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January - July 2016 (most recent first)

19th July

Further pics of an early thick-box diamond-input AC50 .

A note on the bias circuit in early AC50s:

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Click for larger images

Above, parts of the two earliest AC50 schematics that survive - OS/044 for the single channel amps (two-input and diamond-input); and OS/053 for the twin channel amps.

Very few, if any AC50s seem to have been built exactly to OS/044, however. Generally there is a second 220K resistor parallelled across the 220K specified in the schematic, bringing the total resistance at that point to 110K - pretty much in line with OS/053.

The second 220K resistor is piggy-backed across the larger white one at the top of the board

Second, as has been kindly pointed out to me, there is sometimes a third 220K underneath, and in parallel with, the 25K trim pot, limiting the effective value of the pot to 22.5K.

The purpose of these changes was evidently to increase the scope for bias adjustment in the two-input and diamond-input amps - most likely an initiative undertaken at Triumph.

The bias voltages indicated in OS/044, -32 to -35v, will therefore have been brought closer to the values later expressed in OS/053, -32v to -40v. Presumably the range -32v to -35v was felt to be too limiting, running less strong EL34s too hard.

The little neon lamp was a current and voltage regulating device, acting much as the larger lamp limiters (placed in series with mains positive line) that old radio men used to use for testing potentially defective devices.

4th July

The largest number of small-box AC50s together in one place since 1964?

3rd July

Pictures of a new small box, four-input AC50 .

2nd July - part 1

A short note on early Burndept-made AC50s

In late 1964 JMI/Vox, struggling to cope with orders, moved to increase its output of AC50s by setting up a second production line (initially of valve recitified amps) at the Burndept Electronics facility in Erith, taking some of the strain from Triumph in Croydon, a much smaller concern by comparison. Transformers were sourced from Woden, small signal caps from Wima, and larger value caps from Hunts. A register of surviving early amps .

Identifiable date codes - principally on the transformers and potentiometers - are at first of August and September 1964. Parts were presumably ordered a little way in advance. The Woden transformers were certainly specially-made (ie. they are not to be found in other makers' amps):

OUTPUT TRANSFORMER: Part no. 73631. Date code "JV" (Sept. 1964)

MAINS TRANSFORMER: Part no. 73632. Date code "JV" (Sept. 1964)

CHOKE: Part no. 73633. Date code "HV", then "JV" (Aug. and Sept. 1964)

The order placed with Woden evidently also encompassed a second batch of transformers for the AC80/100 - those, in the main with black panels, assembled at Westrex in west London. .

What clearly gave impetus to production of the AC50 at Burndept was the receipt, in late August 1964, of a substantial order from Thomas Organ in the US. Some of the info is gathered on the - entry for 30th May.

Quite whether production of AC50s at Burndept actually began in September is difficult to say for sure - on balance probably not. October seems more likely.

The earliest amps produced by the Burndept factory had serial numbers in the 2200s.

However, Burndept, which had a more extensive range of equipment than Triumph, was already producing part-machine-stamped serial number plates for Triumph-made amps.

Machine-stamped numbers first appear in the high 1700s, though Triumph still hand-stamped the "AC50". Up to that point Triumph had hand-stamped both "AC50" and serial number.

Serial no. 1360, made by Triumph. Hand-stamped.

Serial no. 1796, made by Triumph. Hand-stamped "AC50".

Serial no. 2269, made by Burndept. Machine-stamped.

A futher feature introduced at Burndept was the mains socket on the amp's back panel - a Cannon LNE-11. Triumph had run the mains lead direct from the chassis.

Serial no. 1360, made by Triumph. Hand-stamped.

Serial no. 1796, made by Triumph. Hand-stamped "AC50".

Serial no. 2269, made by Burndept. Machine-stamped.

2nd July - part 2

Pics of an amp with a serial no in the 3500s .

Serial no. 2352, solid state rectified, in a later box .

Pics of serial no. 2555, valve rectified .

Serial no. 4114 .

Entry on serial no. 4131 .

Pics of serial no. 4131, Triumph-made, solid state rectified, early box .

14th May

Goodmans Audiom 90s are likely to have been the first type of speaker adopted for Foundation Bass cabs, soon to be superceded, in early 1964, by the Celestion T1079, sprayed Vox blue. Goodmans began making the Audiom 90 in the late 1950s to accompany electric organs. The driver was initially 6ohms. By the early 1960s a 15ohm version was also available (which is the one that Vox presumably used).

Further pictures of these speakers are available on the .

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The Audiom 90 and Celestion T1079 were later replaced by the Goodmans Audiom 91 (two versions: black label = 50W; blue label = 100W).

7th May

Pics of serial nos 5192, 5692 and 5921.

It is clear now that the new style of transformer with black shrouds (said to have been made by Albion but on no good evidence) came in somewhere between serial numbers 5619 and 5692.

The likelihood is that these were sourced in the first half - perhaps first third - of 1966 by someone at the Burndept factory in Erith (the main contractor for Vox at this point) rather than someone at Triumph Electronics.

6th May

An early thin-edged AC50 that came to light earlier this year in Italy. It is probably close in date to serial no. 1005B (not that no. 1005B was the fourth or fifth amp made). .

2nd May

Pics of serial no. 1485, a small box, twin channel amp and its cab .

2nd May

Above stills from "extra" footage of the Dave Clark Five at the Tottenham Royal in 1964. The Pathe site gives the date: 4th February 1964. This coincides well with the release of "Bits and Pieces", which was the second number in the film, the first being "Glad All Over".

Interesting to see that the thin-edged AC50 in the foreground (one of four on stage) has two speaker outs, no warning plaque, and no screws along the top edge of the backboard.

The Beatles' thin-edged amps also had no warning plaque (though they differed otherwise, having only one speaker out, and two upper backboard screws).

Could it be that warning plaques were not affixed to "non-sale" amps, ie. those given to bands rather than sent out for sale?

Against that, one has to set the fact that Bill Wyman's thin-edged AC50 (pictured above as back up during the Ready Steady Go, Mod Ball, at Wembley in April 1964), does have a plaque. Click for a larger detail.

1st May

Only a small proportion of Foundation Bass cabs survive with their serial number plate intact. Many were presumably knocked off as the units were moved around. The lowest number registered so far is 1135, which probably means that the sequence began at 1000 or 1001. Whether the numbering stretched back to brown fronted cabs remains to be seen. It's possible that it only began when the black grille cloth came in.

Further pics of serial no. 2698, solid state rectified, no brimistor, made by Triumph, .

Two small box (thick edged) four-input AC50s are now known, one serial number 1532 (original black cloth confirmed by the amp's owner), the other unrecorded for the time being. Amps of this sort were evidently produced alongside newer two channel amps, certainly small box, and perhaps early large box.

Pics of serial no. 4540 .

Pics of serial no. 1743 (dome voltage selector) and its cab itemprop="url" registered on this page.

Page on .

1st May

Recently sold on ebay UK, a nice small box (thick edged) four input AC50 from the second quarter of 1964. Evidently a single owner up to the point of sale.

16th April

Above, the cab accompanying serial number 3276. Note the four screws down each side. The majority of Foundation cabs accompanying AC50s have only three. Perhaps this was simply a short-lived experiment - so far only three four-screw cabs have come to light. Foundation Bass cabs supplied with solid state amps have five screws per side.

Further pics of serial no. 1532 - small box AC50, thick edged, black grille cloth - . The amp has its original black ("plasti-leather") cover. The electronics are in excellent original condition. The cab is from late 1966/early 1967.

14th April

The amp pictured above is now known to be serial number 1532, further indicating that late small box and early large box amps (single channel and twin channel) were produced and marketed simultaneously. The usual caveats about the fairly random assigning of serial numbers to AC50s made by Triumph apply.

29th March

Vox Sound Equipment Ltd serial no. 1061. Thanks to Viktor for the pics.

20th March

Serial number 1815 (dome voltage selector).

Serial number 3490 (amp, cab and trolley) .

Further pics of serial number 4756 .

The box of serial number 5016 .

Serial number 7144, still old-style logo , and no. 7214 (?), also old-style logo .

A new page added - Triumph's own design, c. 1968/1969.

19th March

Serial number 6168 . Thanks to Steve for the pics.

7th March

Further pics of 5619, a Foundation bass rig in superb condition and complete with covers and hangtag . Thanks to Heinz for the pictures.

7th March

Pictures of serial numbers 6090 and 6680 and

A new thin-edged two-input AC50 has recently come to light. Pictures to follow soon.

Pictures of serial number 1920

6th March

Gary Thain with "Me and The Others" using a thin-edged AC50 (note the "BASS" logo) and a Foundation Bass cab in 1966. . Note higher up that page, a colour picture of the band outside the Jennings shop on Charing Cross Road.

5th March

Pictures of serial number 5173 Thanks to Gary for the pictures.

2nd March

Advert from the issue of "Beat Monthly" (forerunner of "Beat Instrumental") for August 1963 - the first advert for the Foundation Bass cabinet. A further ad (of different design) featured in the November issue. See the for pictures of surviving examples of the cab.

Note the mention of "separate amplifier cabinet provides a total output of 50 watts undistorted", suggesting that the AC50 was a reality (in some form) - perhaps close to being ready - in August. However, the Beatles, who were usually first in line for new amps, did not get theirs until late December. We do not know, at present, what went on in the intervening months. It may be, though, the design and testing process was a particular long one. An advert from January 1964 gives dimensions for the AC50 that are intriguingly different from those of production amps.

12th February

Serial number 6757 Thanks to Francesco for the pictures.

8th February

Serial number 5652

Serial number 3466

7th February

Serial number 8178

Serial number 7293

Further images of serial number 5533

Serial number 5619 Further pics of 5533 posted too.

Serial number 4750

Serial number 3969, and 3708 here.