Dating Fender Amps By Serial Number, Part 3 Dating Fender Tube Amps by Serial Number, Part 3 by Greg Gagliano Copyright 1999, 20th Century Guitar Magazine. I know, I know. Tamil Fonts Free Download For Mac.

International Booty 5 here. It’s been over a year since Part 2 of this series was printed, but that’s because I’ve been busy collecting data. For those readers who may have joined us recently, I am doing some Fender amp research along with Devin Riebe and Greg Huntington. Our research efforts are now in their fourth year (will it ever end?). Part 1 and Part 2 of this series can be found in the March 1997 and November 1997 issues of TCG, respectively. Again, I would like to thank everyone who responded to our request for data as outlined in Parts 1 and 2.

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The main focus of this article will be speakers found in Fender amps, but before we get into that topic let’s go over a few other areas first. Early silver face amps: Part 2 went into this topic in some detail, but since that article was printed, I have been asked a lot of questions about this subject. 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. Fender never printed any since there were plenty of leftover AB763 tube charts available and these were used well into 1969. Although the cosmetics changed, the circuits remained unaltered from the blackface circuits on the earliest silverface amps.

The date of the change from the blackface circuit to the CBS silverface circuit was dependent on the model, but most of the amps that were modified received the circuit change by mid-year of 1968. The cosmetics of the silverface amps during the transition between circuits was also in transition making it difficult to determine circuit type on cosmetics alone. The amps during this period could have the earliest style silverface grill cloth or the more familiar silverface grill cloth. All of the amps would have the aluminum grill trim and they may or may not have the thin black vertical lines on the control panel. One clue that can be used is the ink stamped chassis date code that is usually located in the chassis, but is sometimes found on the underside of the chassis behind the tubes. These read something like T020366 or F034267 where the '66' and '67' denote the year and the '03' and '42' denote week of the year. Of course, the most foolproof way is to pull the chassis and look at the layout.

In fact, on the 40-watt and 80-watt amps you can simply pull the chassis out about 2 inches and look for the big honking ceramic power resistors that are connected to ground from the cathode (pin 8) of the power tubes. If those resistors are there, the amp has the dreaded CBS silverface circuit. I am also confident that the serial number can be used as a rough guide for determining the circuit, but again, pulling the chassis is the only way to confirm. Transitional circuits: I’ve been getting quite a few reports from amp geeks about circuits that don’t completely match the schematic for a particular model. Leo Fender was notorious for tweaking circuits and the results of some of his tinkering can be found on late examples of an amp prior to the switch to a new circuit.

Also, the different component values could be due to a necessary substitution on the production line when a particular value was out of stock. Ran out of 100K ohm resistors? Stick a 90K in there no one will notice or care (at least not until the mid to late 1990s).

These changes have been observed on tweed, brown/blonde, and blackface models. Often the differences are minor such as small changes in resistor or capacitor values. 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!) is for the early 1968 silverface Showman. Two examples are known to exist; one from the February-March period and one from the March-April period. These amps have AB763 tube charts and normally any silverface amp made prior to May 1968 will have the AB763 (a.k.a. Blackface) circuit.

After April 1968, most of the big Fender amps received the AC568 circuit, which is a semi-cathode biased design. However, these two Showman amps have a fully cathode biased design that is factory stock!!! There is also an unconfirmed report of an early '68 Twin Reverb with the cathode bias circuit. If you see one of these cathode biased amps, please let me know!

These amps do not have a bias trim pot. The wire from the two 220K ohm bias resistors is connected to the brass control panel ground plate. The only thing connected to the bias cap/diode is the tremolo circuit. The cathodes are tied together and connected to a single 165-ohm resistor and 80 mfd 150V bypass capacitor which are both grounded at the other end.