Vox Ac50, large box, serial number

Assembled by Triumph Electronics, Purley, for Vox/JMI. A page on Triumph is .

Format: twin channel; grey control panel; small underchassis tag board; large box with slider board for the chassis. Some amps have 1/4" output socket. Further details are noted below.

The schematic is OS/053. These amps were produced for a time by Triumph side-by-side with twin channel small box AC50s (see .

A parts list was drawn up in the later sixties by the Thomas Organ company (which distributed Vox products in the USA) - .

Vox Ac50, large box, serial number

Above, a detail from the Vox "Precision in Sound" news flyer of late 1964. Note that the twin channel AC50 for guitar is in a small box (accompanied however by the new style of large-box cab), and the AC50 for bass - the Foundation Bass - is large box amp.

The flyer presumably reflects the position around August 1964. Large box speaker cabs for guitar had already been seen in public in July 1964. But set-ups were always changing.

The Beatles, Isstadion, Stockholm, 27th July 1964. The amps are Vox AC80/100s, the cabs, large-box AC50s.

The Searchers at Seaton Carew, June 1964. Click for a larger image. A large-box Foundation rig is visible in the background. For further early large-box amps, .

A few characteristics of the new AC50 chassis:

  • Transformers have unpainted metal shrouds with a squarish profile (sometimes the metal appears bluish in character).

  • There is now a single bias pot (new in circuit).

  • The mains filter capacitors (behind the main power valves on the upper chassis) are axial rather than vertical/radial.

  • Underchassis leads are tightly (and skilfully) twisted.

  • Preamp capacitors are "mustards".

  • Mains cables are normally hard-wired, ie. there is no socket on the back panel.

Serial number plates are not a reliable indication of the relative dates of early two-channel AC50s. For instance, an AC50 with a serial number in the 1700s is recorded as being solid state rectified, that is to say, of an ostensibly later type. Chassis, boxes and serial number plates were brought together in a fairly random fashion by Triumph. For a rough guide to dates, see this page.

The plates on which the speaker output connectors sit are generally, in the very earliest amps, black in colour. One or two crop up in later amps, however - see . Most amps have blue/grey.

Triumph employees often initialled the underside of the chassis in pencil. "DE" is Dave Earp, and "RW" is Roger White. Both signed serial number 1411 below, and .

Slightly frustratingly, the identifying serial numbers of a number of the amps pictured below are indecipherable. Others have had their voltage selectors removed completely - but that is more likely to have happened to amps with a link selector than to those with the later (and safer) plastic dome type.

Below, a register of early amps. If anyone knows of others that have not yet been included, do let me know.

Vox Ac50, large box, serial number

Serial number 1360 - currently in the UK

Large box amp in nice condition (and its original green cover) with a very low serial number. Triumph made. Mustard capacitors have the codes "A4" (first quarter of 1964) and "B4" (second quarter '64). Part of a pot code is just visible in picture 9 - "DL" (?) = April 1964. Accompanying the amp, an 18" Foundation bass cab with brown cloth and old-style 18" Celestion driver, though in silver rather than blue. Silver speakers for guitar amps came into being in late May '64. Bass speakers followed suit. For the "alnico silver (Celestion T1088), .

Serial number 1411 - currently in the USA (?)

A really nice early large box AC50. No replaced components, the only observable changes being an added output jack socket and a few new control knobs. Chassis signed off in pencil "DE" and "RW" = Dave Earp and Roger White. The date codes of the 0.1uf mustard caps are tantalising indistinct, but probably read "B/64" = second quarter of 1964.

For small-box AC50s with grey panels and dome voltage selectors. .

Serial number 1462

Slot voltage selector - .

Serial number 1471 - currently in the USA

Brown grille cloth, white Erie resistors, mullard capacitors with code B4 = second quarter of 1964. Pot codes GL = July 1964. Thanks to Jeff for the details.

Serial number 1499

Dome voltage selector - .

Serial number 1502 - currently in the UK

Mains transformer mounted on its side, but otherwise few changes (the only notable replacement being the voltage selector).

Serial number 15xx - currently in the USA?

White warning plaque, XLR speaker connectors.

Serial number 1620 - currently in Germany

A good early AC50. The voltage selector has been removed.

Serial number 17xx - currently in the USA?

Sold on ebay c. 2009. Brown grille cloth; as above, voltage selector missing unfortunately. Pictures also .

Serial number 1852 - currently in Europe

Slot voltage selector. The serial number is machine stamped on the plate, but the designation "AC50" is hand stamped. Latest code visible on the mustard caps is B64 = second quarter of 1964. One of the pots (normal channel, treble) has the code "BL" = February 1964. The Hunts caps on the bias board have the code "MEW 111T HDT", the date yet to be established.

Serial number unknown - currently in the USA?

Apparently missing its original output transformer (unless it is a short one now remounted on top of its underchassis frame), otherwise in excellent condition. Grille cloth replaced. The mains transformer has the code "JMI M 007A". Glen Lambert has kindly provided pictures of a later amp that has the same paper notes of the transformer codes - serial number 2069, posted on . These codes are regularly found on the transformers of the earlier single channel amps - first and second generation (two input and four input).

Serial number unknown - currently in the USA?

Link voltage selector originally (see the last pic), white warning plaque, but 1/4 inch jack connectors instead of XLRs. Pictures from .

Serial number unknown - currently in Germany?

Sold on ebay in 2005 with a Foundation Bass cab. Replaced grille cloth, handle and output socket plate.

Serial number unknown - currently in the UK?

Brownish grille cloth - or perhaps black faded to brown; link voltage selector visible in the second image, set to 245V.

Serial number unknown

Black grille cloth. A replaced output transformer, and modified back panel - note the jack sockets protruding through the serial number plate. One of the mustard caps has the code "C4N" = third quarter of 1964.

Serial number unknown

Black grille cloth, as on the amp above; also mustard caps dated "C4N" = third quarter of 1964. Voltage selector pill missing.

Serial number unknown - currently in the USA?

Sold on ebay in 2005. Brown grille cloth; voltage selector missing unfortunately.

Serial number unknown

A fabulous assemblage - a portable AC50. Mains transformer replaced, and some new capacitors in the preamp, but otherwise all there. The remaining 0.1uf probably bears the date code 7/64 = July 1964.

Serial number unknown - currently in Europe?

Old images from ebay. The link voltage selector, set at 225V, is just visible in the second image.


Now in box with serial number 3527 - currently in the USA

An early chassis with link voltage selector in a later box. Note the mis-aligned screw holes on the top edge of the back panel.