as later MC100/6s - 19 3/4" wide; 9 1/2" tall; and 11 1/2" deep. The preamp and power sections are arranged on separate sub-assemblies.

General view of the power section underchassis. Component date codes: Mullard caps: "A5N" = first quarter of 1965; Hunts caps: "WWS" 11th week of '65.

The format of the power section is much as those of known later MC100/6s (from 1966 to early 1968). Below, a later example for general reference:

The power section of a MC100/6 from early 1967. In relation to the earlier amp a few differences of construction and one or two necessary changes to the circuit, otherwise identical.

Below, a detail of the upper chassis of the power section of the early MC100/6. The Radiospares capacitors have the date code "PWC" = March 1965. The phase inverter and driver valves are an ECC83 and an ECC82. In front of those, the B9A and octal sockets for the connectors to the preamp. Left at back, the mains transformer. The potentiometers one above the other are for adjusting the bias of the EL34 valves.

Detail of the early Vox MC100/6. The lamination stack of the mains transformer is: 5" (tall) x 4 3/8" (wide) x 2 15/16" (deep).

The output transformer, part code "D55 0/1". It measures (excluding the frame, lamination stack only): 5 5/16" (tall) x 4 1/2" (wide) x 2 1/8" (deep).

Below, the preamp assembly of the amp, arranged on three "decks" - underchassis, plinth, and shelf. The circuit is relative straightforward, but the wiring between levels is fearsomely complex. Prototyping must have taken weeks. The sub-assembly was identical to the one used in the Vox MC50/6.

Unlike later MC100/6s the preamp is all valve. Development/design of a transistor preamp for the model must have been underway however when this unit was being made. The MC100/6 mark 2 was presented at the Russell Hotel Trade Fair of August 1965.

Underchassis: Circuits to drive the ECC83s and microphone transformers (one of each per channel).

Plinth: ECC83s; microphone transformers; and linking circuitry for the volume controls.

Shelf: ECC83 to drive the main volume and tone circuits; input and output lines to the power section via a noval socket. An octal socket brings HT and heater voltages to the valves.

Below, the right hand side of the preamp assembly showing the microphone transformers, the ECC83 valves in their cans; and the fixing screws for the boards underchassis. Typical of Triumph practice, the insulated chassis pass-throughs - effectively junction solder points taking signals or voltages from one level to another.

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A detail of the preamp underchassis - boards for the first gain stage of the five microphone channels (an ECC83 valve for each) and associated impedance matching transformers.

A general view of the back panel.

Note the arrangement of the bias pots, aligned vertically and staggered left and right. The Cannon 4-pin power socket at right is a later addition.

The original speaker connectors were Belling Lee L1469 terminal blocks, used on most Vox PA amplifiers through to the end of 1965. One can just see the legends for the various taps - "0 - 8 - 15" above the sockets. Further below, blocks reinstated in an MC100/4 from mid 1965.

MC100/6 detail.

MC100/4 detail.

On to the page on Vox PA100s, 1966 to 1968.

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