Bright Boost Capacitor Value

Here is a commonly used circuit for achieving bright boost between preamp stages:

The key components are the volume control R_V and the bypass capacitor C_BP. 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 C_G, which is designed to pass audio but block DC, and the grid stopper R_GS 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 C_G 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 R_GS 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).