While designing the Amplifier Test Bed I observed some evidence that suggested that leaving the cathode bypass capacitor off did NOT improve the distortion figure of an RC amplifier. The actual reference is in Phase Inverters under All About Audio Amplifiers.I have soldered up a circuit on one of my metal chassis breadboards and as I type these words I don't know what the outcome will be. Let's start with a 6SN7 circuit.
Triode Amplifier.
I have chosen a set of operating conditions from the resistance coupled amplifier chart in the back of my Sylvania tube manual. The values from the chart along with the measured values are summarized in the table below. 
For a verbal description click here.
6SN7
Quantity
DescriptionSymbol (Units) From RC
ChartMeasured
With
Capacitor.Measured
Without
Capacitor
Same Input.Measured
Without
Capacitor
Same Output.Plate
Supply
Voltage.Ebb (Volts) 250 - - - Plate
Load
ResistorRb (k ohms) 47 - - - Following Stage
Grid ResistorRcf (k ohms) 100 - - - Cathode
ResistorRk (k ohms) 1.5 - - - Gain Av 14.8 15.7 8.8 8.7 Input
VoltageVin (Volts) 1.0 1.0 1.0 1.8 Output
Voltage.Vo (Volts) 14.8 15.7 8.8 15.7 Total
Harmonic
Distortion.THD (%) 1.4 1.75 0.56 0.98
After the bypass capacitor is removed the gain is reduced so there are two ways to make the test.
- Keep the input voltage the same as given in the chart,
- Keep the output voltage the same.
So far so good. Let's try another tube type.
6SL7
Quantity
DescriptionSymbol (Units) From RC
ChartMeasured
With
Capacitor.Measured
Without
Capacitor
Same Input.Measured
Without
Capacitor
Same Output.Plate
Supply
Voltage.Ebb (Volts) 250 - - - Plate
Load
ResistorRb (k ohms) 100 - - - Following Stage
Grid ResistorRcf (k ohms) 270 - - - Cathode
ResistorRk (k ohms) 1.8 - - - Gain Av 40 37 18 18 Input
VoltageVin (Volts) 0.1 0.1 0.1 0.203 Output
Voltage.Vo (Volts) 4.0 3.7 1.8 3.7 Total
Harmonic
Distortion.THD (%) 0.6 0.53 0.14 0.27
What we see is that if the input is held constant, the distortion is very drastically decreased. If the output is held constant, the distortion is still reduced by a significant amount.
We also see that the effect works for low mu (amplification factor) triodes as well as high mu triodes. The effect should transfer as well to a 12AU7 or a 12AX7.
Pentode Amplifier.
The results previously found indicating there was no benefit from removing the cathode bypass capacitor came from a pentode tube. Here are new data for a 6AU6 that show there is a benefit.
