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03-Jun-2018 23:35

This cathode set up is similar to, but simpler than, the AC568 circuit.Every other part of the circuit (power and preamp) is identical to the blackface AB763 schematic.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 hot that the tolex melted under the chassis mounting strap near the power tubes!Also, the amp eats power tubes, does not have much headroom and breaks up early.

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.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.