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On the larger blackface and silverface Fender amps, there are two channels. The Normal channel is a pretty straight-forward preamp, and lacks the tremolo and reverb found on the Vibrato channel. It was originally intended for another guitarist, but usually doesn't get much use.
To add a little flexibility to the amp, it can be voiced differently. This mod changes it to a Fender Tweed style preamp, which uses a single tone control to roll off treble. The insertion loss is less than the treble-middle-bass tone arrangement, so the preamp is hotter than the BF preamp and distorts more easily. The Normal channel has 3 controls, but only two are needed by the tweed circuit, so a master volume can be added for more control over the amount of distortion.
The right side of this diagram shows the schematic of a BF or SF preamp, and the left side is my sketch of the layout. Not everything is shown on the layout diagram, only the parts of interest.
The existing eyelets on the board can be used for the tweed circuit after the blackface components have been removed. The original Bass tone control pot needs to be changed if the master volume control is going to be used, but the other two pots can stay. Study the differences between the two layout diagrams, and it should be obvious what needs to change
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