LARGE BOX AC50s - SOLID STATE RECTIFIED
late 1965 to early 1966
Serial numbers in the 4000s.
Again, AC50 Mark 3s continued to be assembled for Vox by Triumph Electronics 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 brimistor, 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 valve rectified amps.
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, see this page - 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.
Serial number 4040 - currently in the USA
Serial number 4061 - currently in the USA
Sold on ebay.
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 4095 - currently in the USA
NO BRIMISTOR. Chassis number tantalising indistinct. Good condition all round.
Serial number 4114 - currently in the UK
Serial number 4131 - currently in the UK
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 42xx - currently in Europe
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.
Serial number 4316 - currently in the UK
Serial number 4320 - currently in the UK
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.
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 4513 - 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 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 listed here (no affiliation).
Serial number 4684 - currently in the USA?
Serial number 4702 - currently in Europe
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 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.
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 on the next page, 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 posted here. 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 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 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?
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.
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.