Fender uses a 25µF / 25V electrolytic capacitor to bypass the cathode resistor in just about every amp they ever made.


This combination of cathode resistor and bypass capacitor, especially on the V1a first gain stage triode,  essentially determines the frequency range that that amplifier is expected to reproduce.


A problem with this design has to do with the 3dB cutoff of the cap/resistor, especially in smaller amplifiers such as the Fender Princeton Reverb;  the 10" speakers in these amps often suffer from flabbiness, or "farting out".  Although this symptom can be caused by several different things, a change of the 3dB cutoff point can often solve the problem.


The calculation for the cutoff of a 12AX7 tube with a 25µF bypass cap is 4.3 Hz.  Thus the problem.  4.3 Hz is far below the range of human hearing (and dog hearing, for that matter), and is also far below the lowest note that a standard-tuned guitar can produce (about 80 Hz).  The lowest bass guitar note is about 40 Hz.


Here's the thing:  low notes take more energy to reproduce than high notes.  In this case we are asking the amplifier to reproduce low notes that we can't hear, and the speaker can't reproduce anyway.  It's wasting energy.


Changing the value of the ubiquitous 25µF capacitor to 4.7 µF changes the cutoff to 23 Hz;  still a low value, but much improved over the original 4.3 Hz.


The Fender Princeton Reverb in the example shown here was modified with a 4.7 µF capacitor bypassed with a 1µF Tantalum capacitor (this is a way to improve the bass response of the gain stage, described here ;  you can also see the graphs before/after the modification to the 3dB point.


To do the calculations here you need to know the tube type, the plate resistance, the plate load, the resistance through the Cathode, the cathode resistor value, and the cathode bypass capacitor.  The time constant of the RC circuit is important.  Note how "Fast" the Tantalum capacitor is, compared to the aluminum capacitor.


The  modification shown here was on Fred Shuman's 1967 Fender Princeton Reverb that was having bass flabbiness issues.  The modification of the first stage 3dB cutoff point resolved the problem.  

HardWay Vintage Electronics

Princeton Reverb Bass flabby

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