Dating triad transformers
First use the cosmetic features to get a range of possible years.
If it's a one-digit number you need to determine the correct decade of the amp some other way.The sticker is a Black and Silver label, which contains several lines for "sign-offs" at the completion of sound and electrical tests, where applicable.The last line will have 2 letters of the alphabet either printed or written by hand. These letters are the production date code for the amplifier.Date Code: Wide panel, narrow panel, brown, blonde, and black face amps have a date code hand-stamped on the tube chart. The first letter translates to the year and the second to the month. Example: FG=July 1956, QA=January 1967, and a reissue CG=July 1992 EIA Source-Date Codes: Electronic components such as transformers, potentiometers, speakers, and some capacitors are often stampted with a date code indicating who manufactured them and when.The code follows the format: XXXYY ZZ where: XXX = a two or three (possibly four on newer amps) digit number indicating the manufacturer.