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So when the tone potentiometer is turned all the way counter-clockwise and the tone becomes its most dark. The
resistance of VR3 drops down from 10K to practically 0. C4 and C5 are now in parallel with each other, which means the
capacitances of C4 and C5 add together.
=
fc
= 1 / (2
π
RC
) = 1 / (2
π
⋅ R
O
⋅ (
C
5
+ C
4
))
= 1 / (2
π
⋅
998
⋅
0.253
uF
) = 630
Hz
This means that with the tone knob down all the way, frequencies above 630Hz will be cut, giving a darker, wooly tone.
As the pot is turned clockwise, the tone pot becomes a 10K resistor between C4 and C5. This is when the tone becomes
its most bright.
Since C5 is much larger than C4 in capacitance, we can say C5 is a short to ground/rail when calculating the filter. Now
we have a two-pole low pass filter, so we will need to recalculate the value of RO as it will be RO in parallel with the 10K
Tone Pot. This would look like RO-OLD // 10K = 907Ω:
fc
= 1 / (2
π
RC
) = 1 / (2
π
⋅ R
O
⋅
C
4
)
= 1 / (2
π
⋅
907
⋅
0.033
uF
) = 5317
Hz
This means that with the tone knob up all the way, frequencies above 5.3kHz will be cut, which is a much brighter tone
than when the other way. It still cuts highs, but mostly all the abrasively shrill highs, still giving a bit of a warm tone.
