Vox AC30: component date codes
The Woden output transformer of serial number 6097. "66310" is the Jennings part number (for re-ordering). The transformers were specially made for JMI and do not figure in amps made by other makers. The date code is "JT" = September 1962.
In 1960s Britain, electrical component manufacturers almost without fail printed the date of manufacture of their components (transformers, capacitors, wirewound resistors) in the form of a code. Normally this was of two letters: one the year, the other the month.
The reason for using a code was twofold: (1) purely administrative - accounting, tracking production and so on; and (2) providing repairmen with the means to work out how old components, particularly electrolytic capacitors, were. Electrolytic can capacitors by and large have a finite life. The date code was the key.
The standard British year code was: "R" = 1960, "S" = 1961, "T" = 1962, "U" = 1963, "V" = 1964, and so on.
Month of manufacture was normally either given as A-L where "L" was December; or A-H and J-M (missing out the "I"), where "J" was September and "M" December. This last, used by transformer manufacturer Woden among others, was probably most common. Hunts, an important supplier of capacitors, also had its own system, which we will come to.
Woden transformers: first letter = month; second letter = year.
Early Radiospares capacitors: as above, first letter = month; second letter = year.
TCC and Dubilier capacitors : first letter = year; second letter = month.
Green Welwyn wirewound resistors : as above, first letter = year; second letter = month.
Above, the main filter capacitor (made by TCC) in an AC30/6 from 1961. The date code (following TCB/) is "SC" = March 1961. The capacitor is original to the chassis.
The Welwyn date code "TL" (= November 1962) on the cathode bias resistor of an AC30 from early 1963 visible in the mirror. The power rating of the resistor is marked as being 4-5 watts. Only wirewound resistors have date codes.
Egen, which provided thousand upon thousand of potentiometers for JMI, had A-L for month, but the years were: "H" = 1960, "I" = 1961, "J" = 1962, "K" = 1963, "L" = 1964
Egen pots, far right "AK" visible = January 1963. On many pots the code is difficult to see, directly facing the chassis.
The key to the Hunts date code system was worked out some years ago on a British valve radio forum with the help of a former Hunts employee. See also the notes on the Vox AC50 website.
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.
Normally the code runs from left to right: the first two elements are the week number. The third element is the year. The code below is "SWH" = 512 = 51st week of 1962.
Hunts bypass cap. date code "SWH" = 51st week of 1962.
Main preamp filter cap, Hunts date code "YTI" = 043 = 4th week of 1963.
MULLARD "MUSTARD" CAPACITORS
Mullard capacitors were used occasionally in late 1963, more frequently from mid 1964. The code is very simple. The year, indicated either by a single or double digit - "4" or "64" - was normally divided into quarters "A", "B", "C" and "D". Some batches included a third element - a letter indicating the producing factory, almost always "N" = Blackburn.
The two formats were: "A4N = first quarter of 1964, Blackburn factory; and "A/64" = first quarter of 1964.
From time to time, the quarter was indicated by a number - "14N" = first quarter of 1964, Blackburn factory. But instances are fairly rare.