From MetroAmp Wiki
Allen Bradley manufactured electrical components for most of the 20th century. Many of which were OEM parts in USA made musical amplifiers. Read AB company history here: Allen Bradley Wikipedia page.
For guitar amp purposes, we are most interested in AB carbon composition resistors. These parts are now obsolete as AB stopped manufacture in the mid 1980's. The appeal of these parts is they sound great, like most carbon comp resistors. They hitch is that they are very unstable with regard to temperature and are prone to drift up in value and can cause noise when operated at high voltages (such as the plate resistor position in the first gain stage of many Fender amps).
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AB NOS CC
This acronym refers to Allen Bradley New Old Stock Carbon Comp rsistors.
Size and Values
AB made resistors in several sizes: 1/4W 1/2W 1W 2W.
Musical amplifiers typically use only the 1/2 watt and 1 watt varieties.
Manufacturer Part Numbers
AB part numbering is easy to interpret. A 10 digit code indicates component type, size, value and tolerance.
For example:
RC20GF104J = 100k 1/2 watt carbon comp 5% tolerance.
Here, I assume "RC" to stand for "resistor" and "carbon" (as opposed to wirewound).
"20" indicates 1/2 watt.
"GF" I'm not sure but assume it indicates consumer vs military parts.
"104" indicates 10 and 4 zeros, or 100,000 ohms.
"J" indicates 5% tolerance
Military parts are indicated in two ways: they use a 5 band color code and they use a slightly different part numbering code. For example:
RCR20G104KS = military 100k 1/2 watt carbon comp 10% tolerance.
Pictures
AB 68K 1/2 watt resistors in original 50 piece sleeve.
