Perhaps these two Showman amps were field prototypes, experimental units, transitional between the AB763 and AC563 circuit, or just a plain bad idea.
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.
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.
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.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.Also, the different component values could be due to a necessary substitution on the production line when a particular value was out of stock. 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.There is also an unconfirmed report of an early '68 Twin Reverb with the cathode bias circuit.