Gain section frequency response

[GainFreak]

Hi,

So we have this gain section from a real amp. The question is how to recalculate component values if I want to replace the 500k pot with an 1M one and get the same frequency response.
I may be wrong but but I see here a capacitive divider (K5/K4) and a parallel RC filter (K5/P1). At first I thought it would be enough to change to K4 to 1nF but then I found this PDF file (p13-15) where things look more complicated.
So what do you think about this?

[pdf64]

We’re missing the output impedance of the preceding stage.
Lecture 14 probably assumes that it’s 0, whereas for a common cathode stage, it’ll probably be in the range 40k - 100k. So large enough to skew the calculations a bit.
But halving all the cap values should be close enough for a guitar amp.

[GainFreak]

The preceding stage is a common cathode stage.

[tschrama]

That's the Uberschall, isn't it?

ditto what pdf64 said. Alternatively I have wired two 1M resistors from lugs 1 and 3 to lug 2 of the 1Meg pot. not exactly the same, but it works.

[GainFreak]

Yes, it's the Uber. The thing is the gain control is very sensitive with a regular 500k log pot which is ~25% taper, so I decided to use a 1M 10% taper I had by hand.

[J M Fahey]

Do it the right way.

Get a 500k 10% pot, which by the way is a standard part and easy to find.

[GainFreak]

A 5% 500k would be even better but it's a R16 type PCB pot and I have yet to see a 10% one of that type. I've used Bourns PDA24 type for other projects and they were 10% but not suitable in this case.

[pdf64]

…
But halving all the cap values should be close enough for a guitar amp.

On review, it may be better to connect the LPF cap (eg 1nF) K4 directly from anode to 0V or cathode, or at least after C1.