Vox Public Address Amplifiers
50 watt units
1966 - 1968
A Vox MC50/6 public address amplifier from 1966
From early 1964 - 1968 Triumph Electronics produced a series of Public Address units for Vox - designated "Metal Clads" (MC) as the boxes were metal (in contrast to the wooden cabinets of Vox guitar amps). They are beautifully constructed internally and pack a mean punch in terms of sound.
A page on the earliest MC50s - 1964 to 1965 - is available here. For more on Triumph Electronics see this page. Triumph also assembled AC50s for Vox from 1964 - 1965. An extensive series of amps can be viewed on the Vox AC50 website.
In 1965, JMI embarked on an expansion and revision of its range of Vox public address amplifiers. Two versions of the MC50 resulted; the four input MC50/4 (the original format), and the six input MC50/6. The former were generally all-valve; the latter had a transistor preamp and valve power section. The transistor preamps incorporated a meter and selector switch via which one could gain a read-out of the power valve bias voltage.
To judge from survivals, all-valve MC50/4s were produced alongside the MC50/6 hybrids.
A detail from the JMI pricelist of November 1965.
For the 100 watt Vox public address amps and further notes on chronology, see this page.
A page from "Beacon", July 1967, the Journal of the Royston Group of Companies - the group that owned JMI. Pictured is Cyril Windiate, secretary to Tom Jennings. The piece extols the virtues of a PA system set up by Vox.
Dick Denney with a new six input flat-fronted PA in 1964 at the Russell Hotel Trade Fair The unit on top is a "Page-Boy" office PA mixer.
A detail from a catalogue of 1965
In 1966 Triumph Electronics put into production for Vox a new set of 50W and 100W public address amplifiers, some with transistor preamps, though the power sections remained all valve. The blue/grey and white livery was applied also to the growing array of Vox Effects Units, and other devices:.
In terms of electronics, a notable feature of the new 100W PA amps is the provision of individual bias controls for the output valves, similar to those of the Vox UL 4120 and UL 7120, also assembled (and designed) by Triumph.
The PA50s retained their standard AC50 serial number plates.
Speaker outputs were either 1/4" jacks or, unusually, large Bulgin two-pin bakelite sockets. Triumph generally sourced its transformers for these amps from Drake.
The MC50/4, four channels, all valve
Black or grey hammerite case, blue/grey fascia. Four channels: three for microphone, one for music, a dedicated volume control for each. Overall treble and bass controls. In the preamp, four ECC83s; in the power section two EL34s. From 1966, the preamp of the MC50/4 was identical from an electronic standpoint to the preamp of the MC100/4. Transformers were generally made by Drake, but in some instances Woden was used. Changes to the construction of the chassis were introduced from time to time during production, but the size of the units remained the same. Different permutations of speaker output socket were provided, the principal types being large Bulgin two pin, and 1/4" jack socket.
JMI circuit diagram OS/127 is likely to correspond to this version of the MC50/4.
Serial number unknown - currently in the UK
A metal-clad 50 watt version of the 100W amps below. The preamp valves date to 1965. However, one of the pots has the code LN = December 1966; and another FN = June 1966; so the amp was probably produced for Vox early in 1967 by the Triumph works. A few repairs here and there, and fairly scuffed on the outside, but otherwise in good condition. The transformers appear to have been made by Drake and have codes that will be familiar to those who know Marshall amps: output transformer is 784-125; power transformer is 1202-112A and the choke 352-169. From the sticker on the front, the amp seems to have been used somewhere in Northern Ireland.
Serial number unknown - currently in the UK
Sold on ebay.uk in October 2013 - not in working order, but easy enough to remedy. Note the three-line serial number plate. The number is at present unknown.
Serial number unknown - currently in Canada
Rear panel arranged slightly differently from the two PA50 amps pictured above. To judge by the position of the EL34s, the internal chassis must be fairly low.
23rd April 2017. Further pictures of the amp showing the chassis and electronics. Thanks to Will. Note the valve sockets, supported on brackets at a level lower than the rest of the chassis, presumably to obtain as much clearance for the EL34s as possible. A similar solution was applied to the Beatles' 7120 amps, also made by Triumph. The output transformer of this PA, which provides a 100V line for hall or field speakers, is absolutely massive. No maker's stamp is visible on the OT and PT, but they may be Wodens. Note on the underside of the latter the varnished/waxed cloth wrapping - identical to the wrappings in the Woden transformers in AC30s, AC80/100s and so on.
Just to note, the choke certainly is Woden. There is no date code (unusual for Woden), simply the PA model number - MC/50/3, ie. a Vox MC50 mark 3.
The MC50/6, six channels, transistor preamp, valve power section
Grey hammerite cases, blue/grey fascias. These amps were developed from the MC100/6 of 1964. Six channels: five for microphones, one for music, all with dedicated volume controls. Overall volume, treble and bass controls, were arranged top left on the fascia. To their right, a multi-position switch and meter, to display power output and if desired the individual bias voltages of the two EL34s. The preamp was solid state (all transistor), and identical (certainly from 1966) to the one used for the MC100/6. The power amp was all valve. Speaker output sockets were generally large two-pin Bulgins.
Detail from the JMI brochure produced for the German market in March 1967. Although there is no "50" or "100" on the front of the amp pictured, it is probably a MC50/6. The MC100/6 has an indicator lamp above the mains switch. In the brochure the MC50/6 is priced at 1678DM, the equivalent at the time of £150 - substantially more than the price in the UK.
Serial number unknown - currently in the USA?
Whereabouts unknown.
Serial number 7597 - currently in Spain
A metered PA50 - serial number 7597. A transistor preamp. Power section 2 x EL34 with individual bias potentiometers.
The date codes on the bias pots appear to be "HN" = August 1966, pretty much in keeping with the pot codes of AC50 guitar amplifiers in the same serial number range - see this page.