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 .

The new solid state rectified circuit - the AC50 Mark 3 - is set out in schematic (OS/072). For a copy, .

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 .

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 mustard 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. .

Serial number plates are generally machine stamped, but where disorderly 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brimistors (a sort of limiting resistor, standing where the rectifier valve would have been in earlier amps) eventually superceded those without, but the transition was a particularly long one. .

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 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.

NEW

Serial number in the 2000s - currently in Germany

Similar to the amp above. Solid state rectified, no brimistor. Pot codes are "IM" = September 1965. The date codes of the 0.1uf mustard capacitors are "B5N" = second quarter of 1965.

Serial number 2201 - 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.

NEW

Serial number 2216 - currently in the USA

Solid state rectified. Triumph transformers visible through the grille at top. Voltage selector removed.

NEW

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 2252 - currently in Finland

Control panel a deep aqua blue, as in the case of the amp two below.

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.

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, .

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 some 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. In excellent condition, only a few changes here and there. The carrying handle is a replacement.

Serial number unknown - currently in the UK

Vivid blue control panel as on the amp above. Condition good; and few replaced components.

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 in the 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 - see the amp immediately below, for instance. 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.

NEW

Serial number in the 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).

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.

NEW

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.

Serial number unknown - currently in the USA

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 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 . Fane speakers in the cab.

Advertised on Reverb in May 2016. Mustard caps with date codes "C4N" and "D4N" = third and last quarter of 1964.

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

NEW

Serial number 2823 - currently in the USA

Serial number 2861 - currently in the USA

NEW

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 2890 - currently in the USA

In really nice condition. Thanks to Paul for the pictures.

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.

NEW

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.