S.E.X. input impedance

[Tom-s]

What is the effect of the input impedance presented by the attenuator in an amplifier on that amplifier (not on whatever comes before the amp)?

For me Bottlehead kits are all about the journey and i'd like to make most out of them. And the new S.E.X. kit leaves a lot of room inside for eXperimenting.
A ladder type stepped attenuator would be a great step up from standard and present a constant 100K input impedance via 4 resistors.
A shunt attenuator could be a step better with half the amount of resistors in the signal path (or with the same and both a coarse and fine control).
But this would make for a varying and lower input impedance.
Would this be a problem for the S.E.X. 3.0 circuit as lowest would be about 25kOhm?
I'd love to hear the sweetest whispers with S.E.X.. 

[Paul Joppa]

The input impedance by itself does not affect the rest of the amp. It does affect the source device, of course - but you didn't ask about that!

The output impedance of the level control (which is the resistance between the input stage grid and ground) should not be too high. This is to prevent grid leakage currents from disturbing the operating current. Tube spec sheets will usually specify the maximum, often different for fixed or cathode bias. This is hardly ever a problem - for the 6DN7/6FJ7 the maximum resistance from grid to ground is specified as 2.2 Megohms.

If the impedance levels of the control are too high, it is possible to lose some treble due to the input tube's Miller capacitance. That's usually not a problem until the level control is operating at 200K or more, but you'd have to do the math depending on the particular control topology.

[Tom-s]

Now i get it. I bought 2 amps. S.E.X. and Mainline last week.
But couldn't figure out why SEX would use 100K input pot vs 25K in the Mainline.
But the better question would be, why doesn't the Mainline use a 100K pot.
And now looking a datasheet, grid to ground resistance of the 6C45P tube 0.15Mohms, gave (part of) the answer.
Thank you for this clarification. 

[Paul Birkeland]

A ladder type stepped attenuator would be a great step up from standard and present a constant 100K input impedance via 4 resistors.

The pot also presents a constant 100K input impedance.  Do consider the PEC pot also as a potential upgrade.