Pictures of the box of serial number 1015B and its cover posted on this page. Pictures of the cab and its cover will be available soon.
Some more on date codes of Hunts caps in AC50s. See the entries below (6th and 7th Sept.) for details on deciphering.
Legibility naturally depends on the way in which the cap was soldered in. On the whole, bias and cathode bypass caps provide more instances of date codes than the large preamp filter caps, which are held in with clamps that obscure substantial portions of the Hunts legends. But one can sometimes see relevant date code letters peeping out.
Two input, thin edged box - early 1964
1034B - YIW and WAI = 3rd week of 1961 and 19th week of 1963.
Unknown number - ITD = 48th week of 1963.
Diamond input amps - mid 1964
No Hunts date codes are visible in the pics gathered so far - generally speaking, bias caps are oriented so the Hunts name and value of the cap face out.
Large box, twin channel - last third 1964 to 1968
1411 - WST and HWT = 15th and 21st week of 1964.
1502 - IST = 35th week of 1964.
1524 - WST = 15th week of 1964.
1678 - WST (x2) = 15th week of 1964.
1852 - HDT and INT = 28th and 37th week of 1964.
An early amp in Australia - ?NT = probably HNT = 27th week of 1964.
1727 - HNT = 27th week of 1964.
1876 - HUT = 26th week of 1964.
2031 - ?NT = probably HNT = 27th week of 1964.
2069 - TIT = 43rd week of 1964.
2000s - ?TT = probably TTT = 44th week of 1964.
2620 - THT = 42nd week of 1964.
2644 - TWT = 41st week of 1964.
Solid state rectified
Early, unknown - TDT = 48th week of 1964.
Early, unknown - ?SS = probably 5th or 15th week of 1965.
4282 - TST and YUS = 45th week of 1964 and 6th week of 1965.
Further details will be added here as they come to hand. The main pages will then be updated.
Above, a couple of details of AC50 serial number 1411, large box. In the first picture, a Hunts cathode bypass capacitor with the code "WST"; in the second, one in the bias circuit with "HWT".
Using the "WHITSUNDAY" key mentioned below gives 154 and 214 = 15th week (April) of 1964, and 21st week (May) of 1964.
Clearly one does not know how long new capacitors sat on stock room shelves. Any estimation is just a guess or surmise. But in numbers of cases date codes are exceptionally useful for locking down the terminus post quem - the date after which an amp must have been made.
The proviso of course in all cases is that caps are original to the amp.
Above, a Hunts filter cap from AC50 serial number 1034B. Its twin is still in place in the amp. The date code is the three letters: "YIW".
The key to the Hunts code was worked out some years ago on a British valve radio forum with the help of a former Hunts employee.
The system is based on the word "WHITSUNDAY", chosen because it has ten letters, none of which are duplicate. W = 1, H = 2, I = 3, T = 4 and so on, through to Y = 0.
To decipher "YIW": the first two letters give the week of the year, in this case 03, ie. the 3rd week. The third letter gives the last digit of the year of manufacture, in this case "1" = 1961.
There are instances, however, when the code appears to have been given in reverse order, the first letter being the year, and the second and third the week. "ITD", found in another two input amp made a little later in 1964, translates as 348, which must mean 1963, 48th week.
Above, a Hunts cap. with the code "D YW". This is also to be deciphered year first, then week of year: "8 01" = the first week of 1968.
Above, an aerial view of the Hunts factory in south London - Earlsfield, not too far from Wimbledon. The works formerly belonged to (and were originally built for) the Columbia Gramophone Company.
Hunts was taken over by Erie in the late 1960s, probably 1969.
A page now posted with details of the chassis of serial number 1015B - two input, Mark 1, early 1964. Further pages will be added soon
Some quick pics of serial no. 1015B - amp and cab with original green covers. Bought new in 1964, and used for most of its working life in Sussex. Pots have date codes "KK" and "LK" = Nov. and Dec. 1963. Two original valves survive - an ECC83 with date code "B3E4" = fourth week of May 1963; and an EL34 with the code "B3K2" = second week of November '63. Note the serial number plate "Amplifier" rather than "Vox Amplifier".
The mains transformer has the same straight red wires for the 5 volt (not 6.3v as wrongly stated initially) feed to the rectifier valve as no. 1005B. Later amps have twisted wires. The chassis has things in common with 1005B too, not found later on. The output transformer is of the short type found in some diamond input AC50s.
Further pics to follow.
Serial no. 1982, valve rectified, made by Triumph, now registered here.
A short note on solid state rectified AC50s (Mark 3) without the brimistor.
Where evidence is available, the earliest of these amps, which were made by Triumph, have potentiometers with date codes in early 1965 - the one below has "AM" for all = January '65.
No serial number.
The schematic for the AC50 Mark 3 is dated 7th Jan. '65. The latest Triumph-made AC50 Mark 3 *without* a brimistor to have come to light definitively so far has pots with date codes of September '65 ("IM"):
The amp above (Mark III, no brimistor) was given the serial number 2115 at JMI
It seems likely that serial number 4885 with pot codes of October 1965, part of a small batch of Triumph-made units that were given numbers high in the 4000s, has no brimistor too - but as we have no shot of the upper chassis at the mo., it is difficult to be sure. See the foot of this page.
At any rate, the point to draw is that Triumph continued producing chassis *without* a brimistor more or less up to November '65 - November being the month in which the brimistor was apparently brought into the AC50's circuit by JMI. Two physical adjustments to new chassis were required as a result - (i) a large hole for pass-through wiring, and (ii) two smaller holes for screws to secure the two uprights on which the brimistor was to sit.
An original brimistor on its orangey/brown supports, pass-through hole behind.
The brimistor's introduction on the Mark 3 schematic is dated 10th November. Triumph thereafter produced very few units for Vox. The heavy lifting was done by Burndept.
Unfortunately it is not really possible at present to say whether Burndept produced chassis with brimistors earlier than November. There are certainly Burndept amps with brimistors and potentiometers with date codes of July 1965 ("GM") - but those pots could easily have been several or many months old when they were picked from the shelf for use.
The process of updating the pages on later AC50s - from 1967-1968 - is underway. Six new amps have been incorporated to date.
From serial number 7400 a couple of cosmetic changes come in: the new American logo, and the white warning plaque returns. There are also changes to the circuit: the cathode bypass capacitors for the preamp valves are gradually phased out, and the main filter caps are doubled up (as they were in the AC100 at this time). Erie resistors remain standard, but late AC50s were fitted with the makes of pots, coupling caps (yellow Dubillier) and main filter caps (red CCL) adopted for the Vox solid state amp range, also made at the Burndept Works in Erith.
Somewhere there must be a late - or at least intermediate - AC50 schematic recording these changes, not for the assemblers, but for repairmen. The Vox schematics, as we have them, were not drawn up for use on the factory floor. As it is, for some amps only the principal sheets have survived - for the AC50, the Mark 1, Mark 2 and Mark 3. But one often finds AC50s that deviate slightly. For the AC100 the picture would have been much the same were it not for the appearance of a relatively scarce AC100/2 schematic from early 1967 - toward the foot of this page.
Whether anything similar was made for the late AC50 remains to be seen. Possibly it was felt though that any repairman worth his salt, or Thomas Organ service centre in the USA, could sort out what was what using "standard" sheets. The adding of Werth surgistors (a surrogate brimistor) - now and again to valve rectified amps - were really matters of local US service centre practice, mindlessly followed in the case of AC50 mark 2s.
Some additions also on the early solid state rectified page.
13th August (2)
Further updates (still more to follow):
A series of updates:
Early solid state rectified
Further amps to come.
The page on amps with numbers in the 4000s has been recast slightly to take account of a group with a distinctive set of preamp capacitors - green TCC cathode bypass, WIMAs for the tone circuit, and large green Hunts 0.1uf coupling caps.
As a preliminary to arranging the early large-box valve-rectified AC50s in a more accessible way, an index of serial numbers, in order, and main characteristics will be posted soon.
Pictures of the band "The Amen" with a small box thick-edged AC50 Foundation Bass set in 1970 and 1971 now on the bands page (towards the foot).
A copy of an early 1960s Brimar brochure on the brimistor now available here.
A new page begun on the Celestion 18" speakers produced for Vox from early 1964 to early 1968 - at present the earliest types (1964).
Examples of later Celestion bass drivers will be added during the course of this week.
The page on bands with AC50 Mark 1s (two input and diamond input) has been overhauled.
By AC50 serial number 7400 we are into 1967. Amps have the logo adopted for the Vox solid state range, which was brought to market in April 1967.
By serial number 8168 we are well into 1968, and close to the end of JMI.
Most of the changes to the site have now been pushed through. The pages should be easier to view on mobile devices. A few tweaks to follow, but nothing of great note.
Next week, around 12 new AC50s will be added.
The general introductory page has been updated and will be adjusted further over the coming week.
There is some valid concern over who actually "designed" the AC50. The answer is JMI, but with a slight twist.
In the "Vox Story" it is said that: "JMI's design team was already fully committed in 1962 with the Continental organ and the various solid-state amp ventures, so Triumph Electronics, one of the AC30 chassis contractors, offered a 50-watt design based on cathode-biased EL34 valves. This featured two channels, each with its own tone circuit derived from the Top Boost circuit...."
Well there are unfortunately three misconceptions in that: (i) Triumph made AC4 and AC10 chassis, but not the AC30; (2) the AC50 was never cathode biased; and (3) the AC50 was of course at first single channel (in 1963 and early 1964).
Producing a cathode biased AC50 in any case would not have provided much advantage in terms of power over the AC30. A fixed bias circuit was needed.
It seems vanishingly unlikely that Denney, co-author of the "Vox Story", completely mis-remembered Triumph playing some part. Everything falls into place though if one sets the design of the fixed bias circuit at Triumph's door - i.e. the arrangement of lamp, diodes, and resistor/capacitor network.
The working prototype was made at JMI - the tone circuit certainly being the JMI Top Boost circuit - and sent to Triumph for copying, as Graham Huggett notes.
There is no doubt that Denney designed the AC80/100 himself. Cathode bias was his thing.
As a coda: you will find old talk on the internet of cathode biased AC50s actually being seen in the wild. They are unicorns, or yetis, perhaps both.
Above, a detail of the tone circuit of the AC50, and below it, the top-boost circuit of the AC30, designed in 1961 ("borrowed" by Vox from the Gibson GA-70).
The JMI top boost circuit clearly provided the model for the AC50 preamp. It is hard to believe that the latter was not created by Dick Denney at JMI (as has occasionally been claimed.)
The page on early AC50s in thin-edged boxes has been updated with new info.
The page on Triumph Electronics, makers of AC50 chassis from 1964-1967, has now been updated.
11th July (2)
A new page on an AC50 cab from 1966 with Goodmans speakers is now online here.
Some great info from Graham Huggett, who worked at Triumph from 1963 to early 1966. Graham was "chassis beater", making the chassis for AC50s on fly-presses and hand guillotine.
The prototype for the AC50 was sent from Dartford to Purley (Triumph was at 118 and 122 Brighton Road) for copying. Flat masters of the chassis were then made for production.
Around three people worked at Triumph full time. The wirers, mainly women, were part time and trained by Yvonne Andrews.
Control panels (bezels) were sent from JMI for the finished amps. Runs of around 50 chassis were made up on receipt of orders from Dartford.
Graham notes "assuming we had received the transformers from our manufacturer (took some weeks) the staff would easily wire up 15 chassis in a week." Testing was done with a meter, sine wave generator and scope, not with a guitar.
Triumph did not have boxes or serial number plates. The finished chassis were loaded into the back of Geoff Johnson's Ford Zephyr estate - Geoff Johnson was owner of Triumph - and driven by him, often at speed, to JMI.
There the AC50 chassis were given boxes and serial number plates added.
Graham had long-term loan of the prototype AC50 and used it for bass with his band for a couple of years.