Click for larger images
The purpose of these pages is to collect together pictures, information, and stories relating to the Vox AC50 amplifier, used by The Beatles, The Stones, and at one time or other by almost every major 1960s British band. Anyone who has seen the recently restored film of the Beatles' concert at the Washington Coliseum in February 1964 will be in no doubt as to how good these amplifiers sound.
The Washington Coliseum, 11th Feb. 1964.
The Dave Clark Five playing the Tottenham Royal in early February 1964. The band had four early AC50s
The AC50 was brought into being initially with one end in mind: supplying The Beatles with power enough to make themselves heard over legions of screaming fans. John and George were to have 50 watt amps (the AC50), and Paul an 80 watt bass unit - the AC80/100, later the Vox AC100.
The design process evidently began in the summer of 1963, and was well advanced, if not complete, by August 1963, as an advert published the issue of "Beat Monthly" attests.
Click as ever for a larger image. Note the mention in the advert of "separate amplifier cabinet provides a total output of 50 watts undistorted", indicating that the AC50 was ready or close to being ready.
Although the first schematic for the new amp - OS/044 - bears no date, it seems reasonable to assume that the circuit was pretty much settled by late September. The schematic of the Vox AC80/100, which was issued to Paul McCartney at much the same time as John and George's AC50s were issued, is dated 26/09/63.
Although 40-50 watt guitar/bass amps already existed in the form of the cathode-biased Linear/RSC L50 and indeed a number of Fender amps, Dick Denney, designer in chief at Vox, decided to follow his own path. The preamp of the AC50, essentially a reworking the top boost circuitry of the AC30, was created in-house by Denney himself.
Click for larger images. Above, a detail of the tone circuit of the AC50, and below it, the top-boost circuit of the AC30, designed in 1961 ("borrowed" by Vox from the Gibson GA-70).
The power amp, employing a valve rectifier (a GZ34) and two EL34s operating with fixed bias was probably designed at JMI, with input from Triumph Electronics. It is safe to say that despite the occasional reported sighting, no cathode biased version of the AC50 has ever come to light. The circuit diagram as published (OS/044) is fixed.
Manufacture was contracted out to Triumph Electronics, which was based in Purley, near Croydon, in late 1963 - see this page for a brief overview of the premises. Triumph's owner was Geoff Johnson.
Graham Huggett, who worked at Triumph from 1963 to early 1966 as "chassis beater", made the chassis for AC50s on fly-presses and hand guillotine.
The prototype for the AC50 was sent from Dartford to Purley (Triumph was at 118 and 122 Brighton Road) for copying. Flat masters of the chassis were then made for production.
Around three people worked at Triumph full time. The wirers, mainly women, were part time and trained by Yvonne Andrews.
Control panels (bezels) were sent from JMI for the finished amps. Runs of around 50 chassis were made up on receipt of orders from Dartford.
Graham notes "assuming we had received the transformers from our manufacturer (took some weeks) the staff would easily wire up 15 chassis in a week." Testing was done with a meter, sine wave generator and scope, not with a guitar.
Triumph did not have boxes or serial number plates. The finished chassis were loaded into the back of Geoff Johnson's Ford Zephyr estate - Geoff Johnson was owner of Triumph - and driven by him, often at speed, to JMI.
There the AC50 chassis were given boxes and serial number plates added.
Photos indicate that The Beatles received their new amps in the last week of December 1963.
Left, The Beatles at the Wimbledon Palais, 14th Dec. 1963, with their standard set up: two AC30s and an AC30 head for Paul's T60 bass cab. These amps were evidently used at the Gaumont, Bradford, on 21st December too. See the pic on the Savage Young Beatles site. Right, The Beatles on stage at the Astoria, Finsbury Park, last week of December 1963 - the first shots of the AC50s. Note that John has his "back-to-front" on the riser steps (to allow for the connectors sticking out).
A growing selection of adverts and promotional literature on the AC50 is available on this page.
THE EARLIEST AC50s
The AC50 mark 1 (first half of 1964) - schematic OS/044
The earliest AC50s: single channel, two inputs, a copper control panel, and a wooden cabinet with thin edges (3/8" baltic ply). Only around one hundred or so of these were made. After a few months, probably around March/April 1964, the design was changed slightly: two more inputs were introduced, arranged in a diamond formation, and the wooden boxes were made thicker (3/4" ply). Around 300 amps were made in this new format.
Pictures 1 and 2, an early thin-edged two-input AC50. Pictures 3 and 4, a later "diamond-input" amp. Note in the last pic, the two EL34 power valves, and to their right, the smaller and more squat GZ34 rectifier.
The first cabinets were the size of an AC30 but with a full valance front (replicating the arrangement of certain AC30 extension cabs:
Top, The Beatles, Olympia, Paris, Jan. 15th 1964. Immediately above, the full cloth front style also adopted for some AC30 cabs, advertised in 1965, and carried through to 1966/67 for numbers of later "Super Twins", as below (note the new-style plastic logo, centred as in the solid state cabs).
Concerns about the power handling of the Celestion blue speakers (15W each) and the AC50's treble response, however, prompted the inclusion of a Goodmans Midax mid-range horn, accommodated by the ingenious means of a cut-out in the one-piece back board. The Midax, rated at 25W - and thus taking some of the strain from the Celestions - was protected from the deeper bass frequencies, which would have blown it in quick order, by a crossover capacitor.
A still from 'A Hard Day's Night' showing the back of the AC50 cab.
Above, the Washington Coliseum concert.
Very few of these small-box full cloth front AC50 cabs were made. The photographic record to date attests only to use by The Beatles. The Dave Clark Five evidently used AC30 extension cabs.
AC50s for Bass
From early on the AC50 was also envisaged as a bass amp, and could be paired either with T60 cabs or the new 18" Foundation Bass unit.
For further images of bass cabs supplied with AC50s, see the Foundation Bass page.
The AC50 mark 2
Mid 1964 - early 1965.
Twin channel AC50, valve rectified, "large box". Schematic OS/053.
In the early summer of 1964 two developments took place: first the introduction of a new, two channel AC50 with a grey control panel, though still in a small box; then, soon after, the introduction of a larger cabinet with a sliding tray for the amp (the "classic" AC50 arrangement). Around 1700 large box valve rectified amps were produced.
Above, a two channel amp in a small box (serial no. 1436); below, a large box AC50 (serial no. 1678), the amp on a slider board inside the cab, the space below for the plug and cables.
Serial number 1411. Standard chassis. Three main power valves (the smaller one on the left is the GZ34 rectifier valve.
Early instances of large box AC50s
First image, The Searchers playing at Seaton Carew in June 1964, Foundation Bass cab and large box AC50 centre background. The second two images, Swedish tour, Summer 1964 - see this page for further details.
Left, a picture of the prizes offered by Granada TV in September 1964 for a mini battle of the bands contest. Note that the A50s are large box, but the trolleys still have the "old fashioned" baskets in their tops, and a single bar extending up from the pivot, instead of parallel bars all the way round. The middle picture indicates, however, that the small box AC50 was still on offer in September 1964 too (four inputs, single channel). Right, a later brochure describing the AC50 as a two-channel amp, but still using a similar drawing. The small box amp is for the "Super Twin", the large box the "Foundation Bass".
Left and centre, Roy Orbison in the High School gym at Eau Claire, Wisconsin, November 11th 1964. Pictures from this thread. Right, "The Hudsons", a Dutch band, with an AC50 and Foundation Bass cab. with BASS logo, originally published here.
Left, Dick Denney at the Russell Hotel Trade Fair, 1964, thin-edged AC50 to his left - note the large-box AC50 on the shelf in the background. Right, Denney at the Frankfurt Fair, 1964, large box AC50 beside him. Pictures from J. Elyea, Vox Amplifiers, pp. 77-78.
The AC50 mark 3
Early 1965 to end of production.
Twin channel AC50, solid state rectified, "large box". Schematic OS/072.
A typical early Mark 3 chassis - solid state rectified. Note the presence only of the two main power valves. Compare with the valve rectified chassis a little way up on this page.
The Mark 3 (III) is by far the most numerous type of AC50. Around 5000 amps were produced with solid state rectification in the JMI era (ie. up to the point when Vox/JMI folded in 1968). A good number were produced thereafter too, right through to 1976, with only minor differences from the principal schematic of 1965.
A few of these amps were fitted with reverb - an add on unit - on which see this page. But for the most part production was consistent and pretty unvarying. Why change a good formula? AC50s were fantastic workhorses.
Above, serial number 4018, solid state rectified, no brimistor = mid 1965. Note the reverb unit fitted on the shelf. Several of these AC50s survive.
Bill Haley and the Comets on stage in 1966 with two AC50s visible (and an AC30).