“Dating Fender Amps by Serial Number,”20th Century Guitar Magazine, Gagliano, G., March 1997, pp 28 – 29.
“Dating Fender Amps by Serial Number, Part II,” 20th Century Guitar Magazine, Gagliano, G., November 1997, pp 46 – 48.
There still seems to be some confusion about how to distinguish between a silverface Fender amp that has the desirable AB763 circuit and one that has the less-desirable AB568/AC568 circuit.
Part of the confusion stems from the lack of any AB568 or AC568 tube charts.
I had a ’63 Concert with the 6G12-A circuit but had a completely different (factory stock) bias supply circuit than shown on the schematic.
Perhaps the most surprising transitional circuit that has been reported to me (thanks Brian! Two examples are known to exist; one from the February-March period and one from the March-April period.
If those resistors are there, the amp has the dreaded CBS silverface circuit.