Code structure: type | type | batch/construction –  factory | year |month | week

Example: I66 Δ0K4
   I6 = ECC83,    I66 = ECC83 of construction #6
   Δ = Made in Heerlen/Holland
   0K4 = Made 197 0 in, November K,   4th week

The most important factories are:

Δ    Heerlen, Holland –> My favorite!

D    Hamburg, West Germany

B    Blackburn, England (GB)

L    Brussels, Belgium

1 2 3 6 9 Eindhoven, Holland

≠    Munich, Germany Siemens & Halske

X Y Sittard, Holland

Philips code list: PDF Download

Long or short plate ECC83?

I6= Short plate –> 15mm

mC = Long plate –> 17mm

Some says about long or short plate(forum):

” I’d generalize that long-plate 12AX7 (ECC83) are more articulate than short-plate: there’s more air and dynamic potential (less compression) to the sound through long-plates, in my experience. I’ve found bass to be generally more distinct or percussive with long-plates, even though there may not be more bass (depends on the tube). Some long-plate 12AX7 break up earlier than other 12AX7, both long- and short-plates; I don’t know if this is what you mean by “gain,” but it isn’t really the same thing. Oh yeah, the other generalization is that long-plates are more prone to microphonics.”

http://www.tube-classics.de/TC/Tubes/Valvo%20ECC83/ECC83.htm

I just at the begin to find out on my own if I hear these differences…