LARGE BOX AC50s - SOLID STATE RECTIFIED

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, .

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), and only the uprights of the bias pot stems to their left.

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 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. See this page.

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brimistors (a sort of limiting resistor, standing where the rectifier valve would have been in earlier amps) eventually superceded those without, but the transition was a particularly long one.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)

NEW

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. The plate is part hand-stamped and therefore a relatively old one. Nothing should be read into the apparently low number.

Serial number 2201 - currently in the UK

Pretty certainly solid state rectified - only two valves are visible through the top mesh. Excellent condition. The support for the output XLRs has been renewed.

NEW

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. 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 in the late 60s. Thanks to Evert for the pictures.

Two things seem possible: either the power section failed catastrophically (though there doesn't seem to be much sign of a burn-out) and the amp was sent back to Triumph or Vox for repair, hence the Woden transformer marked "AC50"; or the amp had lain unfinished at Triumph for years (for whatever reason) and was completed as part of a process of sorting out long-overdue items. Triumph was certainly not the most orderly of companies.

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.

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Serial number unknown - whereabouts unknown

Pictured above on a cab.

Pictures from another sale.

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

A nice amp - good all round condition.

Serial number 2698 - currently in the USA

A nice rig. Pictures originally posted here. Fane speakers in the cab.


Advertised on Reverb in May 2016.

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 2861 - currently in the USA

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 mustard capacitors and lack of stamped chassis number. Sold in the US in late 2012.