AC50 Mark 3 (Mk III)

Serial numbers in the 4000s - later 1965 to early 1966.

Again, AC50 Mark 3s continued to be assembled for Vox by and in the Vox/Burndept factory at Erith. The majority were produced by the latter. Capacity at Triumph was extremely limited.

It is worth reiterating that Triumph did not box the chassis it made. The finished units were taken to Dartford, where back panels (with serial number plates) and boxes were added in a fairly random way. Late chassis can therefore have early serial number plates and vice versa. Burndept was far more orderly.

Some of the Triumph-made AC50s have boxes with white warning plaques when the norm was red. One simply has to put anomalous amps to one side and take them case by case.

Burndept always stamped its chassis with serial numbers - on the aluminium flat of the preamp underneath the jack sockets.

These numbers do not correspond to any sequence of assembly or registration. Unpopulated chassis, already with numbers, were simply kept in piles in no order. The nearest one was grabbed when a new amp was to be made.

One can see various examples of these stamped numbers below. Triumph chassis are always unstamped.

Many chassis now have the , a heat dependent resistor, added to help delay surges when the amp was switched on. The brimistor is positioned to the left of the main power valves, in effect where the GZ34 rectifier stood in .

Note that the new chassis required three holes (where before there had been none) - two for the brimistor supports, and a large grommeted pass-through behind it to the underchassis. Brimistors were often removed by service engineers, but the tell-tale chassis holes naturally remain.

The brimistor is noted on the schematic OS/072 - for which, - as an addition of 14th October 1965. Howeever, one should not place too much reliance on absolute date. Schematics were drawn up for service engineers, not assembly.

Serial number plates are sometimes two line, sometimes three - there is a definite cross-over between the old and new type.

A small group of amps signalled below, with numbers high in the 4000s, have distinctive capacitors in their preamps: green TCCs for the preamp valve cathode bypass; WIMAs for the tone circuits; and large green Hunts 0.1uf for coupling caps.

Below, a register of amps. If anyone knows of others that have not yet been included, as ever do let me know.

Two-line serial number plates still

Serial number 4007 - currently in the UK

Unusued for 15 years, now expertly restored. One of the blue Hunts filter capacitors has the code "TIT = 43rd week of 1964. Pot codes are "AM" = January 1965. Chassis signed "DE" = Dave Earp. In company with no. 4131 below, the underchassis tagboard is of the eyelet type, rather than hook. The two amps were evidently not given boxes (with serial number plates) immediately after production. A group of similar amps with numbers in the high 2000s . Thanks to Dennis for the pictures.

Serial number 4040 - currently in the USA

The speakers are likely to have come from a different source. Note the wrong orientation of the label and terminals of the T1088 on the left.

Serial number 4061 - currently in the USA

Sold on ebay.


Serial number 4073 - currently in the USA

At (March 2020).

Serial number 4083 - currently in the USA

Assembled by Burndept - the stamped chassis number visible under the input jacks. Blue Hunts cathode bypass caps in the preamp, and Wima coupling caps.

Serial number 4094 - currently in the USA

NO BRIMISTOR. Chassis no. 780. Wima signal caps and blue Hunts electrolytics in the preamp, as serial no. 4083, above. Note the large gold-coloured TCC main filters, also as above. Thanks to Jerry for the pictures.

Serial number 4095 - currently in the USA

NO BRIMISTOR. Chassis number tantalising indistinct. Good condition all round.

Serial number 4131 - currently in the UK

Further pics

NO BRIMISTOR. The amp is Triumph made. Pot codes "AM" and "BM" = Jan. and Feb. 1965. Possibly an early box. Note the white warning plaque; the absence of a mains socket on the back panel; and the position of the serial number plate over to the right. For a similar older box with a Triumph-made AC50, see below, serial number 4918.

Serial number 4137 - currently in Norway

Serial number unknown

BRIMISTOR (subsequently removed). In view of the apparent presence of a cut-out for a rectifier valve - now occupied by a capacitor - it may be that an old Mark 2 chassis was used (and adjusted) for this amp. Serial number plate is two-line. One of the blue Hunts preamp valve bypass capacitors has the date code "YNS" = 7th week of 1965.

From this point, three-line serial number plates

Serial number 42xx - currently in Europe

Serial number in the 4000s (?) - currently in Europe

A three-line serial number plate, but no other details bearing on date. Whether the amp was accompanied this cab from the factory, and whether its T1088s are original to it, are questions. Probably not to the first.

Serial number 4232 - currently in Europe

Serial number 4249 - currently in Europe

On top at front, the shop plaque of "Musikhaus Hummel", probably of Frankfurt. The Frankfurt shop later sold an AC100 and later again a Vox Conqueror - .

Serial number 4252 - currently in Europe

NO BRIMISTOR. Three-line serial number plate, red warning plaque, no brimistor. Chassis number 1292 - so the amp was assembled by the Burndept factory. The amp is currently (Dec. 2012) presented for sale here. Thanks to Detlef for the pictures.

Serial number 4256 - currently in Europe

A nice Foundation bass rig.

Serial number 4282 - currently in the USA

NO BRIMISTOR. Chassis number 1108. A great amp, in good condition. Pot codes are "GM" = July 1965. Assembled by Burndept. Blue Hunts cathode bypass caps, and WIMA coupling caps in the preamp. Date codes on the Hunts caps are: "TST" and "YUS" = 45th week 1964 and 7th week of 1965.

Serial number 4302 - currently in the UK

Chassis no. 1375. Brimistor removed.

Serial number 4316 - currently in the UK

Serial number 4320 - currently in the UK


Serial number 4344 - currently in the USA

Original Amphenol sockets still in place.

Serial number 4414 - currently in the USA

Serial number 4417 - currently in the USA

Thanks to Gene for the note on the existence of this amp

Serial number 4498 - currently in the USA

Paired with a 1970s "Vox Sound Limited" Foundation Bass cab, ported at back. Amphenol connectors still in place on the amp.

Transformers now (certainly) have underside solder lugs

Serial number 4508 - currently in Europe?

BRIMISTOR. Assembled by Burndept - the stamped chassis number visible under the input jacks. Voltage selector set to 225V. Green TCC cathode bypass caps. Smaller coupling caps are WIMAs, the larger were evidently mustards.

Serial number in the 4000s (?) - currently in the USA

NO BRIMISTOR. Assembled by Burndept. Capacitors mounted axially on top of chassis. The transformers have solder tag terminals on their undersides.

Serial number 4513 - currently in Europe

Serial number 4533 - currently in Europe

Serial number 4540 - currently in the UK

Serial number 4553 - currently in Europe?

BRIMISTOR. A couple of WIMA coupling caps remain in the preamp, the others replaced when the amp was renovated.

Serial number 4559 - currently in Europe

Serial number 4566 - currently in the UK?

Serial number 4651 - currently in Europe?

Three-line serial number plate, but white warning plaque. The voltage selector is set to 225V.

Serial number 4677 - currently in the USA?

BRIMISTOR. The cathode bypass caps are green TCCs. Coupling caps are WIMAs and mustards. On the analogy of serial number 4756, below, probably made by Burndept. This amp is (no affiliation).

Serial number 4684 - currently in the USA?

Serial number 4698 - currently in theUK

Serial number 4702 - currently in Europe

Excellent condition

Serial number 4756 - currently in the UK

BRIMISTOR. Chassis no. 1463. Assembled by Burndept. A mixture of WIMA and mustard coupling caps. Sold on ebay in late 2014.


Serial number 4764 - currently in the UK

Currently on (March 2020).

Serial number 4774 - currently in the UK

Amphenol power connector replaced by a Bulgin. The amp retains what looks to be its original cover. Pictures originated here.

Serial number 4784 - currently in the UK

Paired with a 1970s Foundation Bass speaker cabinet.

Green TCC and Hunts capacitors

Below, a small group of amps with green TCC cathode bypass caps, WIMAs for the tone circuit, and large green Hunts 0.1uf coupling caps. All four AC50s have a brimistor and were made by Burndept.

This three-fold array of preamp caps also appears in serial numbers 5173 and 5369 , but not thereafter. Although green TCCs regularly figure in later amps, WIMA 0.1uf coupling caps are the norm.

Serial number 4799 - currently in Europe

BRIMISTOR. Excellent condition. To judge by the transformers, probably made by Burndept. Three-line serial number plate; red warning plaque. Chassis number 1392 (?). The Bulgin mains connector probably, and the 1/4 jacks certainly, are later additions. Pictures originally . Note the large green Hunts coupling caps (also found in some amps below). Smaller WIMA signal caps remain.

Serial number unknown - whereabouts unknown

BRIMISTOR. Chassis no. 1645. Assembled by Burndept. Probably reboxed. Rectifier diodes on the underchassis are of the late cylinder type (rather than "top hat". Green TCC cathode bypass caps. Signal caps for the tone controls are WIMAs, the larger dark green ones are Hunts. The box (with serial number 2300) is probably not original to this chassis.

Serial number 4841 - currently in Holland

Owned at one time by a member of the band "The Endeavours Four", based in Hindeloopen, and possibly bought new by him. Thanks to Martin for the info and pics. On the band, .

Serial number 4852 - currently in the UK

BRIMISTOR. Chassis number 1690. Great condition - retains its original cover. Inspection tag still on the underside tagboard. Green TCC cathode bypass filter caps in the preamp. Date code "WF" = June 1965. One of the TCC 4uf bias caps on the underchassis may have the code "WG" = July 1965. Caps in the tone circuit are WIMA, large ones dark green Hunts.

Serial number in the high 4000s - currently in the USA

BRIMISTOR. Serial number unknown. Chassis no. 1491. Pot codes are "HM" and "JM" = August and October 1965. Note that the preamp has green TCC cathode bypass capacitors, as in the amp above. Its smaller coupling caps are WIMAs. The large dark green ones (0.1uf) are Hunts.

Serial number in the high 4000s - currently in the USA

BRIMISTOR. Chassis 1547. The green Hunts coupling caps and cathode bypass caps can be seen in the third picture. Pot codes are "IM" and "JM" = September and October 1965.

Serial number 4878 - currently in the UK

Serial number 4885 - currently in Holland

Amp and cab set. One replaced speaker in the cab. Pot codes in the amp are "JM" = October 1965. Probably assembled by Triumph. Coupling caps are mustards - visible date codes "B5N" = second quarter of 1965. The large red Hunts cap up near the mains switch indicates that the amp was probably originally exported to the USA.

Serial number 4900 - currently in Europe

Probably a Triumph-made amp. Mustard capacitors in the preamp. The mains lead connectors straight to the amp (ie. not via a connector on the back panel). The Celestion ceramic drivers - either T1225s or T1279s - are probably not original to the cab.

Serial number 4918 - currently in Germany?

NO BRIMISTOR. A Triumph-made amp. Note the early box: white warning plaque; absence of back panel power connector; and serial number plate over to the right. The amp has no chassis number. The preamp is populated with mustard caps.

Serial number 4920 - currently in the USA

Accompanied by a Vox Sound 2x15" cab and repro trolley. Cab and amp regrilled with brown cloth. Note that on the back panel, the holes for the speaker connectors were originally designed to receive Amphenol connectors (which did not come in until mid 1965).

Serial number 4922 - currently in Europe

The same amp as below, but with its back panel returned to original? The pics below are from an old ebay auction. The cab has experienced a large amount of wear, but not the amp, which is in good condition.

Serial number 4922? - currently in Europe?

NO BRIMISTOR. Assembled by Triumph - note the tell-tale transformers, the shrouds with holes at back. A three-line serial number plate, but white warning plaque. The voltage selector is set to 225V. The serial number plate has been moved to the right to cover the hole for the mains socket.


Serial number 4958 - currently in the USA

Externally a good clean amp. The serial number plate has been moved to cover the hole left by the removal of the power socket. The mains cable is now hardwired.

Serial number 4961 - currently in the UK

Currently in the UK - not the USA as previously stated. Thanks to Alan for the info.

Serial number 4987 - whereabouts unknown

Serial number in the high 4000s or 5000s - currently in Europe

BRIMISTOR. A nice amp and cab set, complete with trolley. Voltage selector set for 245v.