LARGE BOX AC50s - 1964
Valve rectified amps, part 1
Assembled by Triumph Electronics
Roy Orbison with "The Candymen" on stage at the high school gym in Eau Claire, Wisconsin: November 11, 1964. Note the AC50 and Foundation Bass cab at right.
Early large box AC50s were assembled by Triumph Electronics, Purley, for Vox/JMI. A page on Triumph is available here.
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 previous page).
A parts list was drawn up in the later sixties by the Thomas Organ company (which distributed Vox products in the USA) - a copy is available on this page.
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, see also this page.
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 JMI in Dartford - see the section on "manufacture" on this page. 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 this page. 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 small box serial number 1449.
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.
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), see this page.
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. One of the Hunts bias caps has the code "HWT" = 21st week of 1964 = May 1964; and one of the preamp cathode bypass caps "WST" = 15th week of 1964 = April '64.
For small-box AC50s with grey panels and dome voltage selectors. See this page.
Serial number 1462
Slot voltage selector - on this page.
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 - on this page.
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). One of the blue Hunts preamp cathode bypass capacitors has the code "IST" = 35th week of 1964 = August '64.
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 on this page.
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. One of the Hunts caps on the bias board has the code "HDT" = 28th week of 1964 = July '64. The main preamp filter cap on the left has "INT" = 37th week of 1964 = September '64. The one on the right has "?UT" - perhaps "IUT" = 36th week of '64.
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.