The key components of this bright-boost circuit are the volume control RV and the bypass capacitor CBP. Bass frequencies are attenuated by the volume control's voltage divider effect, but treble bleeds past via the capacitor. The capacitor has no effect with the volume control set to maximum. It has the most effect at low control settings, when brightening is usually the most desired. Other components in the circuit include the coupling capacitor CG, which is designed to pass audio but block DC, and the grid stopper RGS which combines with the inter-electrode capacitance of the second triode to attenuate radio frequencies.
The calculator above computes the gain at various frequencies based on the capacitor value and volume control setting. It assumes the coupling capacitor CG is large enough to be considered a short circuit for guitar frequencies. This is almost always the case. It also assumes that attenuation due to RGS is only significant above audio frequencies, also a common assumption.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
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