The Brian May Treble Booster has a input inpedance less than the Guild Brian May Box has.
The input circuit of the Box is a bootstraped common collector circuit, which provides a higher input impedance
I calculated it and found an input impedance of about 400 Kiloohms for a hFE of 200.
Therefore, I think a value range from 250 Kiloohms to 500 Kiloohms for P1 make sense.
Ultimately, however, only using trial and error bring the truth to the light.
In comparison to the other circuits result a value range of 4.7nF to 6.8nf fo C6.
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