This 1967 Fender Princeton Reverb was in great shape, but the owner had issues with the bass “bottoming-out” at certain settings.
There are a number of ways to approach the solution to the problem, but the goal of the repair was to not change the great tone of the amp in any way, other than improving the bass.
The fix was simple and quick – change the V1 first gain stage cathode bypass capacitor from the original 25 µF to 4.7 µF.
This completely solved the problem, and the customer is happy.
The modification changes the frequency 3dB point from the original 4.3 Hz to a more reasonable 23 Hz. Humans can’t hear 4.3Hz, and the amp can’t reproduce it, so there is no sense in wasting amp power trying to amplify anything under about 20 Hz.