LARGE BOX AC50s - SOLID STATE RECTIFIED - 1965
Mark 3 schematic without the brimistor.
Serial numbers generally in the 2000s
Amps produced by Triumph Electronics (Purley)
For valve rectified amps with serial numbers in the 2000s, see these pages.
The new solid state rectified circuit - the AC50 Mark 3 - is set out in schematic (OS/072). For a copy, see this page.
In some respects solid state rectification produced a simpler (and cooler-running) amp - two instead of three large valves on the upper chassis - and a reduced demand on the mains transformers.
The rectifying diodes (which replaced the GZ34 valve were positioned on the underchassis tagboard, as they were in the Vox AC80/100.
There are several ways to spot the initial version of this new type of AC50, the easiest being to look at the upper chassis - only two main valves (EL34s), as mentioned, and only the uprights of the bias pot stems beside them.
For examples of the valve rectified chassis, with three main valves, see the many examples on this page.
Two bias pots, rather than a single one for both power valves, was also a new thing, facilitating the balancing of their operating levels. One can spot the Mark 3 circuit too, it should be said, by the presence of a pair of 470K resistors by the preamp valve second to last nearest the voltage selector.
TRIUMPH-made amps are distinguishable by blueish-silver square-topped transformer shrouds, and Mullard mustard-coloured capacitors. There are no stamped chassis numbers underneath the input jacks.
BURNDEPT-made amps are distinguishable by the presence of stampedchassis numbers on the upright of the preamp underneath the input jacks, WIMA capacitors, and heavier-looking transformers with solder-tags under chassis. See this page.
Serial number plates are generally machine stamped, but where Triumph is concerned, there are evident exceptions - no. 2115 below, for instance.
AC50s with brimistors (a sort of limiting resistor, standing where the rectifier valve would have been in earlier amps) eventually superceded those without, but the transition was a particularly long one. See this page for further details about the brimistor.
In terms of component date codes, Mullard mustard capacitors in the preamp generally have "C4N" and/or "D4N", respectively the third and last quarters of 1964. Remember, these are simply the quarters in which the caps were manufactured, NOT the date of manufacture of the chassis. The blue Hunts caps generally have dates in late 1964 - see this page for notes on deciphering the codes.
Mains cables are still hard wired, i.e. there is no socket on the back panel. As with earlier amps, Triumph employees often pencilled initials on the underchassis. The "DE" that appears on two AC50s on this page is Dave Earp.
Below, a register of amps. If anyone knows of others that have not yet been included, do let me know.
Early solid state rectified AC50s (without the brimistor)
Serial number 2098 - currently in the UK
An interesting AC50 from early 1965, built solid state rectified, but on a valve rectified chassis. Note the cathode bypass capacitors with plain metal exteriors on the left-hand side of the preamp. Mullard mustard caps have date codes "C4N" (certainly) = third quarter of 1964, and possibly "D4N" = last quarter of '64. The output transformer is missing.
Serial number 2115 - currently in the UK
Produced by Triumph. Pot codes are "IM" = September 1965, so this is a relatively late non-brimistor amp. Remember that serial numbers on Triumph amps are all over the place. Mustard caps have the date code "C4N" = third quarter of 1964. The plate is part hand-stamped and therefore a relatively old one. Nothing should be read into the apparently low number.
Serial number 2201
Well used but evidently still going strong. Voltage selector, indicator lamp, and the speaker output panel on the back board all replaced. End handles now instead of the original at centre.
Serial number 2216 - currently in the USA
Solid state rectified. The plain-topped transformers used by Triumph visible through the grille at top. Voltage selector removed.
Serial number 2288 - currently in the UK
Good external order. The handle has been replaced, so too the speaker sockets on the back panel.
Serial number 2241 - currently in the UK
A replaced mains transformer (with a unit from a later AC50), but otherwise little changed. Note the black panel for the XLRs on the back panel.
Serial number 2247 - currently in the USA
Serial number 2252 - currently in Finland
Control panel a deep aqua blue, as in the case of the amp two below.
Serial number 2291 - currently in the UK
Pretty certainly solid state rectified - only two valves are visible through the top mesh. The support for the output XLRs has been renewed.
Serial number 2306 - currently in Europe
No photograph of the serial number plate. The box has Amphenol sockets for mains and speaker cabinets, so is later than the ones on this page.
Serial number 2307 - currently in Europe
Probably in Scandinavia as the voltage selector has been removed and replaced with the original Vox black and gold hang-tag. One of the Mullard mustard capacitors in the preamp has the date code "D4N" = last quarter of 1964.
NEW
Serial number 2314 - currently in Sweden
Exported to Denmark early on (as serial number 2307 above). Imported by "Europaisk Musik Import", Lille Værløse (near Copenhagen). Chassis assembled by Triumph. In the preamp, pots with the date code "AM" = January 1965. The cement resistor across the speaker terminal block has "WB" = February 1965.
Serial number 2338 - currently in Sweden
Probably exported to Sweden early on - voltage selector removed to bring the amp into conformity with local electrical legislation. Thanks to Johannes for the pictures.
Serial number 2352 - currently in Japan?
The box is certainly later - Amphenol mains connector (not connected), and red warning plaque. But the amp inside - solid state rectified - may be no. 2532, as indicated by the serial number plate.
NEW
Serial number 2397 - currently in the UK
Used for many years with a pair of Line Source 40 column speakers. The original cover survives. Thanks to Richard for the pictures.
Serial number unknown - currently in the UK
This AC50 was bought from the loans shed in Dartford shortly before the contents of that shed and others at back of the main Vox building were sold to Alan Pyne. The amp was said to have come back from loan to the Chris Barber Jazz Band. For pictures of the Barber band with an AC50 and an AC100, see this page.
The control panel of this amp is of an uncommon sort - deep turquoise-blue instead of blue/grey. For another example, see the amp immediately below. The choke has the sticker JMIX 009A; the output transformer JMID 008A, as in other Triumph-made amps. The pots in the preamp have the code "AM" = January 1965. The mustard caps have date codes "C4N" and D4N" = third and last quarter of 1964. One of the blue Hunts caps on the bias board has the date code: "TDT" = 48th week 1964. In excellent condition, only a few changes here and there. The carrying handle is a replacement.
Serial number unknown - currently in the USA
Pot codes "AM" = January 1965. Mullard mustard caps have the date codes "C4N" and "D4N". The amp may have been exported to the USA early on. At some point, a Thomas Organ service centre added a Werth Surgistor - a sort of brimistor - on the underside of the preamp overhang (next to the voltage selector). The output transformer has been replaced.
Serial number unknown - currently in the UK
Vivid blue control panel as on the amp above. Condition good; and few replaced components.
Serial number 2438 - currently in Denmark
Voltage selector replaced (to conform to Scandinavian regulations), but otherwise in good order externally.
Serial number 2464 - currently in Sweden
Triumph made. Solid state rectified, no brimistor; electronically in good condition. Voltage selector removed (to accord with Swedish regulations - SEMKO). The choke still has its label: JMI X 009A. Box and control panel need some attention, otherwise a really nice amp. Thanks to Niklas for the pictures.
Pictures above showing the amp expertly restored.
Serial number unknown - whereabouts unknown
Similar to the other amps on this page with standard blue/grey panels. Unbranded 25uf caps in the preamp. Voltage selector gone, and a few changes here and there, but otherwise in good condition. Mustard caps have the date codes "D4N" = last quarter of 1964. One of the blue Hunts capacitors has the date code: "TST" = 45th week 1964.
Serial number unknown - currently in the USA
Tell-tale Triumph transformers inside. Solid state rectified, no brimistor. Note that the box apparently has a connector on its back panel, however.
Serial number unknown - whereabouts unknown
Pictured above on a cab.
Pictures from another sale. The mustard caps have date codes "C4N" = third quarter of 1964. Note that the box has an old-style black plate on the back panel for the speaker connectors.
Serial number unknown - currently in the USA
Certainly a Triumph-made amp. The box, with its 3-line serial number plate and Amphenol XLR connectors, is probably from a later amp.
Serial number unknown - currently in Spain
Looks to be in good original order. Mustard caps in the preamp have the code C4N - third quarter of 1964. One of the blue Hunts caps has the date code TIT = 43rd week of 1964.
Serial number unknown - currently in Europe
Excellent condition, the preamp complete and original.
Serial number unknown - currently in Europe
New red jack sockets and preamp filter capacitors mounted off-chassis. Mustard capacitors have the date code: "C4N" = third quarter of 1964.
Serial number 2675 - whereabouts unknown
Sold on ebay in the mid 2000s. One large picture and three thumbnails. Judging by the type of transformer, the amp was assembled by Triumph.
Serial number 2681 - currently in the USA
Still with its shop sticker: The Metronome Music Store, Mansfield, Ohio. In the preamp, the pots have date codes "LL" = December 1964. Thanks to Andrew for the pictures.
Serial number 2687 - currently in the USA
The preamp evidently needed renovation. A two-pin socket supplied on the control panel. The choke retains its original sticker.
Serial number 2698 - currently in the USA
A nice rig. Pictures originally posted here. Fane speakers in the cab. Further, more recent pictures, below.
Advertised on Reverb in May 2016. Mustard caps with date codes "C4N" and "D4N" = third and last quarter of 1964.
Serial number 2776 - currently in the USA
Condition good. Main filter capacitors renewed in the preamp but little else. Mullard mustard capacitors in the preamp have the date codes "C4N" and "D4N" = third and last quarters of 1964. Two holes drilled in the upper chassis, probably for ventilation.
Serial number 2789 - currently in the UK
The handle replaced with a solid metal fitting, and the Cannon XLR speaker sockets replaced with jacks, but otherwise all there externally.
Serial number 2790 - currently in Europe
A Foundation Bass set. A few replaced capacitors in the preamp but otherwise all else appears original. One of the Mullard mustard capacitors has the date code "C4N" = third quarter of 1964. In company with other amps on this page, 2790's mains cable is hardwired internally. The cab now has a new 18" Fane.
Serial number 2793 - location unknown
Serial number 2795 - currently in Europe
The voltage selector is set at 225v.
Serial number 2799 - currently in Europe
Paired with a later cab.
Serial number 280x - currently in the USA
A good looking AC50. Note the extra switch on the control panel - for standby?
Serial number 2809 - currently in Europe
Serial number 2823 - currently in the USA
Serial number 2854 - currently in Europe
The patina of an underlayer showing through wear on the control panel. The body of the box has been recovered.
Serial number 2861 - currently in the USA
EYELET TAGBOARDS FOR THE BIAS CIRCUITRY
Serial number 2878 - currently in the USA
Pot codes "BL" and "AM" = December 1964 and January 1965. Visible date codes on the mustard capacitors are "C4" = third quarter of 1964. The underchassis is initialled "DE" = Dave Earp, who worked at Triumph from c. 1963-1966.
Serial number in the high 2000s (?) - currently in the USA
Produced for export to the USA. Note the Radiospares suppressor capacitor added to the right of the preamp tagboard, and the Werth surgistor (a stand-in for a brimistor) by the voltage selector. The blue Hunts capacitors have the date code: "TST" = 45th week of 1964. Pots have codes "KL" and "AM" = November 1964 and January 1965. Mustard caps have "D4N" = last quarter of 1964. The same type of eyelet underchassis tagboard is found in the amps above and below.
Serial number in the high 2000s (?) - currently in the Netherlands
Assembled by Triumph. In great condition. Pots in the preamp have the code "IM" = September 1965. No brimistor; solid state rectified. The mains transformer and power amp section (underchassis) are later, however. The transformer has the Woden code "KZ", along with "96712" and "AC50", which should mean October 1968. The underchassis tagboard with its BY127 diodes is of a type occasionally used by Triumph. The chassis is signed "DE" = Dave Earp, who left Triumph in late 1966. Thanks to Evert for the pictures.
In view of the Woden transformer marked "AC50", it seems likely that this amp was sent back to Vox or Triumph for repair.
Serial number in the high 2000s (?) - currently in the USA
In common with the amp above, this has a distinctive non-standard eyelet tagboard in its underchassis power section. No date codes are visible. The amp was signed off in pencil by Dave Earp (initials "DE" on the chassis).
Early chassis, later box (number 4007)- currently in the UK
Pot codes "AM" = January 1965. Further pictures here. This is the only amp in the group not to have a hardwired mains lead, a consequence of the later box - note the Cannon LNE-11 socket on the back panel
Early chassis, later box (number 4131)- currently in the UK
The box, bearing serial number plate 4131, is later (Amphenol XLR connectors). Pot codes "AM" and "BM" = Jan. and Feb. 1965. As above and below, an eyelet tagboard. Pictures also here
Serial number 2890 - currently in the USA
In really nice condition. Thanks to Paul for the pictures.
Serial number 2944 - currently in Europe
Some changes, but otherwise in good order.
Serial number 2984 - currently in the USA
Excellent condition. Triumph-made - note the lack of stamped chassis number. The mustard caps have date codes "C4N" = third quarter of 1964. Sold in the US in late 2012.
Serial number unknown - currently in the USA
Outline of the missing serial number plaque still visible. White warning plaque; mains cable hard-wired (ie. no power connector on the back panel). Mustard capacitors have the date code "C4" = third quarter of 1964.
Serial number in the 2000s (?) - currently in Europe
Solid state rectified, two-line serial number plate, white warning plaque, connector (now removed) on the back panel. But it could be that the amp belongs on the next page, serial numbers in the 3000s.
Serial number 2924 - currently in the UK
Sold on ebay UK in 2013. Pictures only of the outside.
