THE SMALL BOX VOX AC50
Single channel; two inputs; copper control panel; thin-edged box
Probably around 100 made
Produced by Triumph Electronics for Vox from late 1963 to early 1964. The Beatles received two of these amps in December 1963.
Above, a detail from a shot of a Beatles Christmas Show, Finsbury Park, December 1963. Note John's new AC50 turned back to front on the step.
Triumph only produced the finished, working, chassis. The control panels were sent over from JMI. The chassis were then driven to Dartford by Geoff Johnson, owner of the business, in the back of his Ford Zephyr estate. See the recollections section on this page. At Dartford, back panels (complete with serial number plates) and boxes were supplied.
As a consequence, serial numbers are not a particularly good guide to relative date or order.
Some 15-20 chassis are said to have been made at Triumph per week. The cost to JMI (ie. what Triumph charged): around £19 each. JMI sold the amplifier for £90.
The amps were supplied separately, as part of a Foundation Bass set, and occasionally together with a cab guitar.
On the left, a standard Foundation Bass pairing. On the right, a rear view of the Beatles' equipment on stage at Versailles, January 1964. For more on the Beatles, see this page.
Some of the serial number plates designate use, the suffix "B" indicating "Bass". See numbers 1005B, 1034B and 1035B, for instance, below. Note that no thin-edged AC50s have a bass flag (unlike AC80/100s).
The schematic for the circuit is OS/044, probably of September - December 1963. The schematic of the Vox AC80/100, the amp with which the AC50 was designed, is dated September '63. The version below is redrawn (sized to print at A4).
Majestic Transformers in Poole have original JMI schemas for a normal wound, and a reverse phase wound, AC50 transformer. Reverse phase winding is said to increase the bass response.
The speaker connectors fitted by JMI were always Cannons - normally at first circular XLR-3-13s, later rectangular XLR-3-32s. The Amphenols on some amps on this page are later (recent) additions
Warning plaques are white. Large Bulgin mains connectors are standard (except in the case of the Beatles' amps, which had Cannon LNE-11s).
Few early AC50s are likely have been exported to the Thomas Organ Company for sale in the United States.
A (temporary) redrawn version of the schematic OS/044. For notes on the bias circuit, see this page. The preamp valve cathode bias caps, marked on the schematic as 25uf, were in production 32uf. Thanks to Glen for pointing this out.
On the left, a detail from a photo taken during the Miami rehearsals for the Ed Sullivan Show, February 1964; right a detail backstage of the Stones at the Empire Hall, Wembley in April 1964. Note the AC50 on the stage floor, large Bulgin mains connector, single speaker output.
Some brief notes
GENERAL. Single channel; two inputs (the second lower gain than the first); GZ34 valve rectifier; two EL34 power valves; two ECC82s and one ECC83 in the preamp; volume, treble and bass controls; output 46-50 watts.
COMPONENTS. Link voltage selector; Belling Lee panel fuseholder; Bulgin indicator lamp; Arrow switch. Main filter caps made by Hunts; red R.T.C filter capacitors in the preamp; white Erie resistors; "mustard" capacitors in the preamp signal path; Mullard BY100 diode; Welwyn cement resistors. The transformers have unpainted metal shrouds.
ASSEMBLY. Copper control panels (bezels) were sent from JMI to Triumph. The chassis were made on fly-presses and a hand guillotine at Triumph by Graham Huggett. In these early amps the bolt holes for the filter capacitor clamps on top of the chassis are always placed diagonally. Six holes were normally provided for the mains transformer fixings to allow for units that had only two (one each side at centre). Most however had four (at the corners).
Transformer leads, where they ran in pairs, were heavily twisted. Also note that, as a quirk of production, the three potentiometers in the preamp are mounted in such a way that their terminals face inwards towards the chassis, making wiring a fiddly operation. In the AC80/100, the terminals of the pots normally face outwards.
FINISHING. At JMI (Dartford Road). The mains and speaker leads, which had been left un-terminated at Triumph, were soldered onto the connectors of the back panel of the amp's box-in-waiting; the chassis and back-panel were then screwed into the box.
A register of surviving amps
Serial number 1005B - currently in Australia
Sold on ebay in mid 2014. Serial no. 1005B - so presumably supplied with a bass cab of some sort - either a Foundation bass, or perhaps a T60. Repaired by Alan Pyne, who took over the old Dartford Road Vox premises (119 Dartford Road) in the very late 60s, remaining there for two decades or so, building and repairing amps and keyboards - see this excellent thread.
The mains transformer is a replacement, probably supplied by Pyne. The Woden number 79806 is actually the part number for the mains transformer of second generation AC80/100s - but the date code (JW = October 1965) indicates that the transformer was made long after the run of AC80/100s had finished. In any case, this unit has a 5v supply for the rectifier valve (not present in the 100 watters).
Condition is great. A couple of replacements parts in the preamp, but otherwise all original.
Visible pot codes are KK and LK = November and December 1963. The font used for the warning plaque is heavier than in later amps. The mustard caps have 1963 date codes.
Serial number 1015B
Now known to exist (2017). Thanks to DB for the info.
Serial number 1018B
Now known to exist (Nov. 2016). Thanks to PCH for the info.
Serial number 1034B - currently in the UK
Original covers and cables. For a matching AC80/100 set (serial number 177), see this page. Bought from new by a band that regularly played in Hastings Pier Ballroom in the 1960s.
Amp and cab are probably the ones seen on stage with The Confederates, a sixties band, for the Hastings Pier "Big Beat" reunion in 1987.
Serial number 1035B - currently in the UK (collection: Mike Handley)
Sold on ebay in 2006. The "B" after the serial number signifies that this is a Foundation Bass model, though there is doubtless no difference in the circuitry. White Erie resistors for the most part throughout, but in company with many early AC50s, the 47K is light green and of a different type. Note that one of the original connectors remained on the back panel, the second now reinstated. The Bulgin mains connector has been reinstated too.
Sold on ebay in September 2015. Pot codes KK and LK = November and December 1963, as number 1005 above. The transformer code stickers are still in place. The chassis has the signing off initial "H" or "I" (others instances further down this page), the initial being the name of the Triumph employee who did the checking.
Serial number unknown
Popped up in Italy earlier this year (2016). Grille cloth renewed (with modern stock), output transformer replaced (some time ago), and box apparently re-covered, but otherwise in good condition. Probably an early one. In company with 1005B above, there are no mounting holes on top of the choke.
Serial number unknown
In the second picture, the back panel is resting upside down. The Amphenol XLR connector is a later addition, as they are in 1035B (above) and 1071 (below).
Serial number unknown
A handsome thin-edged amp formerly belonging to Don Butler. Note the thick feet (similar to those used later on Vox solid state amps).
Serial number unknown - currently in the UK
A two-input AC50 in a later large box. Output transformer, with original paper sticker, mounted on small stand-offs from the plinth
Serial number 1071 - currently in the USA (collection: Jim Elyea)
Pictures from Jim Elyea, Vox Amplifiers. The JMI Years (California, 2009), pp. 426-429. An AC50 in beautiful condition. Note the non-original side-ways turned mains connector on the back panel. Single speaker output. Interesting to see that the chassis has a single centrally-placed hole for attaching the back panel, yet the panel itself (in the second photo) apparently has two screws along its upper edge.
Serial number unknown - currently in the USA
Advertised by Marcus Hardy on his now-defunct "Toneheaven" site. Purchased in 1998. The silver-shrouded transformers are visible through the top grilles.
In great condition. The transformers still have their paper labels: output is JMI D008A; choke JMI X008A; and mains JMI M[007A]. Thanks to Tom for the pics..
CROSS-OVER AND LATER RE-USES
Thin-edged box, copper control panel, diamond inputs
Evidently a few new-style AC50s (with four inputs instead of two) were slipped into old thin-edged boxes by JMI:
Serial number unknown - currently in the UK
Amp in excellent condition, box in course of light restoration. As in the case of the amp above, tolexing had covered the groove for gold string at rear. The black grille cloth a later replacement. Two speaker outputs originally? White warning plaque originally centred. The four inputs on this amp are in parallel. The input sockets, as in the case of the amp with the black panel, are made by the Rendar Company. Possibly a two-holer originally converted (at factory?) into four? The pots have the code "BL" = February 1964. As in the case of other early AC50s, the treble and bass pots are linear instead of logarithmic (as specified in the schematic). Thanks to Stu for the images.
Serial number unknown - currently in the UK (collection: MH)
Another diamond input amp in a thin edged box. Evidently there was some overlap of boxes and new preamp arrangement.
A late sixties "put together" from spare Vox parts - currently in the UK
A strange one. The box, despite drilled holes, recovering, and replacement of three of the four braces at the front, is original. Black cloth a later replacement. Note on the back panel the metal grille is low down, and on the bottom of the box, inside, the grooved slot for the lip. Also "standard" is the plinth of the chassis - capacitor clamp holes set diagonally, and no "spare" holes for bolting in transformers with different fixings (later amps have 6 holes per transformer, allowing for variability of centre fixing and corner fixing). The Woden mains transformer (part no. 73631?), with date code "LU" for November 1963 (as on the choke) seems to be unique among AC50s. It is not borrowed from an AC80/100, as AC80/100s have no rectifier valve.
The aluminium preamp upright is another matter - right size and shape, but holes for the pots are poorly drilled.
The black panel is made of traffalite (traffolyte), a black/white/black sandwich, engraved to reveal the white. This appears to be a an in-house stand-in, produced when the stock of standard panels had run out. At least one other survives. Anecdotal evidence suggests that some were used to finish off amps for the Vox liquidation sale in late 1967 / early 1968.
A repro "Washington" cab made from an AC30 extension cab
Sold on ebay in 2006 as being the remains of a 2 x 12 + midax AC50 cab - but more likely a modified open-backed AC30 extension cab. Originally the baffle had struts in the form of a cross over the speaker openings. The seller cut away two pairs to reproduce the early form of a single vertical bar. The back panels are probably from some other Vox cab but with an opening newly cut for the midax. Note that the Beatles' cabs had one piece backs.
Early box, much later amp - currently in Australia
An early solid state rectified AC50 in an early thin-edged box. Despite its slightly different colour, the back panel is likely to be original.