Almost every vintage amplifier contains at least one part with a date stamped on it. Transformers, speakers, potentiometers and capacitors carry an EIA code: a manufacturer number followed by the year and the week of manufacture. The EIA has published these manufacturer codes since 1924, which makes the system the oldest continuously usable dating tool in the gear world — and the foundation on which this site dates amplifiers that have no serial tables at all.
The format
The usual shape is XXXYY(ZZ): two to four digits for the manufacturer, one or two digits for the year and one or two for the week. Codes on speakers and pots often run to six or seven digits, because the manufacturer code is three digits and the year and week are one digit each, sometimes with a trailing lot number.
Documented examples: 606-4-21 = week 21 of 1964, from Woodward-Schumacher, Fender's main transformer supplier; 606-66-36 = week 36 of 1966; 549-6221 = Midwest Coil and Transformer, week 21 of 1962; 220-27 = Jensen, week 27, year 1951 or 1961; 1376623 = CTS, week 23 of 1966; 304-6110 = Stackpole, week 10 of 1961; 285709 = Rola, week 9 of 1957.
The decade problem
When the year is a single digit, the code names a decade rather than a year. 606-4-xx reads as 1964 or 1974; Jensen's 220-27 as 1951 or 1961; a CTS pot stamped 137634 could be 1956, 1966 or 1976. The decade has to come from somewhere else — cosmetics, circuit, model era — and a code that is newer than the amplifier's cosmetics is evidence of a replacement, not of a later build. One documented case: a pot dated 1966 inside an amplifier built in 1956.
The claimed reversal after 1972
Many guides state that the year and week swap places in EIA codes after 1972. We could not confirm it. Every documented example in our sources reads year first, then week, in every era, including the 1970s and 1980s, so sources disagree with the folk rule and we do not apply it. What is documented as reversed are two different systems: the Fender chassis ink stamp of the late 1970s and early 1980s, where 1831 means week 13 of 1981 rather than week 18 of 1931, and the Celestion "Type 2" speaker codes.
Manufacturer numbers you will actually meet
Transformers: 606 Woodward-Schumacher, 831 Better Coil and Transformer, 830 Triad, 138 Stancor (Chicago-Standard), 549 Midwest Coil, 239 Merit, 394 Foster, 412 General, 418 United Transformer (UTC), 524 Triwec, 757 Grand, 776 Ogden, 892 American and 928 Acme.
Speakers: 220 Jensen, 465 Oxford, 137 CTS, 328 Utah, 285 Rola, 67 Eminence, 73 JBL, 391 Altec-Lansing, 433 Cleveland, 649 Electro-Voice, 1098 Pyle, 117 Credence, 575 Heppner, 918 Oaktron and 1279 WeberVST. On National, Valco and Supro amplifiers the number 550 is not a source code, and reading it as one invents a manufacturer that is not there.
Pots: 137 CTS, 304 Stackpole, 106 Allen-Bradley, 134 CentraLab, 140 Clarostat, 381 Bourns and 615 IRC.
Ampeg and Vox publish their own supplier lists
Ampeg documented its own component suppliers: speakers 220 Jensen, 137 CTS, 67 Eminence, 270 Quam, 285 Rola, 391 Altec and 649 Electro-Voice; capacitors 472 Pyramid and 34 Cornell-Dubilier; transformers 878 Acrosound, 138 Stancor, 682 Electrical Windings, 926 Todd Electric and 668 Standard Electric; pots 137 CTS and 304 Stackpole. Vox sources give the same two pot codes in practice: 1376623 = CTS, week 23 of 1966, and 235-7017 = Mallory, week 17 of 1970.
Why a component code is not a date for the amplifier
Every one of these codes dates a part. That part may be original, or it may have been fitted decades later; and factory stock could sit for six to twelve months before assembly. Pots and capacitors are the worst offenders: CTS pots are documented as date-stamped ahead of use by a month or two, post-dates of up to 18 months are recorded, and distributors in the 1960s returned batches of components as expired. That is why pot codes carry our lowest confidence rating and never date an amplifier on their own.
What this means in practice
Two EIA codes that agree give you a defensible era; one code gives you a part. Enter a transformer or speaker code in the decoder for the brand and you will get the part's year or range, the method used and the caveats that apply. Where to look on the hardware — transformer bells, speaker basket rims, pot cases — is covered in where the codes hide. Every manufacturer number above is attributed on our sources page, drawn mainly from the Blue Book of Guitar Values "Amp Dating Tools" PDF (bluebookofguitarvalues.com), guitarhq.com, jensentone.com, tedweber.com and the Ampeg knowledge base at ampeg.com/kb.