Late AC100s - made by Burndept, summer 1967
On the AC100s made by Triumph Electronics for Vox in 1967, see this page.
During the summer of 1967, two significant changes were made to the AC100 at Burndept, outlined in schematic OS/167 (of 16th July 1967) which notionally supplanted the standard sheet, OS/36.
Click for a larger image.
The first was increased filtering in the power section. The two single 100uf 500v filter capacitors were replaced by pairs of capacitors rated at 200uf 350V each and connected in parallel. This had the effect of creating two capacitors of 100uf capable of standing up to 700V. As a result of this change, the AC100 was better placed to cope with higher wall voltages, especially in the USA.
Although AC100s had taps for 115v, but it was common in the 1960s for wall voltages in the US to be over 120v, which drove up the internal voltages of the amp correspondingly.
The new chassis are instantly recognisable by the four cut-outs for these capacitors. Indeed, their presence has led to the (occasional) designation of this type of AC100 as the "AC100/4". But the name is probably best avoided, as it tends to suggest four inputs (as in the case of the AC30/4, as distinct from the AC30/6).
The cutouts for the four capacitors can be seen in front of and to the right of the cutout for the mains transformer.
The second major change - in August 1967 - was the substitution of an ECC83 (12AX7) valve for the ECC82 (12AU7) in the last position in the preamp (V3) - the "phase splitter" - in other words the valve that sends one part of the music signal to one pair of output valves and the other part of the signal to the second pair.
The overall intention was evidently to increase gain significantly - all well and good with a nice Mullard, Philips or Brimar. But a poor quality or faulty valve would have accentuated any hum present - not so good if your guitar sported cheap and cheerful single coil pickups.
Chassis numbers and serial numbers (on the back panel)
Given that the new design/schematic required additional cut-outs in the chassis, chassis numbers, rather than serial plate numbers, are necessarily key here.
Chassis no. 2271 (serial no. 1971), on page 10, conforms to the standard 1965 design, with cut-outs for only two filter capacitors. Chassis number 2290, recorded and pictured below, has four. So until further information is forthcoming, our bounding range for the change-over is chassis 2271 to 2290.
In terms of serial numbers, the earliest amp known certainly to have been built to the new schematic is no. 2017, below. Serial number 2036, pictured at the foot of this page 10, conforms however to the standard ("old") schematic OS/36.
The latest AC100 serial number known definitively at present is 2104 - the amp pictured second to last on this page.
The upshot is that very few of this final type of AC100 are likely to have been produced, perhaps fewer than 100. No AC100 has yet come to light with a Vox Sound Equipment Ltd plate.
Pricelists and advertising
The AC100 was advertised twice in retail price-lists in the UK during the course of 1967: first in April, then in November:
Retail price-list from April 1967.
Presumably the issuing of a price-list marked the end of a fairly consistent period of production in the factories, during which time amps were finished off, boxed, given covers, and set aside for dispatch. The photo below shows in the foreground AC30s being given their finishing touches, and in the background stacks of them, along with Foundation Bass cabs and AC50 heads waiting to be shipped off. No point issuing a price-list if you didn't have the stock to supply.
Photo from J. Elyea, Vox Amplifiers, p. 337.
Despite the existence of the late schematic OS/167, it seems likely that production slowed drastically or stopped towards the end of 1967, coinciding more or less with the onset of troubles in Royston, the company that had the controlling interest in JMI. One would otherwise expect to find many more AC100s with 4 capacitors. Retail outlets, in the later portions of 1967 and early 1968, were probably simply selling amps that had been made months earlier. One must not forget too, that Supremes and Super Foundation Bass amps were the "lastest" thing.
Triumph Electronics also made a run of units using up boxes and chassis that had for some reason not been fed into the main production run - see the Triumph-made amps page. All of those amps conform to the standard schematic, OS/36.
LATE AC100s (Summer 1967)
Amps produced in the Burndept factory.
Serial number unknown - whereabouts unknown
Sold on ebay in Germany in 2006, the main chassis completely devoid of components and remodelled for some unknown purpose. The pairs of circular cut-outs for the capacitors are clearly visible in the last picture - to the side and fore of the cut-out for the mains transformer (left of chassis).
Serial number 2017 - currently in the USA
This amp came to light on the Plexi Palace boards in 2001 and 2003 - see the discussion here. The control panel is either a later modification, or perhaps a first attempt at prototyping a new format for the AC100. The chassis provides for four filter caps, thereby conforming to the late schematic, OS/167. Although the view is oblique, the back panel of 2017 clearly conforms to the normal arrangement - red warning plaque top left, Amphenol speaker output and mains input, and serial plate dead centre.
Serial number 2089 - currently in the Netherlands
Originally supplied by Servaas Muziekhandel in The Hague, on which, see this wonderful thread. Servaas also sold serial number 936, illustrated on this page. There are no pictures at present of the electronics of number 2089, but it is likely to conform to the late schematic (with paired filter capacitors). Note that the back panel of the box has rounded corners.
Serial numbers 2094 and 2095 - currently in Germany
Two AC100s - serial numbers 2094 and 2095 - and two AC50s from 1966 in unusual boxes. One of the AC100s, probably no. 2905, certainly conforms to the late schematic. One can see the paired capacitors through the top grille.
Serial number 2098 - currently in the USA (collection: Marc Speed)
An interesting amp. Chassis 2290. One of the four original capacitors is shown in picture 2. Note the mini-choke/transformer - part of the bias circuit - on the valve guard shield in picture 1. Resistors throughout are still Eries, but in this amp, one finds the old style of silver "top hat" rectifier diode (either Mullard or Phillips BY100) in the power section - see picture 4. In the preamp, capacitors in the signal path - coupling caps - are either solid yellow polythene affairs (picture 5, next to the tone controls), as used in the solid state amps, or bulbous red Lemcos instead of gold-coloured WIMAs.
The extra capacitance in the power section makes for excellent bass response. This amp puts out 118W at full tilt, a pretty hefty whack. In the last picture, one can see the square corners to the back panel (kindly pointed out to me by the owner), a feature that seems to come in fairly late on, presumably as a cost-saving measure.
Going solely by serial-plates, AC100 no. 1947 is the earliest on these pages to exhibit this, but no. 1980 is still transitional, with two rounded corners, and two square - see page 10.
The output transformer is a Woden, made for cathode biased amps, part number 79806 and date code "JW" = September 1965.
Serial number 2104 - currently in Italy
A really nice late AC100, its 2 x 15" speaker cabinet regrilled with Rose Morris cloth. Double pin corners; square edges to the back panel. Conforms to the late schematic. One can just make out the paired capacitors through the top grille in the sixth picture. Amphenol speaker and mains XLR still in place, also surviving the original mains cable. Pictures from this wonderful site. Thanks to Alberto for permission to post them here.
Serial number 2134 (?)- currently in the USA
Apparently no. 2134 - built on the old-style chassis with two instead of four main filter caps. The amp is in good condition, few changes.
Serial number 2274 (?) - currently in the UK
A fine-looking AC100 atop a 2 x 15 cabinet. The amp's number was reported as being 2274 - but this is likely to be the chassis number (in which case the amp is likely to have a serial number in the 1900s). Chalked inside the box, "10".