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Unrelated question about this schematic

Dear All,

I will use a similar input arrangement in my new amp. The input xfrm will be a Lundahl 1544A. When wired 1:1 the idle impedance @ 40Khz is described as to be 80kOhms, suggested use is 10Kohm / 10Kohm.

So I than conclude that input impedance of this circuit will be 10kOhm(?) I have problem though determining the the source resistance (impedance) the tube sees in this arrangement. Is this 10KOhm? or 80Kohm@40Hz? The 100K pot is in parallel with the secondary, the way I see it the source resistance than changes with the setting of the pot (with zero resistance when the pot is near zero volume), is this correct?

I ask this because I want to calculate the f1 (-3dB) point; 1/( 2 x PI x source resistance x input capacitance) of this stage.

Regards,
NC

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