Early Vox AC50 mark 2s in small boxes

Twin channel amps; brown grille cloth; grey panel

Possibly 60-80 amps issued in this format?

The schematic for the valve rectified two channel AC50 is OS/053. .

Effectively the brilliant channel followed the arrangement of the single channel of the earlier two input circuit (OS/044), but with two small changes - simply the modification of the values of two capacitors.

Above, a detail from the Vox "Precision in Sound" news flyer of late 1964. Note that the two channel AC50 for guitar is in a small box, and the AC50 for bass - the Foundation Bass - is large box.

Small box amps were issued side-by-side with large box probably for a couple of months, if not more. It is also evident that single channel amps were made and sold during this period. See the amps at the foot of the .

Valve rectified AC50s were superceded in very late 1964 / early 1965 by solid state rectified amps - on this page. The schematic for the latter is OS/072.

Besides the obvious changes to the older type of chassis, one finds in the new valve rectified twin channel amps:

  • that the mains filter capacitors (behind the main power valves on the upper chassis) are now axial rather than vertical/radial;

  • the main internal fuse in the preamp is orientated upwards rather than longways;

  • there is now a single bias pot (new in circuit);

  • and the voltage selector is sometimes link, sometimes the newer plastic domed type.

Below, pictures of surviving amps, arranged by serial number - simply for convenience. Finished chassis were sent in batches from Triumph to JMI, where they were put in boxes already fitted with serial number plates. Occasionally new chassis were put in boxes that had been made a good number of weeks before.

Triumph employees often signed the underchassis of amps in pencil. DE = Dave Earp. RW = Roger White. Both Dave and Roger signed serial no. 1449, below.

Relatively few amps were issued in this format. At first it may simply have been a means of using up empty small boxes. But whatever the true impulse, safe to say that the twin channel AC50 amp inside the small box was exactly the same as the twin channel AC50 that went into the large box, the latter being the format that eventually predominated.


Serial number 1311 - currently in the USA

The low serial number is simply a result of a new chassis being put, at JMI, into a box that obviously been made weeks before. Pot codes "GL" and "IL" = July and September 1964. Date code of the blue Hunts capacitors "HDT" = 28th week of 1964 = 6th-12th July. The mustard caps are older stock - "B64" = 2nd quarter of '64.

Signatures are "DE" = Dave Earp, and "SR" = Steve Rolfe, who set the bias at "34" volts. The two holes between the EL34s are likely to be later drillings. The amp is built to the original Mark 2 circuit, and has only one bias pot in the appointed place behind the preamp filter capacitors.

All transformers are in good fettle. At some point in the 2000s, the amp was shipped with the chassis loose in the box: grooves were worn into the wood as it moved. Thanks to Tom for pictures and to Frank for further shots and details of date codes.

Serial number 1429 - currently in the USA

Sold on ebay in 2006. Brown grille cloth; white warning plaque; and what look like the original connectors. Input jack sockets are all black. A white pair for the "normal" channel seems to have been reserved for early large box AC50s.

Serial number 1436 - currently in the USA

With an AC30 cab. Amphenol "doughnut"-style mains connector; single rectangular Cannon output socket. Warning plaque centred. "13" chalked on the back board. Pot codes are "GL" and "HL" = July and August 1964.

Probably the same amp as above

Sold on ebay in 2006. Note the pattern of damage to the serial number plate.

Serial number 1449 - currently in the USA

Brown grille cloth, grey panel, domed voltage selector. Single speaker output. Note the "20" chalked inside the box and on the back board. A nice amp.

Serial number 1485 - currently in the UK

Grille cloth of both amp and cab originally black, but now faded. Note that the cab is of the later type with "handles" let in the side. We pass over the amp's handle in silence.

Serial number unknown

Pics from an old sale. Bulgin mains input socket; dome voltage selector. Overall condition good.

Serial number unknown

Only a shot of the front of the amp. Grille cloth renewed (Rose Morris pattern).

Serial number indecipherable - currently in the USA?

Two Bulgin sockets on the back panel. Domed voltage selector. No shots of the insides unfortunately.

Serial number unknown - currently in the UK?

Black grille cloth. A replaced mains transformer, but otherwise not in bad shape. "5" chalked inside the box and on the back board.

Serial number 1506 - currently in the UK

Sold on ebay. Replaced mains transformer and choke. Condition otherwise good.

Serial number 1524 - currently in the UK

Sold on ebay in 2011. Brown cloth, grey panel. Triumph-made, as all the AC50s pictured on this page - note the highly twisted leads from the transformers. Original connectors on the back panel.

Sold on ebay again in late 2013.