Sodacose
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As long as your source can drive a 5k impedance (worst case input impedance scenario with a 20k pot), the concern with using a different pot value is the input capacitance of the 19J6 and whether it will create high frequency roll-off.
Let's do the math!
From the 19J6 datasheet, Cg-k is 2.6pf and Cg-p is 1.5pf. Let's call stage gain Mu (though it's much lower in reality). So Cmiller is about 60pf. But we have two 19J6s in parallel for each channel so double this to 120pf.
We'll use the full 20k impedance from the pot as Z and remember to express Cmiller in farads, not micromicrofarads (uuf or pf).
1 / (2 * pi * 20,000 * 120 x 10^-12) = a little over 60khz
The 50k pot PM spec'd gives a roll off at about 26khz using the same Cmiller as above. You'll actually have a little more high frequency bandwidth. I don't think we have to worry about oscillation here (60khz is still pretty low in the grand scheme of things), but you could add a 300-1k grid stopper if it worries you.
Let's do the math!
Cmiller = Cg-k + Cg-p * (stage gain + 1)
From the 19J6 datasheet, Cg-k is 2.6pf and Cg-p is 1.5pf. Let's call stage gain Mu (though it's much lower in reality). So Cmiller is about 60pf. But we have two 19J6s in parallel for each channel so double this to 120pf.
f = 1 / (2 * pi * Cmiller * Z)
We'll use the full 20k impedance from the pot as Z and remember to express Cmiller in farads, not micromicrofarads (uuf or pf).
1 / (2 * pi * 20,000 * 120 x 10^-12) = a little over 60khz
The 50k pot PM spec'd gives a roll off at about 26khz using the same Cmiller as above. You'll actually have a little more high frequency bandwidth. I don't think we have to worry about oscillation here (60khz is still pretty low in the grand scheme of things), but you could add a 300-1k grid stopper if it worries you.