Date codes of Hunts capacitors in Vox AC50s
Blue Hunts electrolytic capacitors were used extensively by Triumph Electronics, the company that first assembled AC50s under contract for Vox, throughout 1964 and 1965.
A selection of values were bought in for the circuit, which required: four 25uf 25v for preamp valve cathode bypass; two 8uf 150v for the bias circuit; one 32uf 450v for preamp filtering along with two 16+16uf 450v; and two 32uf 500v as main power amp filters.
Above, a couple of details of AC50 serial number 1411, large box. In the first picture, four Hunts cathode bypass capacitors, one with the code "WST"; also, the main preamp filter caps. In the second picture, the two caps in the bias circuit, one with the date code "HWT".
The Hunts date code system
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.
Some general points
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.
Clearly one does not know how long new capacitors sat on stockroom 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.
An overview of codes in AC50s
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.
Small box, twin channel - mid 1964
1311 - HDT = 28th week of 1964.
Large box, twin channel - valve rectified - mid 1964 to early 1965
1354 - WST and HNT (?) = 15th and 28th week of 1964.
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 and HDT = 27th and 28th week of 1964.
1786 - HUT = 26th 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 1965 to 1968
Early, unknown - TIT = 43rd week of 1964.
2000s - TIT = 43rd week of 1964. (pics to come)
Early, unknown - TST = 45th week of 1964.
Early, unknown - TDT = 48th week of 1964.
Early, unknown - ?SS = probably 5th or 15th week of 1965.
3615 - TWT = 41st week of 1964.
3680 - TAT = 49th week of 1964.
3709 - YNS = 7th week of 1965.
4282 - TST and YUS = 45th week of 1964 and 6th week of 1965.