Ah, good catch!

Yes, just the 22k (R2) in series with 100pF (C2) in parallel yields a Low-Pass Filter with F3 of 72kHz.

But that's still high enough that it should not be causing an F3 of 17kHz from the preamp. I suspect something else is going on here...

Perhaps it's the output impedance of the volume control? If it's a 100k pot, the output impedance could be as high as 25k ohms. The voltage divider formed by the pot has a series resistance which could perhaps be adding to the value of R2, and could also be interacting with the (unnumbered) 470k ohm grid leak resistor (which is unnecessary anyway).

PS - I just thought of something. The Cin of the 12AU7 is likely around 50pF. That's in parallel with C2 (100pF), so the 12AU7's Cin would now be 150pF.

If you calculate a theoretical LPF with Rseries = 47k (the vol put Zout + R2) and 150pF, you get an F3 of only 22.5kHz.

Could that be it?

Yes, just the 22k (R2) in series with 100pF (C2) in parallel yields a Low-Pass Filter with F3 of 72kHz.

But that's still high enough that it should not be causing an F3 of 17kHz from the preamp. I suspect something else is going on here...

Perhaps it's the output impedance of the volume control? If it's a 100k pot, the output impedance could be as high as 25k ohms. The voltage divider formed by the pot has a series resistance which could perhaps be adding to the value of R2, and could also be interacting with the (unnumbered) 470k ohm grid leak resistor (which is unnecessary anyway).

PS - I just thought of something. The Cin of the 12AU7 is likely around 50pF. That's in parallel with C2 (100pF), so the 12AU7's Cin would now be 150pF.

If you calculate a theoretical LPF with Rseries = 47k (the vol put Zout + R2) and 150pF, you get an F3 of only 22.5kHz.

Could that be it?

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