You can see in the Amp Books calculator, above, what the change does. The original Fender design tries to amplify everything from 10 Hz. Well, humans can't hear 10 Hz, and a guitar in normal tuning can't produce anything below 82.41 Hz.
With the cathode capacitor change to 4.7 µF (plus 1 µF for the Tantalum), the amp isn't spending energy to reproduce frequencies that we can't hear and don't need. After the change, we don't see the full gain of the stage until about 80 Hz - before the change we were seeing the same gain at about 20 Hz.
Fred auditioned the change and the bass "flabbiness" is gone. He seems to be happy with the change to his great Princeton Reverb.
This beautiful 1967 Fender Princeton Reverb belongs to Fred Shuman, owner of Durdel's Music, in Toledo, Ohio. Fred is one of the best guitar players I know. This is his favorite amp. The problem Fred was having was the 10" speaker was "farting out" on certain bass notes at certain volume levels. He asked me to take a look at it.
Here the signal into the amp is 84 Hz, which is about the low "E" on a guitar. The distortion here is a bit odd: there is a little peak at the top of the sine wave, and the clipped top is slanted, as with a phase shift.
This may be a capacitive effect, or the Cathodyne distorting, or harmonics, or blocking distortion, or something else, but only the bass frequencies do this, not the treble.
Here I am running a 104 mV, 600 Hz signal (guitar mid-range) into the input of the amp, and looking at the speaker output over a dummy load.
Cathode follower distortion: It was been called the "penultimate" distortion - the positive half of the signal clips first, and the negative half much later. This is typical cathode follower distortion, which sounds creamy and smooth and wonderful. Many "boutique" amp makers try to achieve this in their circuitry.
The Princeton Reverb uses a cathodyne phase inverter which is part of what makes the amp sound so good. Here the amp volume control is at 4.5, and we can see the cathodyne distortion; the top peak is clipping and the bottom peak is just starting to clip. What a great amp!
The amp had been modified over the years. All of the electrolytic capacitors have been replaced, and most of the original blue "Paktron" tubular capacitors. The power transformer had been replaced, and the amp was running a Mullard GZ34 rectifier instead of the stock 5U4GB.
The Fender standard 25 µF cathode bypass capacitors are themselves bypassed by Tantalum 1µF capacitors. This may have been an attempt to improve the bass and treble response of the tube. This is often done in Hi-Fi applications. It attempts to correct the "slow" behavior of the electrolytic cap - when the eCap gets bogged down, the "fast" Tantalum cap takes over. There. How's that for a technical description.
The speaker is original to the amp, and is a Jensen C10P. (Fender also used Jensen C10NS speakers in the Princeton Reverb for 2 years only, 1966 and 1967.)
One way to tame the bass behavior is to install a new, higher power speaker, and/or a larger diameter speaker, but that will definitely change the tone of the amp, which we don't want to do.
I changed the V1 cathode bypass capacitor from 25 µF to 4.7 µF, and left the Tantalum capacitor in the circuit.
If you look closely you can see the "crossover notch"; the little bump between the top and bottoms of the signal. This is crossover distortion. The "notch" is also used by some amp techs to set the idle bias of the amp - you want to set the bias so the notch disappears. In Fred's amp, the crossover notch is absent when the amp is at idle, but as with all AB amps, it will show up with the rest of the distortion when the amp is pushed a bit, as here. Crossover distortion sounds different, but still good.