Serial numbers, unfortunately, are not a reliable means of dating early AC50s, that is to say, amps made from Dec. 1963 to mid 1965.
In large part, this is a result of production practice at Triumph Electronics, which assembled a large proportion of the early amps. Finished chassis, boxes and serial number plates were brought together in a completely random fashion. There may be "patterns" here and there, but nothing that one can rely on.
All is not lost however. One can often get a good general sense of date from the type of box, whether or not the amp has a valve rectifier, and last but not least, from the two-letter date codes on the potentiometers:
Serial number 1727. Some of the pots are replacements, but the one indicated is original and bears the code "GL". "L" = 1964, and "G" = July.
The first letter is the month: "A" = January, and so on. The second letter is the year: "K" = 1963, "L" = 1964, "M" = 1965.
Potentiometers were naturally bought in batches, each batch lasting several months. But one nonetheless has a good terminus post quem ("date after which").
By early to mid 1965, Burndept Electronics, which shared a building with Vox off West Street in Erith, Kent, took over the bulk of production, and serial numbers consequently become more reliable as a means of dating - Burndept was a much more orderly place than Triumph.
It should be said, however, that the Burndept chassis numbers stamped on the aluminium upright underneath the input jacks do not run in sequence with the serial numbers:
Unpopulated chassis, already stamped, were simply picked up randomly from where they were kept.
1964 - early 1965
Common to the types of AC50 mentioned in the sections immediately below (1, 2 and 3), are valve rectification, white warning plaques, and two line serial number plates.
Valve rectified AC50 - note the three main valves from left to right, the GZ34 rectifier, and two EL34 output valves. Later AC50s did away with the rectifier.
Amps produced for Vox by Triumph Electronics do not have chassis numbers.
Thin edged boxes, single channel, copper control panels. 100 made at most. See this page.
Thicker boxes, still single channel but four inputs in diamond formation; copper panels. Around 300 of these amps were made.See this page.
Top, serial number 1524, small box; underneath it serial number 1411, large box.
The very earliest of these amps have link voltage selectors. Next comes a form of voltage selector with a slot. After that, we have the later style of dome selector:
Serial number plates
Serial no. 1360, made by Triumph. Hand-stamped.
Serial no. 1796, made by Triumph. Hand-stamped "AC50", but the serial number itself is machine stamped probably at the Burndept factory, which had a greater array of equipment than Triumph.
Serial no. 2269, made by Burndept. "AC50" and serial number machine-stamped.
1965 and on - general outline.
The biggest change in 1965 was the move to solid state rectification, a gradual change, from the beginning of the year. The detail from the schematic above has the initial date 7th January. The GZ34 valve was abandoned in favour of silicon diodes.
Early solid state rectified AC50 - only two main valves. There is no serial number plate on this amp, but the potentiometers are dated January 1965.
The brimistor - a type of temperature dependent resistor designed to delay in-rush current and voltage - was marked as addition on schematic OS/072 (14th October 1965), and first appears in amps around serial no. 4100. See this page. AC50 number 4133 has a brimistor, but no. 4282 does not. The transition took a little while.
14-10-65 Brimistor added.
The brimistor, on which see this page, was worked into the schematic on 14th November.
The brimistor in AC50 serial no. 6023 - front left by the mains transformer, and the tell-tale chassis pass-through hole behind it.
Naturally, if the AC50 has a brimistor, or has the chassis pass-through hole for one (brimistors were often removed), we have a rough terminus post quem for the amp.
6. Other indicators
After serial number 3450 or so, Burndept-made amps have round Amphenol XLR speaker connectors:
Again in Burndept-made AC50s, the red warning plaque on the backboard was introduced around serial number 3800 in late summer 1965 (after July) - ie. shortly before the brimistor. Some white plaques do persist however on amps with serial numbers in the 3900s.
Serial number 3814. This amp has no brimistor; pot codes are July 1965 - registered on this page.
Also worth mentioning, is the appearance of black-shrouded transformers at around serial number 5600 - an example above (no. 6023). See also the page dedicated to AC50 transformers.