Tube treble booster

Using this spice model, we can simulate a Tube treble booster.

This circuit is for the Banana Booster, which is actually the same as the Valvecaster circuit, as discussed at diystompboxes.com.

To find out how this booster actually performs, we will look at the frequency response chart:

What is peculiar, is the cut-off frequency: it is pretty low for a treble booster. Actually, this circuit is more an overdrive circuit than a treble booster, as is clear from the waveforms: at 440Hz the distortion is quite similar to the distortion at 2744Hz.

The high-pass filter in this circuit is formed by capacitor \(C_1\) and resistor \(R_6\). The cut-off point can be calculated with the formula:

\[f = \dfrac{1}{2πCR} = \dfrac{1}{2πC_1R_6} = \dfrac{1}{2π * 47nF * 47kΩ} = 7Hz\]

7Hz is actually below the audible sound, so no filtering is performed. To achieve an actual treble booster, we need to lower the values of \(C_1\) or \(R_6\). Lowering the value of \(R_6\) will also change the biasing of the tube, so it is better to change the value of \(C_1\). The plot below gives the characteristics for different values of \(C_1\).

\(C_1\)	\(f\)	Output
47nF	7Hz	-1.7dB
4n7F	72Hz	-1.6dB
470pF	720Hz	- 2.0dB
200pF	1693Hz	-2.4 dB
100pF	3386Hz	-3.1 dB

Selecting a value of 470pF or 200pF seems to be the best. Also notice that the amplification of this circuit is much lower than the amplification of the other two treble boosters.