Some things are very obvious such as non-original or reconed speakers, non-original transformers, replaced pots, re-tweed, re-tolex, re-grill, etc.
and these changes are often disclosed and of a non-malicious nature.
Other things to look for include chasses placed in cabinets from a different year, “doctored” tube charts, non-original control plates (usually reproductions) on silverface amps, original transformer bell ends (they have correct date codes, of course) on non-original transformers, and non-original knobs (either repro or silverface knobs on blackface amps).unusual things can be found such as the empty “Pulse Adjust” hole on the rear of early ’60 brown amps, the “middle” volume control, use of tweed style grill cloth, strange non-documented transitional circuits, and changes in tolex color including the super-rare cream colored “brown” tolex that is found on some late ’60 amps. Given that people may refer to this information seeking specific production quantities of amps they are curious about, it should be pointed out that the serial numbers apply to chassis types, and not specifically to amplifier models.
only come into play when you’re trying to value the amplifier in the Blue Book for Guitar Amplifiers per the model description that is provided.
Most post tweed Fenders had trannys manufactured by Schumacher.
They used a code like this: 606 – 645 The date is in the last three digits, after the 606 – .
Fender has used only a handful of different speaker types over the years.
Here’s a list of brands along with their EIA codes.